研究者データベース

渡部 直樹(ワタナベ ナオキ)
低温科学研究所 雪氷新領域部門
教授

基本情報

所属

  • 低温科学研究所 雪氷新領域部門

職名

  • 教授

学位

  • 博士(理学)

ホームページURL

ORCID ID

Researcher ID

  • A-7451-2012

J-Global ID

研究キーワード

  • 原子・分子   星間物質   低温表面反応   atoms & molecules   ice surface   Astrochemistry   

研究分野

  • 自然科学一般 / 宇宙惑星科学
  • 自然科学一般 / 半導体、光物性、原子物理

職歴

  • 2022年04月 - 現在 北海道大学低温科学研究所 所長
  • 2018年04月 - 現在 北海道大学ディスティングイッシュトプロフェッサー
  • 2009年10月 - 現在 北海道大学 低温科学研究所 教授
  • 2020年10月 - 2022年03月 北海道大学総長補佐
  • 2006年 - 2009年09月 北海道大学 低温科学研究所 准教授
  • 1996年07月 - 2006年 北海道大学 低温科学研究所 助手
  • 2000年08月 - 2001年08月 University College London Department of Physics and Astronomy 客員研究員
  • 1994年04月 - 1996年06月 理化学研究所 原子物理研究室 基礎科学特別研究員
  • 1993年10月 - 1994年03月 理化学研究所 奨励研究員

学歴

  • 1990年04月 - 1993年03月   東京都立大学大学院   理学研究科物理学専攻(博士課程)
  • 1988年04月 - 1990年03月   東京都立大学大学院   理学研究科   物理学専攻(修士課程)

所属学協会

  • 日本地球惑星科学連合   原子衝突学会   日本惑星科学会   日本物理学会   

研究活動情報

論文

  • Kenji Furuya, Tetsuya Hama, Yasuhiro Oba, Akira Kouchi, Naoki Watanabe, Yuri Aikawa
    The Astrophysical Journal Letters 2022年07月01日
  • Muneaki Imai, Yoko Oya, Brian Svoboda, Hauyu Baobab Liu, Bertrand Lefloch, Serena Viti, Yichen Zhang, Cecilia Ceccarelli, Claudio Codella, Claire J. Chandler, Nami Sakai, Yuri Aikawa, Felipe O. Alves, Nadia Balucani, Eleonora Bianchi, Mathilde Bouvier, Gemma Busquet, Paola Caselli, Emmanuel Caux, Steven Charnley, Spandan Choudhury, Nicolas Cuello, Marta De Simone, Francois Dulieu, Aurora Durán, Lucy Evans, Cécile Favre, Davide Fedele, Siyi Feng, Francesco Fontani, Logan Francis, Tetsuya Hama, Tomoyuki Hanawa, Eric Herbst, Shingo Hirano, Tomoya Hirota, Andrea Isella, Izaskun Jímenez-Serra, Doug Johnstone, Claudine Kahane, Romane Le Gal, Laurent Loinard, Ana López-Sepulcre, Luke T. Maud, María José Maureira, Francois Menard, Seyma Mercimek, Anna Miotello, George Moellenbrock, Shoji Mori, Nadia M. Murillo, Riouhei Nakatani, Hideko Nomura, Yasuhiro Oba, Ross O’Donoghue, Satoshi Ohashi, Yuki Okoda, Juan Ospina-Zamudio, Jaime Pineda, Linda Podio, Albert Rimola, Takeshi Sakai, Dominique Segura-Cox, Yancy Shirley, Vianney Taquet, Leonardo Testi, Charlotte Vastel, Naoki Watanabe, Yoshimasa Watanabe, Arezu Witzel, Ci Xue, Bo Zhao, Satoshi Yamamoto
    The Astrophysical Journal 934 1 70 - 70 2022年07月01日 
    Abstract The chemical diversity of low-mass protostellar sources has so far been recognized, and environmental effects are invoked as its origin. In this context, observations of isolated protostellar sources without the influence of nearby objects are of particular importance. Here, we report the chemical and physical structures of the low-mass Class 0 protostellar source IRAS 16544−1604 in the Bok globule CB 68, based on 1.3 mm Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of ∼70 au that were conducted as part of the large program FAUST. Three interstellar saturated complex organic molecules (iCOMs), CH3OH, HCOOCH3, and CH3OCH3, are detected toward the protostar. The rotation temperature and the emitting region size for CH3OH are derived to be 131 ± 11 K and ∼10 au, respectively. The detection of iCOMs in close proximity to the protostar indicates that CB 68 harbors a hot corino. The kinematic structure of the C18O, CH3OH, and OCS lines is explained by an infalling–rotating envelope model, and the protostellar mass and the radius of the centrifugal barrier are estimated to be 0.08–0.30 M and <30 au, respectively. The small radius of the centrifugal barrier seems to be related to the small emitting region of iCOMs. In addition, we detect emission lines of c-C3H2 and CCH associated with the protostar, revealing a warm carbon-chain chemistry on a 1000 au scale. We therefore find that the chemical structure of CB 68 is described by a hybrid chemistry. The molecular abundances are discussed in comparison with those in other hot corino sources and reported chemical models.
  • Masashi Tsuge, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 933 2 138 - 138 2022年07月01日 [査読有り]
     
    Abstract Chemical processes on the surface of icy grains play an important role in the chemical evolution in molecular clouds. In particular, reactions involving nonenergetic hydrogen atoms accreted from the gaseous phase have been extensively studied. These reactions are believed to effectively proceed only on the surface of the icy grains; thus, molecules embedded in the ice mantle are not considered to react with hydrogen atoms. Recently, Tsuge et al. suggested that nonenergetic hydrogen atoms can react with CO molecules even in ice mantles via diffusive hydrogenation. This investigation was extended to benzene and naphthalene molecules embedded in amorphous solid water (ASW) in the present study, which revealed that a portion of these molecules could be fully hydrogenated in astrophysical environments. The penetration depths of nonenergetic hydrogen atoms into porous and nonporous ASW were determined using benzene molecules to be >50 and ∼10 monolayers, respectively (1 monolayer ≈ 0.3 nm).
  • W. M. C. Sameera, Bethmini Senevirathne, Thanh Nguyen, Yasuhiro Oba, Atsuki Ishibashi, Masashi Tsuge, Hiroshi Hidaka, Naoki Watanabe
    Frontiers in Astronomy and Space Sciences 9 2022年05月30日 [査読有り]
     
    Heterogeneous radical processes on ice surfaces play a vital role in the formation of building blocks of the biologically relevant molecules in space. Therefore, quantitative mechanistic details of the radical binding and radical reactions on ices are crucial in rationalizing the chemical evolution in the Universe. The radical chemistry on ice surfaces was explored at low temperatures by combining quantum chemical calculations and laboratory experiments. A range of binding energies was observed for OH, HCO, CH3, and CH3O radicals binding on ices. Computed reaction paths of the radical reactions on ices, OCS + H and PH3 + D, explained the experimentally observed products. In both radical reactions, quantum tunnelling plays a key role in achieving the reactions at low temperatures. Our findings give quantitative insights into radical chemistry on ice surfaces in interstellar space and the planetary atmospheres.
  • Satoshi Ohashi, Claudio Codella, Nami Sakai, Claire J. Chandler, Cecilia Ceccarelli, Felipe Alves, Davide Fedele, Tomoyuki Hanawa, Aurora Durán, Cécile Favre, Ana López-Sepulcre, Laurent Loinard, Seyma Mercimek, Nadia M. Murillo, Linda Podio, Yichen Zhang, Yuri Aikawa, Nadia Balucani, Eleonora Bianchi, Mathilde Bouvier, Gemma Busquet, Paola Caselli, Emmanuel Caux, Steven Charnley, Spandan Choudhury, Nicolas Cuello, Marta De Simone, Francois Dulieu, Lucy Evans, Siyi Feng, Francesco Fontani, Logan Francis, Tetsuya Hama, Eric Herbst, Shingo Hirano, Tomoya Hirota, Muneaki Imai, Andrea Isella, Izaskun Jímenez-Serra, Doug Johnstone, Claudine Kahane, Romane Le Gal, Bertrand Lefloch, Luke T. Maud, Maria Jose Maureira, Francois Menard, Anna Miotello, George Moellenbrock, Shoji Mori, Riouhei Nakatani, Hideko Nomura, Yasuhiro Oba, Ross O’Donoghue, Yuki Okoda, Juan Ospina-Zamudio, Yoko Oya, Jaime Pineda, Albert Rimola, Takeshi Sakai, Dominique Segura-Cox, Yancy Shirley, Brian Svoboda, Vianney Taquet, Leonardo Testi, Charlotte Vastel, Serena Viti, Naoki Watanabe, Yoshimasa Watanabe, Arezu Witzel, Ci Xue, Bo Zhao, Satoshi Yamamoto
    The Astrophysical Journal 927 1 54 - 54 2022年03月01日 [査読有り]
     
    Abstract We report a study of the low-mass Class 0 multiple system VLA 1623AB in the Ophiuchus star-forming region, using H13CO+ (J = 3–2), CS (J = 5–4), and CCH (N = 3–2) lines as part of the ALMA Large Program FAUST. The analysis of the velocity fields revealed the rotation motion in the envelope and the velocity gradients in the outflows (about 2000 au down to 50 au). We further investigated the rotation of the circumbinary VLA 1623A disk, as well as the VLA 1623B disk. We found that the minor axis of the circumbinary disk of VLA 1623A is misaligned by about 12° with respect to the large-scale outflow and the rotation axis of the envelope. In contrast, the minor axis of the circumbinary disk is parallel to the large-scale magnetic field according to previous dust polarization observations, suggesting that the misalignment may be caused by the different directions of the envelope rotation and the magnetic field. If the velocity gradient of the outflow is caused by rotation, the outflow has a constant angular momentum and the launching radius is estimated to be 5–16 au, although it cannot be ruled out that the velocity gradient is driven by entrainments of the two high-velocity outflows. Furthermore, we detected for the first time a velocity gradient associated with rotation toward the VLA 16293B disk. The velocity gradient is opposite to the one from the large-scale envelope, outflow, and circumbinary disk. The origin of its opposite gradient is also discussed.
  • Akira Kouchi, Yuki Kimura, Kensei Kitajima, Hiroyasu Katsuno, Hiroshi Hidaka, Yasuhiro Oba, Masashi Tsuge, Tomoya Yamazaki, Kazuyuki Fujita, Tetsuya Hama, Yukihiro Takahashi, Shunichi Nakatsubo, Naoki Watanabe
    Frontiers in Chemistry 9 2021年12月08日 [査読有り]
     
    The occurrence of hydrogen atom-ordered form of ice Ih, ice XI, in the outer Solar System has been discussed based on laboratory experiments because its ferroelectricity influences the physical processes in the outer Solar System. However, the formation of ice XI in that region is still unknown due to a lack of formation conditions at temperatures higher than 72 K and the effect of UV-rays on the phase transition from ice I to ice XI. As a result, we observed the UV-irradiation process on ice Ih and ice Ic using a newly developed ultra-high vacuum cryogenic transmission electron microscope. We found that ice Ih transformed to ice XI at temperatures between 75 and 140 K with a relatively small UV dose. Although ice Ic partially transformed to ice XI at 83 K, the rate of transformation was slower than for ice Ih. These findings point to the formation of ice XI at temperatures greater than 72 K via UV irradiation of ice I crystals in the Solar System; icy grains and the surfaces of icy satellites in the Jovian and Saturnian regions.
  • Masashi Tsuge, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 923 1 71 - 71 2021年12月01日 [査読有り]
     
    Abstract Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.
  • Thanh Nguyen, Yasuhiro Oba, W. M. C. Sameera, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 2021年12月01日 [査読有り]
  • A. Ishibashi, H. Hidaka, Y. Oba, A. Kouchi, N. Watanabe
    The Astrophysical Journal Letters 2021年11月01日 [査読有り]
  • Akira Kouchi, Masashi Tsuge, Tetsuya Hama, Yasuhiro Oba, Satoshi Okuzumi, Sin-iti Sirono, Munetake Momose, Naoki Nakatani, Kenji Furuya, Takashi Shimonishi, Tomoya Yamazaki, Hiroshi Hidaka, Yuki Kimura, Ken-ichiro Murata, Kazuyuki Fujita, Shunichi Nakatsubo, Shogo Tachibana, Naoki Watanabe
    The Astrophysical Journal 918 2 45 - 45 2021年09月01日 [査読有り]
  • Thanh Nguyen, Yasuhiro Oba, W. M. C. Sameera, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 918 2 2021年09月01日 [査読有り]
     
    Phosphine (PH3) is an important molecule in the chemistry of phosphorus (P)-bearing species in the interstellar medium. Interstellar PH3 is thought to primarily form on icy grains, where several surface processes may occur during and after its formation. To better understand the physicochemical behavior of PH3 on icy grains in dense molecular clouds, we performed experimental and computational studies on the reactions of phosphine with H and D atoms at low temperatures. We found that phosphine and its deuterated isotopologue PD3 are released into the gas phase from icy surfaces (porous amorphous, compact amorphous, and crystalline ice) via chemical desorption. Even though the effective desorption cross section did not vary between the different icy surfaces, the desorption fraction was smallest for porous amorphous ice. We confirmed that, at 10 K on icy surfaces, H-D substitution reactions of PH3 and D-H substitution reactions of PD3 occurred following reactions with D and H atoms, respectively. Because the activation barrier for the abstraction of H or D atoms from phosphine is similar to 1500 K, quantum tunneling should play a role at low temperatures. Under the present experimental conditions, the H-D substitution reaction of PH3 was slightly slower than the D-H substitution reaction of PD3. Therefore, the rates of the H-D and D-H substitution reactions should not be constrained by tunneling reactions but rather by other elementary processes such as atom diffusion on ice. Our experimental results suggest the possible presence of a deuterated phosphine isotopologue (PH2D) in PH3-rich interstellar environments.
  • Akira Kouchi, Masashi Tsuge, Tetsuya Hama, Hiromasa Niinomi, Naoki Nakatani, Takashi Shimonishi, Yasuhiro Oba, Yuki Kimura, Sin-iti Sirono, Satoshi Okuzumi, Munetake Momose, Kenji Furuya, Naoki Watanabe
    Monthly Notices of the Royal Astronomical Society 505 1 1530 - 1542 2021年06月03日 [査読有り]
     
    ABSTRACT The crystallinity and morphology of solid carbon monoxide (CO) on icy interstellar grains were examined by observing the deposition, crystallization, and UV and electrons irradiation of solid CO using transmission electron microscopy. Herein, we found that solid CO deposited in molecular clouds was crystalline, and that even if amorphous CO was deposited, amorphous CO crystallized within 103 yr at 10 K. Conversely, crystalline CO was not amorphized by UV rays or electron beam at 10 K. These results indicated the occurrence of chiral crystalline CO instead of amorphous CO in space. Furthermore, the large surface diffusion coefficients of CO on eamorphous H2O and crystalline CO at 10 K facilitated the morphological equilibration of crystalline CO. Bad wetting of crystalline CO with amorphous H2O proved that the morphology of the ice grains was not spherical with an onion-like structure, as hitherto assumed, but rather it was a polyhedral crystalline CO attached to amorphous H2O. This has important implications for phenomena associated with the collision and subsequent sticking between ice grains, surface chemical reactions, non-thermal desorption of molecules and the origin of homochirality in interstellar biomolecules.
  • G. Molpeceres, V. Zaverkin, N. Watanabe, J. Kästner
    Astronomy & Astrophysics 2021年04月 [査読有り]
  • Yuki Okoda, Yoko Oya, Logan Francis, Doug Johnstone, Shu-ichiro Inutsuka, Cecilia Ceccarelli, Claudio Codella, Claire Chandler, Nami Sakai, Yuri Aikawa, Felipe O. Alves, Nadia Balucani, Eleonora Bianchi, Mathilde Bouvier, Paola Caselli, Emmanuel Caux, Steven Charnley, Spandan Choudhury, Marta De Simone, Francois Dulieu, Aurora Durán, Lucy Evans, Cécile Favre, Davide Fedele, Siyi Feng, Francesco Fontani, Tetsuya Hama, Tomoyuki Hanawa, Eric Herbst, Tomoya Hirota, Muneaki Imai, Andrea Isella, Izaskun Jímenez-Serra, Claudine Kahane, Bertrand Lefloch, Laurent Loinard, Ana López-Sepulcre, Luke T. Maud, María José Maureira, Francois Menard, Seyma Mercimek, Anna Miotello, George Moellenbrock, Shoji Mori, Nadia M. Murillo, Riouhei Nakatani, Hideko Nomura, Yasuhiro Oba, Ross O’Donoghue, Satoshi Ohashi, Juan Ospina-Zamudio, Jaime E. Pineda, Linda Podio, Albert Rimola, Takeshi Sakai, Dominique Segura-Cox, Yancy Shirley, Brian Svoboda, Vianney Taquet, Leonardo Testi, Charlotte Vastel, Serena Viti, Naoki Watanabe, Yoshimasa Watanabe, Arezu Witzel, Ci Xue, Yichen Zhang, Bo Zhao, Satoshi Yamamoto
    The Astrophysical Journal 910 1 11 - 11 2021年03月01日 [査読有り]
     
    We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 to 1800 au, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and (CO)-O-18 line emission, which extends from the source in a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blueshifted with respect to the systemic velocity. A velocity gradient of 1.2 km s(-1) over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.
  • Masashi Tsuge, Naoki Watanabe
    Accounts of Chemical Research 54 3 471 - 480 2021年02月02日 [査読有り]
  • M. Tsuge, T. Namiyoshi, K. Furuya, T. Yamazaki, A. Kouchi, N. Watanabe
    The Astrophysical Journal 908 2 234 - 234 2021年02月01日 [査読有り]
  • Kensei Kitajima, Yoichi Nakai, W. M. C. Sameera, Masashi Tsuge, Ayane Miyazaki, Hiroshi Hidaka, Akira Kouchi, Naoki Watanabe
    The Journal of Physical Chemistry Letters 12 1 704 - 710 2021年01月14日 [査読有り]
  • W. M. C. Sameera, Bethmini Senevirathne, Stefan Andersson, Muhsen Al-lbadi, Hiroshi Hidaka, Akira Kouchi, Gunnar Nyman, Naoki Watanabe
    The Journal of Physical Chemistry A 125 1 387 - 393 2021年01月14日 [査読有り]
  • Masashi Tsuge, Thanh Nguyen, Yasuhiro Oba, Tetsuya Hama, Akira Kouchi, Naoki Watanabe
    Chemical Physics Letters 760 137999 - 137999 2020年12月 [査読有り]
  • Hideyuki Nakano, Naoki Hirakawa, Yasuhiro Matsubara, Shigeru Yamashita, Takuo Okuchi, Kenta Asahina, Ryo Tanaka, Noriyuki Suzuki, Hiroshi Naraoka, Yoshinori Takano, Shogo Tachibana, Tetsyua Hama, Yasuhiro Oba, Yuki Kimura, Naoki Watanabe, Akira Kouchi
    Scientific Reports 10 7755  2020年12月 [査読有り][通常論文]
  • A. Miyazaki, N. Watanabe, W. M. C. Sameera, Y. Nakai, M. Tsuge, T. Hama, H. Hidaka, A. Kouchi
    Physical Review A 102 5 2020年11月20日 [査読有り]
  • Tetsuya Hama, Akira Kouchi, Naoki Watanabe, Nobutaka Shioya, Takafumi Shimoaka, Takeshi Hasegawa
    Biophysical Chemistry 266 106459 - 106459 2020年11月 [査読有り][通常論文]
  • Tetsuya Hama, Atsuki Ishibashi, Akira Kouchi, Naoki Watanabe, Nobutaka Shioya, Takafumi Shimoaka, Takeshi Hasegawa
    The Journal of Physical Chemistry Letters 11 18 7857 - 7866 2020年09月17日 [査読有り]
  • Masashi Tsuge, Hiroshi Hidaka, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 900 2 187 - 187 2020年09月15日 [査読有り]
  • Thanh Nguyen, Yasuhiro Oba, Takashi Shimonishi, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 898 2 2020年08月03日 [査読有り]
     
    Phosphine (PH3), an important molecule for the chemistry of phosphorus (P)-bearing species in the interstellar medium (ISM) is considered to form primarily on interstellar grains. However, no report exists on the processes of PH(3)formation on grains. Here, we experimentally studied the reactions of hydrogen (H) atoms and PH(3)molecules on compact amorphous solid water, with a particular focus on the chemical desorption of PH(3)at 10-30 K. After exposure to H atoms for 120 minutes, up to 50% of solid PH(3)was lost from the icy surface. On the basis of experiments using deuterium atoms, it was concluded that the loss of PH(3)resulted from chemical desorption through the reactions PH3+ H -> PH2 + H(2)and/or PH2+ H -> PH3. The effective desorption cross-section was similar to 5 x 10(-17)cm(2), which is three times larger than that of hydrogen sulfide measured under similar experimental conditions. The present results suggest that the formation of PH3, and possibly PH(2)and PH, followed by their desorption from icy grains, may contribute to the formation of PN and PO in the gas phase, and thus may play a role in the P chemistry of the ISM.
  • Naoki Watanabe, Masashi Tsuge
    Journal of the Physical Society of Japan 2020年05月15日
  • Akira Kouchi, Kenji Furuya, Tetsuya Hama, Takeshi Chigai, Takashi Kozasa, Naoki Watanabe
    ASTROPHYSICAL JOURNAL LETTERS 891 1 2020年03月 [査読有り][通常論文]
     
    The importance of the activation energy of surface diffusion (E-sd) of adsorbed molecules on amorphous solid water (ASW) has been widely discussed in terms of chemical reactions on ASW at low temperatures. However, in previous work, E-sd has not been measured directly but estimated from indirect experiments. It has been assumed in chemical network calculations that E-sd is between 0.3 and 0.8 of the desorption energies of a molecule. It remains important to obtain direct measurements of E-sd. We performed in situ observations of the deposition process of CO and CO2 on ASW using transmission electron microscopy (TEM) and deduced the E-sd of CO and CO2 on ASW to be 350 50 and 1500 100 K, respectively. The value of E-sd of CO is approximately 0.3 of the total adsorption energy of CO on ASW, i.e., much smaller than assumed in chemical network calculations, where the corresponding figure is 575 K, assuming approximately 0.5 of the desorption energy. We demonstrated that TEM is very useful not only for the observation of ices but also for the measurement of some physical properties that are relevant in astrochemistry and astrophysics. Using the E-sd of CO measured in the present study (350 K), we have updated the chemical network model of Furuya et al., confirming that CO2 could be efficiently formed by the reaction CO + OH -> CO2 + H in the initial stages of the evolution of molecular clouds.
  • Furuya Kenji, Aikawa Yuri, Hama Tetsuya, Watanabe Naoki
    ASTROPHYSICAL JOURNAL 882 2 2019年09月10日 [査読有り][通常論文]
     
    We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H-2 via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to star- and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous H-2 on grain surfaces, which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed H-2. It is found that the spin conversion efficiency depends on the H-2 gas density and the surface temperature. As a general trend, enhanced H-2 gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy H-2 is inefficient (i.e., adsorbed H-2 does not desorb and hinders another gaseous H-2 to be adsorbed), while at warm temperatures, the residence time of H-2 on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas phase in star- and planet-forming regions. Finally, we establish a simple, but accurate way to implement the H-2 spin conversion on grain surfaces in existing gas-ice astrochemical models.
  • Y. Oba, Y. Takano, H. Naraoka, N. Watanabe, A. Kouchi
    Nature Communications 10 1 4413  2019年09月 [査読有り][通常論文]
     
    The synthesis of nucleobases in natural environments, especially in interstellar molecular clouds, is the focus of a long-standing debate regarding prebiotic chemical evolution. Here we report the simultaneous detection of all three pyrimidine (cytosine, uracil and thymine) and three purine nucleobases (adenine, xanthine and hypoxanthine) in interstellar ice analogues composed of simple molecules including H2O, CO, NH3 and CH3OH after exposure to ultraviolet photons followed by thermal processes, that is, in conditions that simulate the chemical processes accompanying star formation from molecular clouds. Photolysis of primitive gas molecules at 10 K might be one of the key steps in the production of nucleobases. The present results strongly suggest that the evolution from molecular clouds to stars and planets provides a suitable environment for nucleobase synthesis in space.
  • Hama, Tetsuya, Seki, Kousuke, Ishibashi, Atsuki, Miyazaki, Ayane, Kouchi, Akira, Watanabe, Naoki, Shimoaka, Takafumi, Hasegawa, Takeshi
    PLANT AND CELL PHYSIOLOGY 60 7 1567 - 1580 2019年07月 [査読有り][通常論文]
     
    The surface of most aerial plant organs is covered with the cuticle, a membrane consisting of a variety of organic compounds, including waxes, cutin (a polyester) and polysaccharides. The cuticle serves as the multifunctional interface between the plant and the environment, and plays a major role in protecting plants against various environmental stress factors. Characterization of the molecular arrangements in the intact cuticle is critical for the fundamental understanding of its physicochemical properties; however, this analysis remains technically challenging. Here, we describe the nondestructive characterization of the intact cuticle of Brassica oleracea L. leaves using polarization modulation-infrared (IR) reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS has a probing depth of less than several hundreds of nanometers, and reveals the crystalline structure of the wax covering the cuticle surface (epicuticular wax) and the nonhydrogen-bonding character of cutin. Combined analysis using attenuated total reflection- IR spectra suggested that hemicelluloses xylan and xyloglucan are present in the outer cuticle region close to the epicuticular wax, whereas pectins are dominant in the inner cuticle region (depth of <= 2 mu m). PM-IRRAS can also determine the average orientation of the cuticular molecules, as indicated by the positive and negative spectral peaks. This unique advantage reveals the orientational order in the intact cuticle; the hydrocarbon chains of the epicuticular wax and cutin and the backbones of hemicelluloses are oriented perpendicular to the leaf surface. PM-IRRAS is a versatile, informative and easy-to-use technique for studying plant cuticles because it is nondestructive and does not require sample pretreatment and background measurements.
  • Masashi Tsuge, Tetsuya Hama, Yuki Kimura, Akira Kouchi, Naoki Watanabe
    The Astrophysical Journal 878 1 23  2019年06月 [査読有り][通常論文]
     
    The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H-2 formation at relatively high temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and the desorption energetics of H-2 molecules on an amorphous diamond-like carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H-2 detection, the H-2 coverage-dependent activation energies for H-2 desorption are determined. The activation energies decrease with increasing H-2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H-2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H-2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H-2 and D-2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H-2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H atoms might play a role in H-2 formation at relatively high temperatures.
  • Naoki Watanabe
    Proceedings of the International Astronomical Union 15 S350 116 - 122 2019年04月 [査読有り]
  • Y. Oba, T. Tomaru, A. Kouchi, N. Watanabe
    Astrophysical Journal 874 8pp  2019年04月 [査読有り][通常論文]
  • N. Watanabe, W. M. C. Sameera, H. Hidaka, A. Miyazaki, A. Kouchi
    Chemical Physics Letters 737 136820  2019年 [査読有り][通常論文]
  • Y. Kimura, M. Tsuge, V. Pirronello, A. Kouchi, N. Watanabe
    Astrophys. J. Lett. 858 L23 5pp  2018年05月 [査読有り][通常論文]
  • Tetsuya Hama, Akira Kouchi, Naoki Watanabe
    Astrophysical Journal Letters 857 2 6pp  2018年04月20日 [査読有り][通常論文]
     
    Water has two nuclear-spin isomers: ortho- and para-H2O. Some observations of interstellar space and cometary comae have reported the existence of gaseous H2O molecules with anomalous ortho-to-para ratios (OPRs) less than the statistical value of three. This has been often used to estimate the formation temperature of ice on dust, which is inferred to be below 50 K. The relation between the nuclear-spin dynamics of H2O in ice at low temperatures and the OPR of gaseous H2O desorbed from the ice has yet to be explored in a laboratory. Consequently, the true meaning of the observed OPRs remains debated. We measure the OPR of H2O photodesorbed from ice made from para-H2O monomers at 11 K, which was prepared by the sublimation of Ne from a para-H2O/Ne matrix. The photodesorbed H2O molecules from the ice have the statistical OPR value of three, demonstrating the immediate nuclear-spin-state mixing of H2O toward the statistical value of ice even at 11 K. The OPR of H2O thermally desorbed from the ice also shows the expected statistical value. Our results indicate that the OPR of H2O desorbed from interstellar ice should be the statistical value regardless of the formation process of the ice, which cannot be used to deduce the ice-formation temperature. This study highlights the importance of interstellar gas-phase processes in understanding anomalous abundance ratios of nuclear-spin isomers of molecules in space.
  • Y. Oba, T. Tomaru, T. Lamberts, A. Kouchi, N. Watanabe
    Nature Astronomy 2 3 228 - 232 2018年03月01日 [査読有り][通常論文]
     
    In molecular clouds at temperatures as low as 10 K, all species except hydrogen and helium should be locked in the heterogeneous ice on dust grain surfaces. Nevertheless, astronomical observations have detected over 150 different species in the gas phase in these clouds. The mechanism by which molecules are released from the dust surface below thermal desorption temperatures to be detectable in the gas phase is crucial for understanding the chemical evolution in such cold clouds. Chemical desorption, caused by the excess energy of an exothermic reaction, was first proposed as a key molecular release mechanism almost 50 years ago 1 . Chemical desorption can, in principle, take place at any temperature, even below the thermal desorption temperature. Therefore, astrochemical network models commonly include this process 2,3 . Although there have been a few previous experimental efforts 4-6, no infrared measurement of the surface (which has a strong advantage to quantify chemical desorption) has been performed. Here, we report the first infrared in situ measurement of chemical desorption during the reactions H + H2S → HS + H2 (reaction 1) and HS + H → H2S (reaction 2), which are key to interstellar sulphur chemistry 2,3 . The present study clearly demonstrates that chemical desorption is a more efficient process for releasing H2S into the gas phase than was previously believed. The obtained effective cross-section for chemical desorption indicates that the chemical desorption rate exceeds the photodesorption rate in typical interstellar environments.
  • Tetsuya Hama, Akira Kouchi, Naoki Watanabe, Shinichi Enami, Takafumi Shimoaka, Takeshi Hasegawa
    Journal of Physical Chemistry B 121 49 11124 - 11131 2017年12月14日 [査読有り][通常論文]
     
    The outermost surface of the leaves of land plants is covered with a lipid membrane called the cuticle that protects against various stress factors. Probing the molecular-level structure of the intact cuticle is highly desirable for understanding its multifunctional properties. We report the in situ characterization of the surface structure of Kalanchoe pinnata leaves using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Without sample pretreatment, PM-IRRAS measures the IR spectra of the leaf cuticle of a potted K. pinnata plant. The peak position of the CH2-related modes shows that the cuticular waxes on the leaf surface are mainly crystalline, and the alkyl chains are highly packed in an all-trans zigzag conformation. The surface selection rule of PM-IRRAS revealed the average orientation of the cuticular molecules, as indicated by the positive and negative signals of the IR peaks. This unique property of PM-IRRAS revealed that the alkyl chains of the waxes and the main chains of polysaccharides are oriented almost perpendicular to the leaf surface. The nondestructive, background-free, and environmental gas-free nature of PM-IRRAS allows the structure and chemistry of the leaf cuticle to be studied directly in its native environment.
  • Yasuhiro Oba, Yoshinori Takano, Hiroshi Naraoka, Akira Kouchi, Naoki Watanabe
    ASTROPHYSICAL JOURNAL 849 2 9pp  2017年11月 [査読有り][通常論文]
     
    Hexamethylenetetramine (HMT) is a representative product after the photolysis of interstellar ice analogs containing methanol followed by warming-up to room temperature. Since interstellar methanol is often significantly enriched in deuterium (D), the HMT photoproduct is expected to inherit D atoms from deuterated methanol. However, D fractionation upon the formation of HMT is not well understood, especially when it is produced from partly deuterated methanol isotopologues such as CH2DOH and CH3OD. Here, we experimentally studied the composition of deuterated HMT (d(n)-HMT, where n is the number of D atoms) at the isotopologue level formed by the photolysis of ice mixtures containing deuterated methanol, CH2DOH or CH3OD, at 10 or 77 K. The analyses were performed using a state-of-the-art high-resolution mass spectrometer coupled with a compoundspecific separation technique. The formation of dn-HMT (n = 0-8) was confirmed under all experimental conditions. In addition, methyl-and hydroxyl-substituted HMT and their deuterated isotopologues were also obtained in the products. The deuterium enrichment was outstanding when CH2DOH was used rather than CH3OD, and when photolysis was performed at 77 K rather than 10 K. We found that the deuteration level of the formed HMT far exceeded that of the reactants under the present experimental conditions. These results obtained during stable isotope probing of deuterium strongly suggest that HMT can play a role as an organic pool of interstellar D atoms. These may be distributed into other chemical species through molecular evolution in space.
  • Shogo Tachibana, Akira Kouchi, Tetsuya Hama, Yasuhiro Oba, Laurette Piani, Iyo Sugawara, Yukiko Endo, Hiroshi Hidaka, Yuki Kimura, Ken-Ichiro Murata, Hisayoshi Yurimoto, Naoki Watanabe
    Science advances 3 9 eaao2538  2017年09月 [査読有り][通常論文]
     
    Interstellar ice is believed to be a cradle of complex organic compounds, commonly found within icy comets and interstellar clouds, in association with ultraviolet (UV) irradiation and subsequent warming. We found that UV-irradiated amorphous ices composed of H2O, CH3OH, and NH3 and of pure H2O behave like liquids over the temperature ranges of 65 to 150 kelvin and 50 to 140 kelvin, respectively. This low-viscosity liquid-like ice may enhance the formation of organic compounds including prebiotic molecules and the accretion of icy dust to form icy planetesimals under certain interstellar conditions.
  • Tetsuya Hama, Shinnosuke Ishizuka, Tomoya Yamazaki, Yuki Kimura, Akira Kouchi, Naoki Watanabe, Toshiki Sugimoto, Valerio Pirronello
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19 27 17677 - 17684 2017年07月 [査読有り][通常論文]
     
    Crystalline ice formation requires water molecules to be sufficiently mobile to find and settle on the thermodynamically most stable site. Upon cooling, however, diffusion and rearrangement become increasingly kinetically difficult. Water ice grown by the condensation of water vapor in laboratory is thus generally assumed to be in a metastable amorphous form below 100 K. Here, we demonstrate the possibility of crystalline ice formation at extremely low temperature using a water/neon matrix (1/1000, 30000 monolayers) prepared at 6 K, which is subsequently warmed to 11-12 K. In situ infrared spectroscopy revealed the assembly of the dispersed water molecules, forming crystalline ice I during the sublimation of the neon matrix for 40-250 seconds. This finding indicates that the high mobility of the water molecules during matrix sublimation can overcome the kinetic barrier to form crystals even at extremely low temperature.
  • Rafael Escribano, Emilio Artacho, Akira Kouchi, Tetusya Hama, Yuki Kimura, Hiroshi Hidaka, Naoki Watanabe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 19 10 7280 - 7287 2017年03月 [査読有り][通常論文]
     
    Models for the inclusion of water molecules in carbon monoxide matrices are developed using density functional theory applied to amorphous solid systems. The models cover a large range of systems for smaller or larger CO matrices with different water content, consisting of either individual H2O molecules or small clusters linked by H-bonds. The vibrational spectra of the samples are predicted at the minimum of their potential energy surface. The spectra allow instances where the water molecules remain isolated or form aggregates to be discerned, and they also provide an indication of the strength of the H-bonding, when present. The calculations support recent experimental observations that linked IR bands at 3707 cm(-1) and 3617 cm(-1) to the presence of unbound water molecules in water-poor CO/H2O mixed ices. Assignment of some observed bands to water dimers or trimers is suggested as well. The residual static pressure in fixed-volume simulation cells is also calculated.
  • 大場 康弘, 高野 淑識, 奈良岡 浩, 渡部 直樹, 香内 晃
    日本地球化学会年会要旨集 64 131 - 131 一般社団法人日本地球化学会 2017年 

    重水素一置換メタノールCH2DOHを含む疑似星間塵氷の光化学反応で生成するヘキサメチレンテトラミン(HMT:C6H12N4)の重水素存在度を報告する。10Kの反応基板上に混合ガス(H2O:CO:NH3:CH2DOH=5:2:2:2)を蒸着し,疑似星間塵氷を作製した。それと同時に真空紫外光を照射し,星間塵上での光化学反応を再現した。生成したHMTには非常に多くの重水素が含まれていたことから,本研究はHMTが化学進化における優れた重水素キャリアとなることを強く示唆した。

  • Tetsuya Hama, Akira Kouchi, Naoki Watanabe
    XXX INTERNATIONAL CONFERENCE ON PHOTONIC, ELECTRONIC, AND ATOMIC COLLISIONS (ICPEAC2017) 875 2017年 [査読有り][通常論文]
     
    The ortho-to-para ratio of water photodesorbed from ice at 10 K shows a statistical value of 3, and does not reflect the formation temperature of the ice.
  • Shinnosuke Ishizuka, Yuki Kimura, Tomoya Yamazaki, Tetsuya Hama, Naoki Watanabe, Akira Kouchi
    CHEMISTRY OF MATERIALS 28 23 8732 - 8741 2016年12月 [査読有り][通常論文]
     
    Multiple pathways in crystal nucleation are now known to be more common than previously predicted; it is, therefore, crucial to understand the early stages of crystallization. Even in relatively simple vapor-phase homogeneous nucleation, the process has significant potential diversity. Here, we experimentally show crystalline Al2O3 nanoparticles forming via precisely two steps in the nucleation process from supersaturated vapor with a moderate cooling rate. In situ FT-IR measurement of nucleation allowed us to observe the formation of Al2O3 nanoparticles. Liquid-like particles first nucleated from the vapor before crystallizing. The crystalline phase was preserved by quenching without further transformation into the most stable alpha-Al2O3 polymorph. The precipitated phase changed from delta-Al2O3 for pure Al2O3 to gamma-Al2O3 or theta-Al2O3 by adding Sb or Cr, respectively. We demonstrate that a two-step process occurs in homogeneous nucleation of refractory materials from supersaturated vapor, which may facilitate polymorphic control in industry formation in space. and improve understanding of cosmic dust
  • Yasuhiro Oba, Naoki Watanabe, Akira Kouchi
    CHEMICAL PHYSICS LETTERS 662 14 - 18 2016年10月 [査読有り][通常論文]
     
    We report an experimental study on the catalytic effect of solid water on the reactivity of hydrogen abstraction (H-abstraction) from dimethyl ether (DME) in the low-temperature solid DME-H2O complex. When DME reacted with deuterium atoms on a surface at 15-25 K, it was efficiently deuterated via successive tunneling H-abstraction and deuterium (D)-addition reactions. The 'effective rate constant for DME-H2O + D was found to be about 20 times smaller than that of pure DME + D. This provides the first evidence that the presence of solid water has a negative catalytic effect on tunneling H-abstraction reactions. (C) 2016 Elsevier B.V. All rights reserved.
  • Yasuhiro Oba, Kazuya Osaka, Takeshi Chigai, Akira Kouchi, Naoki Watanabe
    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 462 1 689 - 695 2016年10月 [査読有り][通常論文]
     
    Ethanol (CH3CH2OH) is one of the most abundant complex organic molecules in star-forming regions. Despite its detection in the gas phase only, ethanol is believed to be formed by low temperature grain-surface reactions. Methanol, the simplest alcohol, has been a target for observational, experimental, and theoretical studies in view of its deuterium enrichment in the interstellar medium; however, the deuterium chemistry of ethanol has not yet been an area of focus. Recently, deuterated dimethyl ether, a structural isomer of ethanol, was found in star forming regions, indicating that deuterated ethanol can also be present in those environments. In this study, we performed laboratory experiments on the deuterium fractionation of solid ethanol at low temperatures through a reaction with deuterium (D) atoms at 10 K. Hydrogen (II)-D substitution, which increases the deuteration level, was found to occur on the ethyl group but not on the hydroxyl group. In addition, when deuterated ethanol (e.g. CD3CD2OD) solid was exposed to H atoms at 10 K, D-H substitution that reduced the deuteration level occurred on the ethyl group. Based on the results, it is likely that deuterated ethanol is present even under H-atom-dominant conditions in the interstellar medium.
  • Yasuhiro Oba, Yoshinori Takano, Naoki Watanabe, Akira Kouchi
    ASTROPHYSICAL JOURNAL LETTERS 827 1 L18 (7pp)  2016年08月 [査読有り][通常論文]
     
    Deuterium (D) atoms in interstellar deuterated methanol might be distributed into complex organic molecules through molecular evolution by photochemical reactions in interstellar grains. In this study, we use a state-of-the-art high-resolution mass spectrometer coupled with a high-performance liquid chromatography system to quantitatively analyze amino acids and their deuterated isotopologues formed by the photolysis of interstellar ice analogs containing singly deuterated methanol CH2DOH at 10 K. Five amino acids (glycine, alpha-alanine, beta-alanine, sarcosine, and serine) and their deuterated isotopologues whose D atoms are bound to carbon atoms are detected in organic residues formed by photolysis followed by warming up to room temperature. The abundances of singly deuterated amino acids are in the range of 0.3-1.1 relative to each nondeuterated counterpart, and the relative abundances of doubly and triply deuterated species decrease with an increasing number of D atoms in a molecule. The abundances of amino acids increase by a factor of more than five upon the hydrolysis of the organic residues, leading to decreases in the relative abundances of deuterated species for alpha-alanine and beta-alanine. On the other hand, the relative abundances of the deuterated isotopologues of the other three amino acids did not decrease upon hydrolysis, indicating different formation mechanisms of these two groups upon hydrolysis. The present study facilitates both qualitative and quantitative evaluations of D fractionation during molecular evolution in the interstellar medium.
  • A. Kouchi, T. Hama, Y. Kimura, H. Hidaka, R. Escribano, N. Watanabe
    CHEMICAL PHYSICS LETTERS 658 287 - 292 2016年08月 [査読有り][通常論文]
     
    A novel method for the formation of amorphous ice involving matrix sublimation has been developed. A CO-rich CO:H2O mixed ice was deposited at 8-10 K under ultra-high vacuum condition, which was then allowed to warm. After the sublimation of matrix CO at 35 K, amorphous ice remained. The amorphous ice formed exhibits a highly porous microscale texture; however, it also rather exhibits a density similar to that of high-density amorphous ice formed under high pressure. Furthermore, unlike conventional vapor-deposited amorphous ice, the amorphous ice is stable up to 140 K, where it transforms directly to cubic ice Ic. (C) 2016 Elsevier B.V. All rights reserved.
  • Yoichi Nakai, Hiroshi Hidaka, Naoki Watanabe, Takao M. Kojima
    JOURNAL OF CHEMICAL PHYSICS 144 22 224306(11pp)  2016年06月 [査読有り][通常論文]
     
    We measured equilibrium constants for H3O+(H2O)(n-1) + H2O <-> H3O+(H2O)(n) (n = 4-9) reactions taking place in an ion drift tube with various applied electric fields at gas temperatures of 238-330 K. The zero-field reaction equilibrium constants were determined by extrapolation of those obtained at non-zero electric fields. From the zero-field reaction equilibrium constants, the standard enthalpy and entropy changes, Delta H-n,n-1(0) and Delta S-n, n-1(0), of stepwise association for n = 4-8 were derived and were in reasonable agreement with those measured in previous studies. We also examined the electric field dependence of the reaction equilibrium constants at non-zero electric fields for n = 4-8. An effective temperature for the reaction equilibrium constants at non-zero electric field was empirically obtained using a parameter describing the electric field dependence of the reaction equilibrium constants. Furthermore, the size dependence of the parameter was thought to reflect the evolution of the hydrogen-bond structure of H3O+(H2O)(n) with the cluster size. The reflection of structural information in the electric field dependence of the reaction equilibria is particularly noteworthy. Published by AIP Publishing.
  • Hirokazu Ueta, Naoki Watanabe, Tetsuya Hama, Akira Kouchi
    PHYSICAL REVIEW LETTERS 116 25 253201(5pp)  2016年06月 [査読有り][通常論文]
     
    The surface temperature dependence of the ortho-to-para conversion of H-2 on amorphous solid water is first reported. A combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques allowed us to sensitively probe the conversion on the surface of amorphous solid water at temperatures of 9.2-16 K. Within a narrow temperature window of 8 K, the conversion time steeply varied from similar to 4.1 x 10(3) to similar to 6.4 x 10(2) s. The observed temperature dependence is discussed in the context of previously suggested models and the energy dissipation process. The two-phonon process most likely dominates the conversion rate at low temperatures.
  • 植田 寛和, 渡部 直樹, 羽馬 哲也, 香内 晃
    日本物理学会講演概要集 71 742 - 742 一般社団法人日本物理学会 2016年
  • 羽馬 哲也, 香内 晃, 渡部 直樹
    日本物理学会講演概要集 71 2570 - 2571 一般社団法人日本物理学会 2016年
  • Hydrogenation and Deuteration of C2H2 and C2H4 on Cold Grains: Clue to the Formation Mechanism of C2H6 with Astronomical Interest
    H. Kobayashi, H. Hidaka, T. Lamberts, T. Hama, H. Kawakita, J. Kästner, N. Watanabe
    The Astrophysical Journal 837 155  2016年 [査読有り][通常論文]
  • Tetsuya Hama, Akira Kouchi, Naoki Watanabe
    SCIENCE 351 6268 65 - 67 2016年01月 [査読有り][通常論文]
     
    The anomalously low ortho-to-para ratios (OPRs) exhibited by gaseous water in space have been used to determine the formation temperature (<50 kelvin) of ice on cold interstellar dust. This approach assumes that the OPR of water desorbed from ice is related to the ice formation temperature on the dust. However, we report that water desorbed from ice at 10 kelvin shows a statistical high-temperatureOPRof 3, even when the ice is produced in situ by hydrogenation of O-2, a known formation process of interstellar water. This invalidates the assumed relation between OPR and temperature. The necessary reinterpretation of the low OPRs will help elucidate the chemical history of interstellar water from molecular clouds and processes in the early solar system, including comet formation.
  • K. Kuwahata, T. Hama, A. Kouchi, N. Watanabe
    PHYSICAL REVIEW LETTERS 115 13 133201  2015年09月 [査読有り][通常論文]
     
    Reported here is the first observation of the tunneling surface diffusion of a hydrogen (H) atom on water ice. Photostimulated desorption and resonance-enhanced multiphoton ionization methods were used to determine the diffusion rates at 10 Kon amorphous solid water and polycrystalline ice. H-atom diffusion on polycrystalline ice was 2 orders of magnitude faster than that of deuterium atoms, indicating the occurrence of tunneling diffusion. Whether diffusion is by tunneling or thermal hopping also depends on the diffusion length of the atoms and the morphology of the surface. Our findings contribute to a better understanding of elementary physicochemical processes of hydrogen on cosmic ice dust.
  • 羽馬哲也, 香内晃, 渡部直樹
    日本物理学会誌 70 8 608 - 613 一般社団法人日本物理学会 2015年08月 [査読有り][通常論文]
     
    近年の天文観測により,恒星や惑星が誕生する以前の星間分子雲には水分子や有機分子を含む140種を超える分子の存在が確認されている(星間分子に関するデータベースとして,http://www.astro.uni-koeln.de/cdms/moleculesがある).これらの分子は,単純な構造を持つ水素分子(H_2)や一酸化炭素分子などから化学反応を経て進化したものである.恒星が存在しない分子雲中心部は,大量の星間塵により外部からの可視光や紫外光が遮蔽されるため10K程度の極低温に保たれる.そのため,活性化エネルギーを必要とする化学反応は起こりにくく,分子雲は分子生成の場としてはきわめて不利な環境にある.それにもかかわらず,多くの分子種が存在するのは,外部エネルギーを必要としない効率的な分子生成メカニズムが存在するからである.ひとつは活性化エネルギーを必要としない気相でのイオン分子反応であり,これまでの多くの研究でその有効性が確認されてきた.その一方で,気相反応ではその存在量が説明できない分子が存在する.その代表例がH_2や水分子といった恒星や氷惑星・彗星の形成に必須である分子に加え,ホルムアルデヒドやメタノールなどの有機分子である.これらの分子の生成には低温の星間塵表面に物理吸着した原子・分子への水素(H)原子の付加反応が決定的な役割を果たしていると考えられており,低温表面における物理吸着系の化学反応という基礎化学的な興味とも相まって,さまざまな分野の研究者が精力的に研究を進めている.希薄な分子雲の星間塵上で化学反応が起こるためには,H原子が星間塵表面を拡散し,反応相手と出会うことが必要となる.ところが,反応の前過程であるH原子の表面拡散やH_2分子生成に関する実験研究は困難であり,拡散のメカニズム(熱拡散・トンネル拡散)や活性化エネルギーについては依然として不明な点が多い.著者らは最近,レーザー刺激脱離法と共鳴多光子イオン化法を用いて,疑似星間塵表面物質である8Kのアモルファス氷表面上に照射・吸着させたH原子,重水素(D)原子数をモニターする手法を開発した.表面上のH(D)原子は,拡散を経て再結合しH_2(D_2)を生成するため,表面原子数の時間変化は拡散の情報を含んでいる.本手法により,表面拡散のメカニズムを明らかにすると同時に拡散の活性化エネルギーを見積もることに成功した.表面拡散は同位体効果が大きくあらわれる量子トンネル効果によるものでなく,熱的拡散であることが示唆され,その活性化エネルギーはH,D原子に対して,それぞれ22,23meVという値が得られた.また,照射した原子の一部は照射中に速やかに拡散・分子化していること,逆に,吸着後90分以上にわたり表面に残存している原子が存在することも同時にわかった.これらは,アモルファス氷表面には,吸着原子が非常に速く拡散し別の原子との再結合に至る浅い吸着ポテンシャルサイトから,8Kでは拡散できない深いポテンシャルサイトまでさまざまなポテンシャルサイトが分布していることを意味する.本研究により,星間塵表面における水素原子の振る舞いが明らかになりつつある.
  • Y. Oba, N. Watanabe, Y. Osamura, A.Kouchi
    Chemical Physics Letters 634 53 - 59 2015年08月 [査読有り][通常論文]
     
    We report experimental evidence that chiral glycine (NH2CHDCOOH) is formed by the surface reaction of normal glycine (NH2CH2COOH) solid with deuterium (D) atom at 12K under the simulative conditions of interstellar molecular clouds. Chiral glycine formation is most likely initiated by the tunneling abstraction reaction of H atom by D atom followed by the addition of D atom to the glycine radical (NH2CHCOOH). Given that chiral glycine can form in such a primordial low-temperature environment, it might source molecular chirality as molecular clouds evolve into planetary systems. (C) 2015 Elsevier B.V. All rights reserved.
  • Tetsuya Hama, Hirokazu Ueta, Akira Kouchi, Naoki Watanabe
    PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 112 24 7438 - 7443 2015年06月 [査読有り][通常論文]
     
    Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (greater than or similar to 100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.
  • 中井 陽一, 日高 宏, 渡部 直樹, 小島 隆夫
    日本物理学会講演概要集 70 632 - 632 一般社団法人日本物理学会 2015年
  • Tetsuya Hama, Hirokazu Ueta, Akira Kouchi, Naoki Watanabe, Hiroto Tachikawa
    JOURNAL OF PHYSICAL CHEMISTRY LETTERS 5 21 3843 - 3848 2014年11月 [査読有り][通常論文]
     
    Despite the rapid accumulation of structural information about organic materials, the correlation between the surface structure of these materials and their chemical properties, a potentially important aspect of their chemistry, is not fully understood. Here, we show that the amorphous or crystalline structure of a solid benzene surface controls its chemical reactivity toward hydrogen. In situ infrared spectroscopy revealed that cold hydrogen atoms can add to an amorphous benzene surface at 20 K to form cyclohexane by tunneling. However, hydrogenation is greatly reduced on crystalline benzene. We suggest that the origin of the high selectivity of this reaction is the large difference in geometric constraints between the amorphous and the crystalline surfaces. The present findings can lead us to a more complete understanding of heterogeneous reaction systems, especially those involving tunneling, as well as to the possibility of nonenergetic surface chemical modification without undesired side reactions or physical processes.
  • Yasuhiro Oba, Kazuya Osaka, Naoki Watanabe, Takeshi Chigai, Akira Kouchi
    FARADAY DISCUSSIONS 168 185 - 204 2014年08月 [査読有り][通常論文]
     
    We performed laboratory experiments on the formation of water and its isotopologues by surface reactions of hydrogen peroxide (H2O2) with hydrogen (H) atoms and their deuterated counterparts (D2O2, D) at 10-30 K. High-purity H2O2 (>95%) was prepared in situ by the codeposition of molecular oxygen and H atoms at relatively high temperatures (45-50 K). We determined that the high-purity H2O2 solid reacts with both H and deuterium (D) atoms at 10-30 K despite the large activation barriers (similar to 2000 K). Moreover, the reaction rate for H atoms is approximately 45 times faster than that for D atoms at 15 K. Thus, the observed large isotope effect indicates that these reactions occurred through quantum tunneling. We propose that the observed HDO/H2O ratio in molecular clouds might be a good tool for the estimation of the atomic D/H ratio in those environments.
  • Yasuhiro Oba, Takeshi Chigai, Yoshihiro Osamura, Naoki Watanabe, Akira Kouchi
    METEORITICS & PLANETARY SCIENCE 49 1 117 - 132 2014年01月 [査読有り][通常論文]
     
    We experimentally studied hydrogen (H)-deuterium (D) substitution reactions of solid methylamine (CH3NH2) under astrophysically relevant conditions. We also calculated the potential energy surface for the H-D substitution reactions of methylamine isotopologues using quantum chemical methods. Despite the relatively large energy barrier of more than 18kJmol(-1), CH3NH2 reacted with D atoms to yield deuterated methylamines at 10K, suggesting that the H-D substitution reaction proceeds through quantum tunneling. Deuterated methylamines reacted with H atoms as well. On the basis of present results, we propose that methylamine has potential for D enrichment through atomic surface reactions on interstellar grains at very low temperatures in molecular clouds. D enrichment would occur in particular in the methyl group of methylamine.
  • Yasuhiro Oba, Takeshi Chigai, Yoshihiro Osamura, Naoki Watanabe, Akira Kouchi
    METEORITICS & PLANETARY SCIENCE 49 1 117 - 132 2014年01月 [査読有り][通常論文]
     
    We experimentally studied hydrogen (H)-deuterium (D) substitution reactions of solid methylamine (CH3NH2) under astrophysically relevant conditions. We also calculated the potential energy surface for the H-D substitution reactions of methylamine isotopologues using quantum chemical methods. Despite the relatively large energy barrier of more than 18kJmol(-1), CH3NH2 reacted with D atoms to yield deuterated methylamines at 10K, suggesting that the H-D substitution reaction proceeds through quantum tunneling. Deuterated methylamines reacted with H atoms as well. On the basis of present results, we propose that methylamine has potential for D enrichment through atomic surface reactions on interstellar grains at very low temperatures in molecular clouds. D enrichment would occur in particular in the methyl group of methylamine.
  • 大場 康弘, 渡部 直樹, 香内 晃
    日本地球化学会年会要旨集 61 168 - 168 日本地球化学会 2014年 
    本講演では,星間塵表面反応で生成する水分子の水素同位体組成について,これまでの実験結果をもとに詳細な議論をおこなう。一般的に星間分子は分子生成時および分子生成後に重水素濃集可能であるが,水分子に関しては,極低温の星間塵表面では分子生成時に限定される。水分子は3つの経路でおもに生成するが,うち2つの生成反応は極低温では熱的におこりえず,量子力学的なトンネル効果が不可欠である。量子力学的トンネル反応には大きな同位体効果が期待されるため,生成する水分子の水素同位体組成を議論する際には注意が必要である。
  • Tetsuya Hama, Naoki Watanabe
    CHEMICAL REVIEWS 113 12 8783 - 8839 2013年12月 [査読有り][通常論文]
  • N. Watanabe, T. Hama, A. Kouchi
    AIP Conference Proceedings 1543 308 - 316 2013年 [査読有り][招待有り]
  • Naoki Watanabe, Tetsuya Hama, Akira Kouchi
    FIRST INTERNATIONAL CONFERENCE ON CHEMICAL EVOLUTION OF STAR FORMING REGION AND ORIGIN OF LIFE (ASTROCHEM2012) 1543 308 - 316 2013年 [査読有り][招待有り]
     
    To clarify the meaning of the nuclear spin temperatures of H-2 and H2O molecules associated with various astronomical targets, it is important to understand the mechanisms that could alter these temperatures; i.e., the molecules' ortho/para nuclear-spin ratio (OPR). We have performed a series of experiments to investigate how the OPRs of H-2 and H2O behave on the surface of amorphous solid water (ASW), which is analogous to cosmic ice dust. The OPR of H-2 initially shows a high temperature limit of 3 upon its formation through H-H recombination at similar to 10 K and gradually decreases toward lower temperatures on the surface. The spin temperatures of H2O molecules that are thermally desorbed from various types of ASW at similar to 10 K always return the high-temperature limit.
  • H. Hidaka, M. Watanabe, A. Kouchi, N. Watanabe
    FIRST INTERNATIONAL CONFERENCE ON CHEMICAL EVOLUTION OF STAR FORMING REGION AND ORIGIN OF LIFE (ASTROCHEM2012) 1543 318 - 325 2013年 [査読有り][通常論文]
     
    The elucidation of formation pathways and mechanisms of deuterated species that have been detected by astronomical observations is important in studying chemical evolution in space. Formaldehyde and methanol are well-known molecules not only as reaction products on grain surfaces, but also as highly deuterated molecules. We have conducted experimental studies to clarify the formation mechanisms of deuterated formaldehyde and methanol on low temperature solid surfaces. In this paper, we summarize our experimental findings of the exposure of solid H2CO and D2CO on amorphous solid water to D and H atoms at 10-20 K, respectively. The isotope exchange reactions were observed in both reaction systems of D + H2CO and H + D2CO. In contrast, the addition reactions were only observed in the H + D2CO system because D addition to H2CO is much slower than isotope exchange. This isotope dependence on the reaction pathways can be explained by the quantum tunneling effect.
  • Tetsuya Hama, Kazuaki Kuwahata, Naoki Watanabe, Akira Kouchi, Yuki Kimura, Takeshi Chigai, Valerio Pirronello
    ASTROPHYSICAL JOURNAL 757 2 185 - (12pp) 2012年10月 [査読有り][通常論文]
     
    To understand elementary processes leading to H-2 formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solidwater (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H-2 (J = 0 and 1) and D-2 (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of <= 18, 22 (23 meV for D atoms), and >= 30 meV, respectively. The present study pictured the outline of H-2 formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with such trapped H atoms to yield H-2 molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.
  • Tetsuya Hama, Kazuaki Kuwahata, Naoki Watanabe, Akira Kouchi, Yuki Kimura, Takeshi Chigai, Valerio Pirronello
    ASTROPHYSICAL JOURNAL 757 2 185 - (12pp) 2012年10月 [査読有り][通常論文]
     
    To understand elementary processes leading to H-2 formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solidwater (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H-2 (J = 0 and 1) and D-2 (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of <= 18, 22 (23 meV for D atoms), and >= 30 meV, respectively. The present study pictured the outline of H-2 formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with such trapped H atoms to yield H-2 molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.
  • Y. Oba, N. Watanabe, T. Hama, K. Kuwahata, H. Hidaka, A. Kouchi
    ASTROPHYSICAL JOURNAL 749 1 67 - (12pp) 2012年04月 [査読有り][通常論文]
     
    The present study experimentally demonstrated that solid H2O is formed through the surface reaction OH + H-2 at 10 K. This is the first experimental evidence of solid H2O formation using hydrogen in its molecular form at temperatures as low as 10 K. We further found that H2O formation through the reaction OH + H-2 is about one order of magnitude more effective than HDO formation through the reaction OH + D-2. This significant isotope effect results from differences in the effective mass of each reaction, indicating that the reactions proceed through quantum tunneling.
  • Y. Oba, N. Watanabe, T. Hama, K. Kuwahata, H. Hidaka, A. Kouchi
    ASTROPHYSICAL JOURNAL 749 1 67 - (12pp) 2012年04月 [査読有り][通常論文]
     
    The present study experimentally demonstrated that solid H2O is formed through the surface reaction OH + H-2 at 10 K. This is the first experimental evidence of solid H2O formation using hydrogen in its molecular form at temperatures as low as 10 K. We further found that H2O formation through the reaction OH + H-2 is about one order of magnitude more effective than HDO formation through the reaction OH + D-2. This significant isotope effect results from differences in the effective mass of each reaction, indicating that the reactions proceed through quantum tunneling.
  • 渡部 直樹, 香内 晃, 羽馬 哲也, 日高 宏, 大場 康弘, 千貝 健
    表面科学 33 662 - 668 2012年 [査読有り][通常論文]
  • Yasuhiro Oba, Naoki Watanabe, Akira Kouchi, Tetsuya Hama, Valerio Pirronello
    Physical Chemistry Chemical Physics 13 35 15792 - 15797 2011年09月21日 [査読有り][通常論文]
     
    We investigated the OH-related formation routes of two astrophysically important molecules, H2O and CO2, under relatively warm astrophysical conditions. OH radicals, together with other neutral species such as H, O, H2, and O2, were produced in H2O microwave-discharge plasma and cooled to 100 K before being deposited on an Al substrate at 40-60 K. H2O formed at 40 and 50 K, but not at 60 K. Taking the experimental conditions into account, a possible route of H 2O formation is via reactions involving OH + OH, which yield H 2O2 as the main reaction product. The present study is the first to show experimentally that surface reactions of two OH radicals can yield H2O at low temperatures. The products' branching ratio was 0.2 and 0.8 for H2O and H2O2, respectively. When CO was co-deposited with neutral species that formed in the H2O plasma, CO2 was formed at 40-60 K. H2CO3 formed at 40 and 50 K. The present results may suggest that chemical reactions related to OH radicals are effective at yielding various molecules in relatively warm astrophysical environments, such as protostars. © 2011 the Owner Societies.
  • T. Hama, N. Watanabe, A. Kouchi, M. Yokoyama
    ASTROPHYSICAL JOURNAL LETTERS 738 1 L15 - (5pp) 2011年09月 [査読有り][通常論文]
     
    The nuclear-spin temperature of molecules observed in interstellar space or cometary coma is an important key to understanding physical and chemical histories of the molecules. The present Letter reports measurements of nuclear-spin states and rotational temperatures of thermally desorbed H2O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H2O molecules were measured at similar to 150 K from vapor-deposited ASW at 8 K. The nuclear-spin temperature of desorbed H2O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H2O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O-2 molecules, nor upon exposure of ASW to vacuum-ultraviolet (VUV) photons. Desorbed H2O molecules were also investigated from ASW produced by VUV irradiation of a CH4/O-2 mixture at 8 K. The resulting H2O spectrum showed a high nuclear-spin temperature. These results suggest that the nuclear-spin temperatures of gaseous H2O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H2O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H2O in water ice. The present study advocates the importance of further studies to give an interpretation of nuclear-spin temperatures of molecules observed in interstellar space or cometary coma.
  • T. Hama, N. Watanabe, A. Kouchi, M. Yokoyama
    ASTROPHYSICAL JOURNAL LETTERS 738 1 L15 - (5pp) 2011年09月 [査読有り][通常論文]
     
    The nuclear-spin temperature of molecules observed in interstellar space or cometary coma is an important key to understanding physical and chemical histories of the molecules. The present Letter reports measurements of nuclear-spin states and rotational temperatures of thermally desorbed H2O molecules from amorphous solid water (ASW) by combining temperature-programmed desorption and resonance-enhanced multiphoton ionization (REMPI). REMPI spectra of desorbed H2O molecules were measured at similar to 150 K from vapor-deposited ASW at 8 K. The nuclear-spin temperature of desorbed H2O molecules exhibits almost an upper limit to the ortho-to-para ratio of close to 3. No discernible change was observed in the H2O REMPI spectrum, neither after leaving the deposited ASW for 9 days in a vacuum chamber at 8 K, nor in the presence of O-2 molecules, nor upon exposure of ASW to vacuum-ultraviolet (VUV) photons. Desorbed H2O molecules were also investigated from ASW produced by VUV irradiation of a CH4/O-2 mixture at 8 K. The resulting H2O spectrum showed a high nuclear-spin temperature. These results suggest that the nuclear-spin temperatures of gaseous H2O molecules thermally desorbed from ice do not necessarily reflect the surface temperature at which H2O molecules condensed or formed under some laboratory conditions. We discuss the possibility of nuclear-spin conversion of H2O in water ice. The present study advocates the importance of further studies to give an interpretation of nuclear-spin temperatures of molecules observed in interstellar space or cometary coma.
  • Tetsuya Hama, Masaaki Yokoyama, Akihiro Yabushita, Masahiro Kawasaki, Naoki Watanabe
    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS 269 9 1011 - 1015 2011年05月 [査読有り][通常論文]
     
    Water ice is the major solid component in a variety of astrophysical environments, e.g., cold and dense molecular clouds. Photodesorption plays a dominant role in consuming ice in such cold regions. In this study, photodesorption of vibrationally ground-state H(2)O(v = 0) from amorphous solid water has been investigated at 157 nm. Using a resonance-enhanced multiphoton ionization technique, the translational and rotational energy distributions of photodesorbed H(2)O(v = 0) were measured, i.e., Boltzmann distributions at 1800 and 300 K, respectively. These energies are in good accordance with those predicted by classical molecular calculations for water photodesorption due to a kick-out mechanism following absorption of a single photon; hot H atom released by photodissociation of H(2)O in ice transfers enough momentum to another H(2)O molecule to kick it off the surface. Desorption of D(2)O(v = 0) following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was investigated to provide further direct evidence for the operation of a kick-out mechanism. The other desorption mechanisms were also discussed in the context of possible photodesorption of vibrationally excited H(2)O. (c) 2010 Elsevier B.V. All rights reserved.
  • Tetsuya Hama, Masaaki Yokoyama, Akihiro Yabushita, Masahiro Kawasaki, Naoki Watanabe
    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS 269 9 1011 - 1015 2011年05月 [査読有り][通常論文]
     
    Water ice is the major solid component in a variety of astrophysical environments, e.g., cold and dense molecular clouds. Photodesorption plays a dominant role in consuming ice in such cold regions. In this study, photodesorption of vibrationally ground-state H(2)O(v = 0) from amorphous solid water has been investigated at 157 nm. Using a resonance-enhanced multiphoton ionization technique, the translational and rotational energy distributions of photodesorbed H(2)O(v = 0) were measured, i.e., Boltzmann distributions at 1800 and 300 K, respectively. These energies are in good accordance with those predicted by classical molecular calculations for water photodesorption due to a kick-out mechanism following absorption of a single photon; hot H atom released by photodissociation of H(2)O in ice transfers enough momentum to another H(2)O molecule to kick it off the surface. Desorption of D(2)O(v = 0) following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was investigated to provide further direct evidence for the operation of a kick-out mechanism. The other desorption mechanisms were also discussed in the context of possible photodesorption of vibrationally excited H(2)O. (c) 2010 Elsevier B.V. All rights reserved.
  • 大場 康弘, 渡部 直樹, 香内 晃, 羽馬 哲也, ピロネロ バレリオ
    地球化学 45 4 213 - 226 日本地球化学会 2011年 [査読有り][通常論文]
     
    Since the first detection in interstellar medium in 1989, solid CO2 has been found in various lines of sight and regarded as one of the main constituent in icy grain mantles in interstellar clouds. Due to the low efficiency in the formation of CO2 in the gas phase and relatively high abundance of CO in icy grain mantles, it is generally accepted that CO2 forms on the surface of icy grain mantles in interstellar clouds. CO2 formation on/in icy grain mantles has been extensively studied experimentally, with the aid of energetic sources such as UV or cosmic-rays, and also without them. In this review, we summarize experimental results on the formation of CO2 on cold surfaces through energetic and non-energetic processes under the simulative conditions of interstellar molecular clouds.
  • Takashi Kinugawa, Akihiro Yabushita, Masahiro Kawasaki, Tetsuya Hama, Naoki Watanabe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 13 35 15785 - 15791 2011年 [査読有り][通常論文]
     
    Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O(P-3(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO2. Two different adsorption states of CO2, i.e., physisorbed CO2 on the surface of amorphous solid water and trapped CO2 in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H2O. Since the absorption cross section of CO2 is smaller than that of H2O at 157 nm, the CO2 surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.
  • Hiroshi Hidaka, Motohiro Watanabe, Akira Kouchi, Naoki Watanabe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 13 35 15798 - 15802 2011年 [査読有り][通常論文]
     
    A Fourier transform infrared absorption spectroscopy (FTIR) study showed that NH3 was formed by the successive reaction of hydrogen atoms with nitrogen atoms in an N-2 matrix at 10 K. Reactions appeared to proceed via the Langmuir-Hinshelwood mechanism because NH3 formation was not observed at 20 K. At this temperature, H atoms did not adsorb significantly onto the N-2 matrix; i.e., the surface residence times were short. Furthermore, NH3 yields via the successive hydrogenation of N atoms were significant, even after H atom deposition onto the N-2 matrix containing trapped N atoms onto which had been deposited a superficial pure solid N-2 adlayer. This result clearly indicates that H atoms diffuse in pure solid N-2 matrices at 10 K.
  • 日高 宏, 香内 晃, 渡部 直樹
    日本地球化学会年会要旨集 58 292 - 292 日本地球化学会 2011年 
    分子雲でアンモニア分子が生成される過程として提案されている,低温星間塵表面における窒素原子への水素原子付加反応の検証を実験的手法により行った.実験は,超高真空槽内に設置した低温基板(10K)にNおよびH原子を照射し,赤外吸収分光により生成分子の観測を行った.その結果,10Kという低温表面でも上記表面反応によりアンモニアが生成されることが明らかになった.
  • 大場 康弘, 渡部 直樹, 香内 晃
    日本地球化学会年会要旨集 58 291 - 291 日本地球化学会 2011年 
    H2Oは星間塵アイスマントルの主要な構成成分である。H2Oはその存在量を気相反応のみでは説明できないことから,星間塵表面での反応で生成したと考えられている。本研究では,高密度星間分子雲で主要な経路だと考えられている,ヒドロキシルラジカル(OH)と水素分子(H2)の反応による,H2O生成に関する実験結果を紹介する。実験は,超高真空・極低温環境のアルミニウム基板表面でおこなった。
  • Laboratory investigations of physico-chemical processes occurring in icy mantles on interstellar dust grains
    V. Pirronello, M. Accolla, T. Hama, H. Hidaka, A. Kouchi, G. Manico, Y. Oba, N. Watanabe
    Physics and Chemistry of ice 2010 361 - 368 2011年 [査読有り][通常論文]
  • Hiroshi Hidaka, Motohiro Watanabe, Akira Kouchi, Naoki Watanabe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 13 35 15798 - 15802 2011年 [査読有り][通常論文]
     
    A Fourier transform infrared absorption spectroscopy (FTIR) study showed that NH3 was formed by the successive reaction of hydrogen atoms with nitrogen atoms in an N-2 matrix at 10 K. Reactions appeared to proceed via the Langmuir-Hinshelwood mechanism because NH3 formation was not observed at 20 K. At this temperature, H atoms did not adsorb significantly onto the N-2 matrix; i.e., the surface residence times were short. Furthermore, NH3 yields via the successive hydrogenation of N atoms were significant, even after H atom deposition onto the N-2 matrix containing trapped N atoms onto which had been deposited a superficial pure solid N-2 adlayer. This result clearly indicates that H atoms diffuse in pure solid N-2 matrices at 10 K.
  • Y. Oba, N. Watanabe, A. Kouchi, T. Hama, V. Pirronello
    Phys. Chem. Chem. Phys. 13 35 15792 - 15797 2011年 [査読有り][通常論文]
  • Takashi Kinugawa, Akihiro Yabushita, Masahiro Kawasaki, Tetsuya Hama, Naoki Watanabe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 13 35 15785 - 15791 2011年 [査読有り][通常論文]
     
    Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O(P-3(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO2. Two different adsorption states of CO2, i.e., physisorbed CO2 on the surface of amorphous solid water and trapped CO2 in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H2O. Since the absorption cross section of CO2 is smaller than that of H2O at 157 nm, the CO2 surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.
  • Yasuhiro Oba, Naoki Watanabe, Akira Kouchi, Tetsuya Hama, Valerio Pirronello
    ASTROPHYSICAL JOURNAL 722 2 1598 - 1606 2010年10月 [査読有り][通常論文]
     
    We present the experimental results of carbonic acid (H2CO3) formation through surface reactions of CO molecules with non-energetic hydroxyl (OH) radicals at 10-40 K. The formation of H2CO3 was clearly identified both in the IR spectra and in the thermally programmed desorption mass spectra. The H2CO3 yield was rather high, amounting to approximately 40%-70% relative to that of CO2 formed by the reaction of CO with OH. The structure of H2CO3 formed by reactions of CO with OH may differ from that formed by energetic processes such as UV irradiation, ion irradiation, and electron irradiation of H2O/CO2 binary ices. In this paper, we envisage some of the possible roles H2CO3 may have in the interstellar medium, such as enriching grain mantles of new molecules via acid-base reactions with basic species and contributing to the formation of the unidentified band at 6.8 mu m; we suggest possible reasons for its non-detection yet and discuss the restoration of carbonic acid molecules in the gas phase.
  • Yasuhiro Oba, Naoki Watanabe, Akira Kouchi, Tetsuya Hama, Valerio Pirronello
    ASTROPHYSICAL JOURNAL 722 2 1598 - 1606 2010年10月 [査読有り][通常論文]
     
    We present the experimental results of carbonic acid (H2CO3) formation through surface reactions of CO molecules with non-energetic hydroxyl (OH) radicals at 10-40 K. The formation of H2CO3 was clearly identified both in the IR spectra and in the thermally programmed desorption mass spectra. The H2CO3 yield was rather high, amounting to approximately 40%-70% relative to that of CO2 formed by the reaction of CO with OH. The structure of H2CO3 formed by reactions of CO with OH may differ from that formed by energetic processes such as UV irradiation, ion irradiation, and electron irradiation of H2O/CO2 binary ices. In this paper, we envisage some of the possible roles H2CO3 may have in the interstellar medium, such as enriching grain mantles of new molecules via acid-base reactions with basic species and contributing to the formation of the unidentified band at 6.8 mu m; we suggest possible reasons for its non-detection yet and discuss the restoration of carbonic acid molecules in the gas phase.
  • Naoki Watanabe, Yuki Kimura, Akira Kouchi, Takeshi Chigai, Tetsuya Hama, Valerio Pirronello
    ASTROPHYSICAL JOURNAL LETTERS 714 2 L233 - L237 2010年05月 [査読有り][通常論文]
     
    Physicochemical processes (H-atom sticking, diffusion, recombination, and the nuclear spin temperature of nascent H-2 molecules) important in the formation of molecular hydrogen have been experimentally investigated on amorphous solid water (ASW). A new type of experiment is performed to shed light on a longstanding dispute. The diffusion rate of H atom is directly measured at 8 K and is found to consist of a fast and a slow component due to the presence of at least two types of potential sites with the energy depths of similar to 20 and >50 meV, respectively. The fast diffusion at the shallow sites enables efficient H-2 formation on interstellar ice dust even at 8 K, while H atoms trapped in the deeper sites hardly migrate. The spin temperature of nascent H-2 formed by recombination on ASW has been obtained for the first time and is higher than approximately 200 K. After formation, H-2 molecules are trapped and their spin temperature decreases due to the conversion of spin states on ASW.
  • Eui-Seong Moon, Heon Kang, Yasuhiro Oba, Naoki Watanabe, Akira Kouchi
    ASTROPHYSICAL JOURNAL 713 2 906 - 911 2010年04月 [査読有り][通常論文]
     
    We present direct evidence for ammonium ion (NH4+) formation through ultraviolet (UV) photolysis of NH3-H2O mixture ice that does not contain acids. NH4+ forms by the reaction of NH3 with protonic defects (H3O+) in the UV-photolyzed ice. Our observations may explain the deficient counter-anions in interstellar ice relative to the abundance of NH4+. Also, H3O+ may play an important role in the acid-base chemistry of interstellar ice in UV-irradiating environments. IR absorption results suggest that NH4+ is a potential contributor to the interstellar 6.85 mu m band but is not a dominant component.
  • Tetsuya Hama, Masaaki Yokoyama, Akihiro Yabushita, Masahiro Kawasaki, Stefan Andersson, Colin M. Western, Michael N. R. Ashfold, Richard N. Dixon, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 132 16 164508  2010年04月 [査読有り][通常論文]
     
    Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H(2)O and D(2)O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D(2)O following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism. (C) 2010 American Institute of Physics. [doi:10.1063/1.3386577]
  • Yasuhiro Oba, Naoki Watanabe, Akira Kouchi, Tetsuya Hama, Valerio Pirronello
    ASTROPHYSICAL JOURNAL LETTERS 712 2 L174 - L178 2010年04月 [査読有り][通常論文]
     
    Surface reactions between carbon monoxide and non-energetic hydroxyl radicals were carried out at 10 K and 20 K in order to investigate possible reaction pathways to yield carbon dioxide in dense molecular clouds. Hydroxyl radicals, produced by dissociating water molecules in microwave-induced plasma, were cooled down to 100 K prior to the introduction of CO. The abundances of species were monitored in situ using a Fourier transform infrared spectrometer. Formation of CO2 was clearly observed, even at 10 K, suggesting that reactions of CO with OH proceed with little or no activation barrier. The present results indicate that CO2 formation, due to reactions between CO and OH, occurs in tandem with H2O formation, and this may lead to the formation of CO2 ice in polar environments, as typically observed in molecular clouds.
  • Tetsuya Hama, Masaaki Yokoyama, Akihiro Yabushita, Masahiro Kawasaki, Stefan Andersson, Colin M. Western, Michael N. R. Ashfold, Richard N. Dixon, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 132 16 164508  2010年04月 [査読有り][通常論文]
     
    Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H(2)O and D(2)O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D(2)O following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism. (C) 2010 American Institute of Physics. [doi:10.1063/1.3386577]
  • Y. Oba, N. Watanabe, A. Kouchi, T. Hama, V. Pirronello
    Astrophys. J. Lett. 712 2,Pt.2 L174 - L178 2010年 [査読有り][通常論文]
  • N. Watanabe, Y. Kimura, A. Kouchi, T. Chigai, T. Hama, V. Pirronello
    Astrophys. J. Lett. 714 2,Pt.2 L233 - L237 2010年 [査読有り][通常論文]
  • N. Watanabe, Y. Kimura, A. Kouchi, T. Chigai, T. Hama, V. Pirronello
    Astrophys. J. Lett. 714 2,Pt.2 L233 - L237 2010年 [査読有り][通常論文]
  • E-S. Moon, H. Kang, Y. Oba, N. Watanabe, A. Kouchi
    Astrophys. J. 713 2,Pt.1 906 - 911 2010年 [査読有り][通常論文]
  • Y. Oba, N. Watanabe, A. Kouchi, T. Hama, V. Pirronello
    Astrophys. J. Lett. 712 2,Pt.2 L174 - L178 2010年 [査読有り][通常論文]
  • Hiroshi Hidaka, Motohiro Watanabe, Akira Kouchi, Naoki Watanabe
    Journal of Physics: Conference Series 194 13 132024  2009年12月24日 [査読有り][通常論文]
     
    Reaction routes when H2CO and D2CO were exposed to the D and H atoms on amorphous solid water (ASW) at 10-20 K, respectively, were studied experimentally by IR spectroscopy. For D + H2CO on ASW, H2CO was converted to HDCO and D2CO by the H-D substitution reactions. Although CD3OD was slightly observed, doubly- and triply-deuterated methanol, CH2DOD and CHD2OD, were not observed. This implies that the D additions to H2CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D2CO, H addition reactions to form CHD2OH proceed significantly as much as conversions to HDCO and H2CO, which evolve to CH2DOH and CH3OH by the further H additions, respectively. Therefore, in this reaction system, two kind of reaction routes for D2CO HDCO H2CO CH3OH and D2CO CHD2OH are competitive.
  • Hiroshi Hidaka, Motohiro Watanabe, Akira Kouchi, Naoki Watanabe
    Journal of Physics: Conference Series 194 13 132024  2009年12月24日 [査読有り][通常論文]
     
    Reaction routes when H2CO and D2CO were exposed to the D and H atoms on amorphous solid water (ASW) at 10-20 K, respectively, were studied experimentally by IR spectroscopy. For D + H2CO on ASW, H2CO was converted to HDCO and D2CO by the H-D substitution reactions. Although CD3OD was slightly observed, doubly- and triply-deuterated methanol, CH2DOD and CHD2OD, were not observed. This implies that the D additions to H2CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D2CO, H addition reactions to form CHD2OH proceed significantly as much as conversions to HDCO and H2CO, which evolve to CH2DOH and CH3OH by the further H additions, respectively. Therefore, in this reaction system, two kind of reaction routes for D2CO HDCO H2CO CH3OH and D2CO CHD2OH are competitive.
  • H. Hidaka, M. Watanabe, A. Kouchi, N. Watanabe
    ASTROPHYSICAL JOURNAL 702 1 291 - 300 2009年09月 [査読有り][通常論文]
     
    Grain surface reaction has been expected to be a key process for deuterium enrichment in interstellar molecules. We focus on formaldehyde, which is predicted to be formed on cold grain surface in astrophysical models and is known to be deuterium-enriched in a molecular cloud. Reaction routes and effective reaction rate constants are experimentally investigated when H2CO and D2CO are exposed to D and H atoms on amorphous solid water (ASW) at 10-20 K, respectively. For D + H2CO on ASW, H2CO was converted to HDCO and D2CO by the H-D substitution reactions. Although CD3OD was slightly observed, doubly and triply deuterated methanol, CH2DOD and CHD2OD, were not observed. This implies that D addition to H2CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D2CO, H addition reactions to form CHD2OH proceed at a significant rate. Simultaneously, a competitive reaction, the substitution reaction by H atoms and subsequent H addition (D2CO -> HDCO -> H2CO -> CH3OH) also proceed at a significant rate. However, no H addition to HDCO was observed. The effective surface reaction routes when CO is exposed to H and D atoms are summarized using the present experimental results and the previous results of our group.
  • Tetsuya Hama, Akihiro Yabushita, Masaaki Yokoyama, Masahiro Kawasaki, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 131 11 114510  2009年09月 [査読有り][通常論文]
     
    Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O(D-1) and OH(nu= 0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O(D-1) atoms from the ice surface. One is unimolecular dissociation of H2O to H-2+O(D-1) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H2O2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3194798]
  • Tetsuya Hama, Akihiro Yabushita, Masaaki Yokoyama, Masahiro Kawasaki, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 131 11 114511  2009年09月 [査読有り][通常論文]
     
    Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O(D-1) and OH(nu= 0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O(D-1) atoms from the ice surface. One is unimolecular dissociation of H2O to H-2+O(D-1) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H2O2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3194798]
  • Tetsuya Hama, Akihiro Yabushita, Masaaki Yokoyama, Masahiro Kawasaki, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 131 11 114511  2009年09月 [査読有り][通常論文]
     
    Desorption of ground state O(P-3(J= 2,1,0)) atoms following the vacuum ultraviolet photolysis of water ice in the first absorption band was directly measured with resonance-enhanced multiphoton ionization (REMPI) method. Based on their translational energy distributions and evolution behavior, two different formation mechanisms are proposed: One is exothermic recombination reaction of OH radicals, OH+OH -> H2O+O(P-3(J)) and the other is the photodissociation of OH radicals on the surface of amorphous solid water. The translational and internal energy distributions of OH radicals as well as the evolution behavior were also measured by REMPI to elucidate the roles of H2O2 and OH in the O(P-3(J)) formation mechanisms. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3194797]
  • Tetsuya Hama, Akihiro Yabushita, Masaaki Yokoyama, Masahiro Kawasaki, Naoki Watanabe
    JOURNAL OF CHEMICAL PHYSICS 131 11 114510  2009年09月 [査読有り][通常論文]
     
    Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O(D-1) and OH(nu= 0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O(D-1) atoms from the ice surface. One is unimolecular dissociation of H2O to H-2+O(D-1) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H2O2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3194798]
  • H. Hidaka, M. Watanabe, A. Kouchi, N. Watanabe
    ASTROPHYSICAL JOURNAL 702 1 291 - 300 2009年09月 [査読有り][通常論文]
     
    Grain surface reaction has been expected to be a key process for deuterium enrichment in interstellar molecules. We focus on formaldehyde, which is predicted to be formed on cold grain surface in astrophysical models and is known to be deuterium-enriched in a molecular cloud. Reaction routes and effective reaction rate constants are experimentally investigated when H2CO and D2CO are exposed to D and H atoms on amorphous solid water (ASW) at 10-20 K, respectively. For D + H2CO on ASW, H2CO was converted to HDCO and D2CO by the H-D substitution reactions. Although CD3OD was slightly observed, doubly and triply deuterated methanol, CH2DOD and CHD2OD, were not observed. This implies that D addition to H2CO (formation of deuterated methanol) is a minor reaction route. On the other hand, for H + D2CO, H addition reactions to form CHD2OH proceed at a significant rate. Simultaneously, a competitive reaction, the substitution reaction by H atoms and subsequent H addition (D2CO -> HDCO -> H2CO -> CH3OH) also proceed at a significant rate. However, no H addition to HDCO was observed. The effective surface reaction routes when CO is exposed to H and D atoms are summarized using the present experimental results and the previous results of our group.
  • Y. Oba, N. Miyauchi, H. Hidaka, T. Chigai, N. Watanabe, A. Kouchi
    ASTROPHYSICAL JOURNAL 701 1 464 - 470 2009年08月 [査読有り][通常論文]
     
    Formation of H2O molecules through the codeposition of oxygen molecules and hydrogen atoms is examined in situ using IR spectroscopy at 10-40 K under various O-2 and H fluxes. It is found that H2O and H2O2 are continuously formed by reaction, even at 40 K. The H2O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H2O ice, because dangling OH bonds are not observed in the IR spectrum. This is consistent with astronomical observations in molecular clouds and theoretical predictions, which suggest that hydrogenation of O-2 is one of the potential routes for reproducing these IR spectral characteristics. The composition of the ice formed by codeposition varies with the O-2/H ratio and temperature. Although no data are available at present for the H2O/H2O2 ratio of ice in molecular clouds, this study suggests that hydrogenation of O-2 has a potential to yield a H2O/H2O2 ratio of 5 or more in molecular clouds.
  • Y. Oba, N. Miyauchi, H. Hidaka, T. Chigai, N. Watanabe, A. Kouchi
    ASTROPHYSICAL JOURNAL 701 1 464 - 470 2009年08月 [査読有り][通常論文]
     
    Formation of H2O molecules through the codeposition of oxygen molecules and hydrogen atoms is examined in situ using IR spectroscopy at 10-40 K under various O-2 and H fluxes. It is found that H2O and H2O2 are continuously formed by reaction, even at 40 K. The H2O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H2O ice, because dangling OH bonds are not observed in the IR spectrum. This is consistent with astronomical observations in molecular clouds and theoretical predictions, which suggest that hydrogenation of O-2 is one of the potential routes for reproducing these IR spectral characteristics. The composition of the ice formed by codeposition varies with the O-2/H ratio and temperature. Although no data are available at present for the H2O/H2O2 ratio of ice in molecular clouds, this study suggests that hydrogenation of O-2 has a potential to yield a H2O/H2O2 ratio of 5 or more in molecular clouds.
  • Akihiro Yabushita, Tetsuya Hama, Masaaki Yokoyama, Masahiro Kawasaki, Stefan Andersson, Richard N. Dixon, Michael N. R. Ashfold, Naoki Watanabe
    ASTROPHYSICAL JOURNAL LETTERS 699 2 L80 - L83 2009年07月 [査読有り][通常論文]
     
    For interstellar grains coated with water ice, the most important desorption mechanism at the edge of molecular clouds is photodesorption of water. To reveal details of the photodesorption mechanism, we have measured the translational and rotational energies of H(2)O (v = 0) molecules photodesorbed from amorphous solid water and polycrystalline ice following excitation within the first absorption band using a 157 nm laser. The measured translational and rotational temperatures are 1800 K and 300 K, respectively. These energies are in good accord with those predicted by classical molecular dynamics calculations for the "kick-out" of an H(2)O molecule in the ice by an energetic H atom. The statistical ortho: para ratio of g(OPR) = 3 is appropriate for the Boltzmann rotational distribution of the H(2)O molecules.
  • Akihiro Yabushita, Tetsuya Hama, Masaaki Yokoyama, Masahiro Kawasaki, Stefan Andersson, Richard N. Dixon, Michael N. R. Ashfold, Naoki Watanabe
    ASTROPHYSICAL JOURNAL LETTERS 699 2 L80 - L83 2009年07月 [査読有り][通常論文]
     
    For interstellar grains coated with water ice, the most important desorption mechanism at the edge of molecular clouds is photodesorption of water. To reveal details of the photodesorption mechanism, we have measured the translational and rotational energies of H(2)O (v = 0) molecules photodesorbed from amorphous solid water and polycrystalline ice following excitation within the first absorption band using a 157 nm laser. The measured translational and rotational temperatures are 1800 K and 300 K, respectively. These energies are in good accord with those predicted by classical molecular dynamics calculations for the "kick-out" of an H(2)O molecule in the ice by an energetic H atom. The statistical ortho: para ratio of g(OPR) = 3 is appropriate for the Boltzmann rotational distribution of the H(2)O molecules.
  • 宇宙における水分子生成過程の解明に至まで
    香内晃, 渡部直樹
    雪氷 71 395  2009年 [査読有り][通常論文]
  • 大場康弘, 宮内直弥, 千貝健, 日高宏, 渡部直樹, 香内 晃
    地球化学 43 4 117 - 131 日本地球化学会 2009年 [査読有り][通常論文]
     
    Formation of water by reaction of oxygen molecules (O2) with hydrogen atoms (H) were investigated at low temperatures (10-40 K) in order to reveal the possible reaction pathways to yield water on grain surfaces in molecular clouds. Two types of experiments for surface reaction between O2 and H were performed: one of which was that solid O2 was deposited onto the surface, followed by H irradiations to the O2, another of which was that gaseous O2 was continuously codeposited with H onto the surface and was reacted with each other. H2O (D2O) and H2O2 (D2O2) were quickly produced through successive reactions of O2 with H (D). Reaction kinetics and isotope effect are discussed for the former experiment. For the latter experiment, the dependence of compositions of the products on the temperature and O2/H ratio, and also the structure of the H2O ice produced are examined.
  • Tetsuo Koizumi, Takao M. Kojima, Mutsumi Sano, Naoki Watanabe
    14TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF HIGHLY CHARGED IONS (HCI 2008) 163 1 12099  2009年 [査読有り][通常論文]
     
    Using an ion-photon merged-beam apparatus, photoion yield spectra of Cs2+ and Cs3+ from the 4d photoionization of Cs+ have been measured as a function of photon energy in the range 80-140 eV. Above 90 eV, the 4d giant resonance peak is observed clearly for Cs3+ spectrum. Below 90 eV, some prominent discrete peaks are observed for both Cs2+ and Cs3+. These discrete peaks can be attributed to the 4d-np and 4d-nf transitions.
  • N. Watanabe, A. Kouchi, H. Hidaka, Y. Oba, N. Miyauchi
    Journal of Physics: Conference Series 194 1 12044 - (5pages) 2009年 [査読有り][通常論文]
     
    Reactions of atomic hydrogen with CO and O2 on amorphous solid water (ASW), relevant to chemical evolution on cosmic ice dust, were experimentally investigated at around 10 K. Successive addition of hydrogen atoms to CO and O2 produces H2CO, CH3OH, and H2O2, H2O, respectively. At such low temperatures, some of hydrogen additions such as H + CO HCO proceed via tunneling reactions rather than thermally-activated reactions. Effective reaction rates and isotope effect of the tunneling reactions to produce HCO and H2O were measured. The surface of ASW was found to enhance the effective rate of hydrogen addition to CO at relatively higher temperatures, namely around 20 K. © 2009 IOP Publishing Ltd.
  • 大場 康弘, 渡部 直樹, 日高 宏, 香内 晃, ピロネロ バレリオ
    日本地球化学会年会要旨集 56 66 - 66 日本地球化学会 2009年 
    二酸化炭素は分子雲中星間塵アイスマントルの主要な構成成分であるが、気相反応によってその存在量を説明することができないため、その生成には塵表面での反応が重要だと考えられている。これまでの研究では、UVなどの外部エネルギーをH2OやCOなどを含む混合氷に照射してCO2生成が確認されてきたが、外部エネルギー無しにCO2が生成するかどうかは明らかにされていなかった。本講演では、100Kに冷却されたOHラジカルとCOの固体表面上での反応でCO2生成が確認されたので、その結果について報告する。
  • A. Kouchi, Y. Oba, N. Miyauchi, H. Hidaka, N. Watanabe
    Cryobiology 59 3 370  Elsevier {BV} 2009年 [査読有り][通常論文]
  • N. Watanabe, A. Kouchi, H. Hidaka, Y. Oba, N. Miyauchi
    J. Phys. Conf. Ser. 194 1 012044 - (5pages) 2009年 [査読有り][通常論文]
  • Tetsuo Koizumi, Takao M. Kojima, Mutsumi Sano, Naoki Watanabe
    14TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF HIGHLY CHARGED IONS (HCI 2008) 163 1 012099  2009年 [査読有り][通常論文]
     
    Using an ion-photon merged-beam apparatus, photoion yield spectra of Cs2+ and Cs3+ from the 4d photoionization of Cs+ have been measured as a function of photon energy in the range 80-140 eV. Above 90 eV, the 4d giant resonance peak is observed clearly for Cs3+ spectrum. Below 90 eV, some prominent discrete peaks are observed for both Cs2+ and Cs3+. These discrete peaks can be attributed to the 4d-np and 4d-nf transitions.
  • Naoki Watanabe, Akira Kouchi
    PROGRESS IN SURFACE SCIENCE 83 10-12 439 - 489 2008年12月 [査読有り][通常論文]
     
    Chemical reactions on the surface of cosmic ice dust play an important role in chemical evolution in space. Arnong the many kinds of molecules observed, the abundances of some major species such as hydrogen molecules cannot be explained by gas-phase synthesis; therefore, surface reactions oil cosmic dust are considered for the synthesis of such molecules. Experimental research on surface reactions relevant to cosmic ice dust has been conducted since the late 1980s. Early experiments were rather qualitative, and so the details of reactions were not clear. In the last decade, many scientists from various fields including surface science have been motivated to extend the knowledge in this area of research, and have performed sophisticated experiments regarding surface reactions on cosmic ice dust. The aim of this article is to review the recent progress in surface astrochemistry. In particular, focus is made on experiments concerning the synthesis of abundant molecules such as H-2, H2O, CO2, H2CO and CH3OH oil ice surfaces at very low temperatures. (C) 2008 Elsevier Ltd. All rights reserved.
  • Naoki Watanabe, Akira Kouchi
    Progress in Surface Science 83 10-12 439 - 489 2008年12月 [査読有り][通常論文]
     
    Chemical reactions on the surface of cosmic ice dust play an important role in chemical evolution in space. Among the many kinds of molecules observed, the abundances of some major species such as hydrogen molecules cannot be explained by gas-phase synthesis therefore, surface reactions on cosmic dust are considered for the synthesis of such molecules. Experimental research on surface reactions relevant to cosmic ice dust has been conducted since the late 1980s. Early experiments were rather qualitative, and so the details of reactions were not clear. In the last decade, many scientists from various fields including surface science have been motivated to extend the knowledge in this area of research, and have performed sophisticated experiments regarding surface reactions on cosmic ice dust. The aim of this article is to review the recent progress in surface astrochemistry. In particular, focus is made on experiments concerning the synthesis of abundant molecules such as H2, H2O, CO2, H2CO and CH3OH on ice surfaces at very low temperatures. © 2008 Elsevier Ltd. All rights reserved.
  • 大場 康弘, 宮内 直弥, 千貝 健, 日高 宏, 渡部 直樹, 香内 晃
    日本惑星科学会秋期講演会予稿集 2008 0 93 - 93 日本惑星科学会 2008年11月 [査読無し][通常論文]
  • Akihiro Yabushita, Tetsuya Hama, Daisuke Iida, Noboru Kawanaka, Masahiro Kawasaki, Naoki Watanabe, Michael N. R. Ashfold, Hans-Peter Loock
    ASTROPHYSICAL JOURNAL LETTERS 682 1 L69 - L72 2008年07月 [査読有り][通常論文]
     
    We demonstrate experimentally that photodissociation of amorphous solid water at 100 K results in formation of H-2 molecules with an ortho/para ratio of g(OPR) = 3. Two distinct mechanisms can be identified: endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H-2 products, whereas exothermic recombination of two H-atom photoproducts yields translationally and internally hot H-2. These results are in accord with predictions by molecular dynamics calculations and their astrophysical implications are discussed.
  • Akihiro Yabushita, Tetsuya Hama, Daisuke Iida, Noboru Kawanaka, Masahiro Kawasaki, Naoki Watanabe, Michael N. R. Ashfold, Hans-Peter Loock
    JOURNAL OF CHEMICAL PHYSICS 129 4 044501  2008年07月 [査読有り][通常論文]
     
    The production of H(2) in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H(2)(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H(2)(v=0 and 1) populations in this case were below the detection limit. The H(2) products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H(2)O by a photolytically produced H atom yields vibrationally cold H(2) products, whereas exothermic recombination of two H-atom photoproducts yields H(2) molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations. (C) 2008 American Institute of Physics.
  • Akihiro Yabushita, Tetsuya Hama, Daisuke Iida, Noboru Kawanaka, Masahiro Kawasaki, Naoki Watanabe, Michael N. R. Ashfold, Hans-Peter Loock
    JOURNAL OF CHEMICAL PHYSICS 129 4 044501  2008年07月 [査読有り][通常論文]
     
    The production of H(2) in highly excited vibrational and rotational states (v=0-5, J=0-17) from the 157 nm photodissociation of amorphous solid water ice films at 100 K was observed directly using resonance-enhanced multiphoton ionization. Weaker signals from H(2)(v=2,3 and 4) were obtained from 157 nm photolysis of polycrystalline ice, but H(2)(v=0 and 1) populations in this case were below the detection limit. The H(2) products show two distinct formation mechanisms. Endothermic abstraction of a hydrogen atom from H(2)O by a photolytically produced H atom yields vibrationally cold H(2) products, whereas exothermic recombination of two H-atom photoproducts yields H(2) molecules with a highly excited vibrational distribution and non-Boltzmann rotational population distributions as has been predicted previously by both quantum-mechanical and molecular dynamics calculations. (C) 2008 American Institute of Physics.
  • Akihiro Yabushita, Tetsuya Hama, Daisuke Iida, Noboru Kawanaka, Masahiro Kawasaki, Naoki Watanabe, Michael N. R. Ashfold, Hans-Peter Loock
    ASTROPHYSICAL JOURNAL LETTERS 682 1 L69 - L72 2008年07月 [査読有り][通常論文]
     
    We demonstrate experimentally that photodissociation of amorphous solid water at 100 K results in formation of H-2 molecules with an ortho/para ratio of g(OPR) = 3. Two distinct mechanisms can be identified: endothermic abstraction of a hydrogen atom from H2O by a photolytically produced H atom yields vibrationally cold H-2 products, whereas exothermic recombination of two H-atom photoproducts yields translationally and internally hot H-2. These results are in accord with predictions by molecular dynamics calculations and their astrophysical implications are discussed.
  • H. Hidaka, N. Miyauchi, A. Kouchi, N. Watanabe
    CHEMICAL PHYSICS LETTERS 456 1-3 36 - 40 2008年04月 [査読有り][通常論文]
     
    Experiments on the hydrogenation of CO on crystalline and amorphous ice at 15 K were carried out to investigate the structural effects of the ice surface. The effective rate of H atom addition to CO on the amorphous ice was found to be larger than that on the crystalline ice, while CO depletion on crystalline ice became larger after long exposure. We demonstrated that the CO-coverage on the ice surfaces dominates the effective reaction rate rather than the surface structure. The larger depletion of CO on crystalline ice, as compared to amorphous ice, suggests easier desorption of CO and/or products by the heat of the reaction. (C) 2008 Elsevier B.V. All rights reserved.
  • N. Miyauchi, H. Hidaka, T. Chigai, A. Nagaoka, N. Watanabe, A. Kouchi
    CHEMICAL PHYSICS LETTERS 456 1-3 27 - 30 2008年04月 [査読有り][通常論文]
     
    The reactions of cold H atoms with solid O-2 molecules were investigated at 10 K. The formation of H2O2 and H2O has been confirmed by in situ infrared spectroscopy. We found that the reaction proceeds very efficiently and obtained the effective reaction rates. This is the first clear experimental evidence of the formation of water molecules under conditions mimicking those found in cold interstellar molecular clouds. Based on the experimental results, we discuss the reaction mechanism and astrophysical implications. (C) 2008 Elsevier B.V. All rights reserved.
  • N. Miyauchi, H. Hidaka, T. Chigai, A. Nagaoka, N. Watanabe, A. Kouchi
    CHEMICAL PHYSICS LETTERS 456 1-3 27 - 30 2008年04月 [査読有り][通常論文]
     
    The reactions of cold H atoms with solid O-2 molecules were investigated at 10 K. The formation of H2O2 and H2O has been confirmed by in situ infrared spectroscopy. We found that the reaction proceeds very efficiently and obtained the effective reaction rates. This is the first clear experimental evidence of the formation of water molecules under conditions mimicking those found in cold interstellar molecular clouds. Based on the experimental results, we discuss the reaction mechanism and astrophysical implications. (C) 2008 Elsevier B.V. All rights reserved.
  • H. Hidaka, N. Miyauchi, A. Kouchi, N. Watanabe
    CHEMICAL PHYSICS LETTERS 456 1-3 36 - 40 2008年04月 [査読有り][通常論文]
     
    Experiments on the hydrogenation of CO on crystalline and amorphous ice at 15 K were carried out to investigate the structural effects of the ice surface. The effective rate of H atom addition to CO on the amorphous ice was found to be larger than that on the crystalline ice, while CO depletion on crystalline ice became larger after long exposure. We demonstrated that the CO-coverage on the ice surfaces dominates the effective reaction rate rather than the surface structure. The larger depletion of CO on crystalline ice, as compared to amorphous ice, suggests easier desorption of CO and/or products by the heat of the reaction. (C) 2008 Elsevier B.V. All rights reserved.
  • 分子雲で生じる低温表面反応:表面実験で探る分子進化
    日高宏, 渡部直樹, 香内晃
    低温科学 66 13  2008年 [査読有り][通常論文]
  • 大場 康弘, 宮内 直弥, 千貝 健, 日高 宏, 渡部 直樹, 香内 晃
    日本地球化学会年会要旨集 55 371 - 371 日本地球化学会 2008年 
    水分子は星間塵アイスマントル中で最も多く存在する分子であるが、その生成メカニズムはいまだはっきりしない。本研究では星間塵上表面原子反応による水分子生成反応の検証を行った。酸素分子を10Kに冷却された基板上に蒸着させ、そこに水素分子をプラズマ解離させて生成する水素原子を照射した。水素原子照射により酸素分子へ水素原子が付加され、過酸化水素、水の生成が確認された。これは酸素分子への水素原子逐次付加反応O2+H->HO2、HO2+H->H2O2、H2O2+H->H2O+OHが起こったことによる。
  • N. Watanabe, O. Mouri, A. Nagaoka, T. Chigai, A. Kouchi, V. Pirronello
    ASTROPHYSICAL JOURNAL 668 2 1001 - 1011 2007年10月 [査読有り][通常論文]
     
    Hydrogenation and photolysis of H2O-CO binary ice mixtures at 10-50 K have been revisited in order to quantitatively evaluate their relative importance in the chemical evolution of interstellar dust icy mantles. The dominant product of photolysis was CO2, with lower yields of formaldehyde, methanol, and formic acid, while only formaldehyde and methanol were obtained by hydrogenation reactions. Hydrogenation has higher formation efficiencies and yields of formaldehyde and methanol than photolysis. However, the contribution of photolysis should not be negligible for the formation of these molecules in molecular clouds. The simultaneous irradiation of binary ice mixtures with hydrogen atoms and UV photons resulted in relative abundances of CO2, formaldehyde, methanol, and formic acid that are consistent with the observed abundances. Our results show that the composition and structure of ice are crucial in the chemical evolution of ice mantles, as much as the temperature and the type of irradiation.
  • N. Watanabe, O. Mouri, A. Nagaoka, T. Chigai, A. Kouchi, V. Pirronello
    ASTROPHYSICAL JOURNAL 668 2 1001 - 1011 2007年10月 [査読有り][通常論文]
     
    Hydrogenation and photolysis of H2O-CO binary ice mixtures at 10-50 K have been revisited in order to quantitatively evaluate their relative importance in the chemical evolution of interstellar dust icy mantles. The dominant product of photolysis was CO2, with lower yields of formaldehyde, methanol, and formic acid, while only formaldehyde and methanol were obtained by hydrogenation reactions. Hydrogenation has higher formation efficiencies and yields of formaldehyde and methanol than photolysis. However, the contribution of photolysis should not be negligible for the formation of these molecules in molecular clouds. The simultaneous irradiation of binary ice mixtures with hydrogen atoms and UV photons resulted in relative abundances of CO2, formaldehyde, methanol, and formic acid that are consistent with the observed abundances. Our results show that the composition and structure of ice are crucial in the chemical evolution of ice mantles, as much as the temperature and the type of irradiation.
  • H. Hidaka, A. Kouchi, N. Watanabe
    JOURNAL OF CHEMICAL PHYSICS 126 20 204707  2007年05月 [査読有り][通常論文]
     
    An experiment on the addition reaction of a D atom (deuteration) to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples (pure solid CO, CO-capped H2O ice, and CO-H2O mixed ice) at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H2O ice and CO-H2O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H2O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant k(D)/k(H) obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H+CO reaction at the surface by using the ratio k(D)/k(H) and by a model calculation of the potential tunneling with the asymmetric Eckart potential. (C) 2007 American Institute of Physics.
  • Akihiro Nagaoka, Naoki Watanabe, Akira Kouchi
    JOURNAL OF PHYSICAL CHEMISTRY A 111 16 3016 - 3028 2007年04月 [査読有り][通常論文]
     
    The surface reactions of CH3OH, CH2DOH, and CHD2OH with cold D atoms at 10 K were investigated using an atomic beam source and FTIR. Methyl-deuterated isotopologues CH2DOH, CHD2OH, and CD3OH were produced by exposure of amorphous solid CH3OH to D atoms at 10 K, and the pseudo-first-order rates for the reactions CH3OH + D -> CH2OH + HD, CH2DOH + D -> CHDOH + HD, and CHD2OH + D -> CD2OH + HD were estimated. The ratios of the reaction rates of the second and third reactions to the first reaction were 0.69 +/- 0.11 and 0.52 +/- 0.14, respectively. The difference in reaction rates is thought to be due to a secondary kinetic isotope effect on the H-abstraction reaction from the methyl side by D atoms.
  • Akihiro Nagaoka, Naoki Watanabe, Akira Kouchi
    JOURNAL OF PHYSICAL CHEMISTRY A 111 16 3016 - 3028 2007年04月 [査読有り][通常論文]
     
    The surface reactions of CH3OH, CH2DOH, and CHD2OH with cold D atoms at 10 K were investigated using an atomic beam source and FTIR. Methyl-deuterated isotopologues CH2DOH, CHD2OH, and CD3OH were produced by exposure of amorphous solid CH3OH to D atoms at 10 K, and the pseudo-first-order rates for the reactions CH3OH + D -> CH2OH + HD, CH2DOH + D -> CHDOH + HD, and CHD2OH + D -> CD2OH + HD were estimated. The ratios of the reaction rates of the second and third reactions to the first reaction were 0.69 +/- 0.11 and 0.52 +/- 0.14, respectively. The difference in reaction rates is thought to be due to a secondary kinetic isotope effect on the H-abstraction reaction from the methyl side by D atoms.
  • 渡部直樹, 香内 晃, 毛利織絵, 長岡明宏, 日高 宏
    J. Vac. Soc. Jpn. 50 4 282 - 290 2007年 [査読有り][通常論文]
     
    Surface reaction on interstellar ice grains is a key process for the chemical evolution in molecular clouds. We have performed a series of experiments to simulate the chemical evolution of the primordial CO molecule on the ice grains. Photochemical and hydrogen atom reactions of CO-H 2O binary ice at 15 K were studied because those are anticipated to be the most dominant chemical processes on the surface of grains. It was found that the photochemical reactions produce CO 2 with the highest yield, HCOOH, H 2CO, CH 3OH, and so on and that even at very low temperatures (much lower than activation energies), addition reactions of hydrogen atoms to CO proceed efficiently due to the tunneling effect and produce H 2CO and CH 3OH. Branching ratio of the yields in photochemical reactions strongly depends on the structure of ice, while hydrogen atom reactions are very sensitive to both the structure and temperature of ice. Considering the physical and chemical conditions of molecular clouds, the experimental results are consistent with the astronomical observation of ice grains.
  • H. Hidaka, A. Kouchi, N. Watanabe
    J. Chem. Phys. 126 20 204707  2007年 [査読有り][通常論文]
  • N. Watanabe, A. Nagaoka, H. Hidaka, T. Shiraki, T. Chigai, A. Kouchi
    PLANETARY AND SPACE SCIENCE 54 11 1107 - 1114 2006年09月 [査読有り][通常論文]
     
    Experiments on the hydrogenation of CO on pure CO and CO-H2O mixed ice have been performed at temperatures between 8 and 20 K. We obtained temperature and compositional dependence of the effective reaction rate constants. Results indicate that hydrogenation proceeds efficiently on pure solid CO and CO-H2O mixed ice at temperatures below 10 and 20 K, respectively. Rate constants for pure CO decreased significantly at 12 K compared to those obtained with CO-H2O mixed ice. Hydrogenation of CO at temperatures greater than 12 K were catalyzed by the H2O adjacent to the CO. The importance of the experimental results for some relevant astrophysical environments has also been outlined. (c) 2006 Elsevier Ltd. All rights reserved.
  • 星間塵表面での低温水素原子反応による分子進化
    渡部直樹, 長岡明宏, 日高宏, 香内晃
    固体物理11 41 755 - 764 2006年 [査読有り][通常論文]
  • Hiroshi Hidaka, Naoki Watanabe, Akira Kouchi
    ASTROCHEMISTRY: FROM LABORATORY STUDIES TO ASTRONOMICAL OBSERVATIONS 855 107 - + 2006年 [査読有り][通常論文]
     
    The reaction of cold H and D atoms with solid formaldehyde (D2CO and H2CO) deposited on amorphous H2O ice at 15 K was studied experimentally by in-situ FTIR spectroscopy. New routes of formation of deuterated formaldehyde (HDCO and D2CO) were identified. In the reaction of D atoms with H2CO, H-D substitution was observed, H2CO -> HDCO -> D2CO, and for the reaction of H atoms with D2CO, the analogous D-H substitution was observed, D2CO -> HDCO -> H2CO. In the reaction of D atoms with H2CO, the rate of H-D substitution was greater than that of formation of deuterated methanol-d(2-4) by addition of 2D atoms to formaldehyde-d(0-2). However, in the reaction of H atoms with D2CO, the rate of D-H substitution was comparable to that of addition of 2H atoms to D2CO.
  • Akihiro Nagaoka, Naoki Watanabe, Akira Kouchi
    ASTROCHEMISTRY: FROM LABORATORY STUDIES TO ASTRONOMICAL OBSERVATIONS 855 69 - + 2006年 [査読有り][通常論文]
     
    We performed an experiment in which pure solid CO was simultaneously exposed to cold H and D atoms at 10 K, and observed efficient formation of H2CO, CH3OH and deuterated isotopologues HDCO, D2CO, CH2DOH, CHD2OH, and CD3OH. The D/H ratios obtained for formaldehyde and methanol are in good agreement with previously reported observations. We found that the H-D substitution reaction in methanol is a key process in the formation of deuterated methanol, especially multi-deuterated isotopologues CHD2OH and CD3OH.
  • Naoki Watanabe, Hiroshi Hidaka, Akira Kouchi
    ASTROCHEMISTRY: FROM LABORATORY STUDIES TO ASTRONOMICAL OBSERVATIONS 855 122 - + 2006年 [査読有り][通常論文]
     
    Using cold atomic hydrogen and deuterium beams, hydrogenation and deuteration of solid CO on the surface of H2O ice were investigated in the surface-temperature range 8-20 K. In hydrogenation experiments, formaldehyde and methanol were produced efficiently by successive hydrogenation of CO below 20 K in conditions similar to those in molecular clouds. In deuteration experiments, we found, for the first time, that D2CO and CD3OD are formed from CO, with the reaction proceeding similarly to hydrogenation although at a much slower rate. From the attenuation curve of the parent CO at 15 K, the relative reaction rate of CO-deuteration to -hydrogenation was determined to be about 0.1. This isotope effect is considered to be due to tunneling reaction.
  • 星間塵表面での低温水素原子反応による分子進化
    渡部直樹, 長岡明宏, 日高宏, 香内晃
    固体物理 特集号 <新しい水素の科学> 41 1 59 - 68 2006年 [査読有り][通常論文]
  • TM Kojima, FF Chen, M Kitajima, T Koizumi, Y Nakai, H Yamaoka, N Watanabe
    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 144 71 - 74 2005年06月 [査読有り][通常論文]
     
    The relative photoion yield in the 4d photoionization of Eu+ has been measured with an energy resolution of E/Delta E similar or equal to 820. The obtained spectrum shows that the 4d 'giant resonance' of Eu+ is not a single peak but consists of many resonances. The result is compared with a recent theoretical calculation and our previous result which has been measured with lower resolution. The calculation well reproduces the experimental result in the region of E >= 140 eV, while some peaks are not corresponding in lower energy region. (c) 2004 Elsevier B.V. All rights reserved.
  • TM Kojima, FF Chen, M Kitajima, T Koizumi, Y Nakai, H Yamaoka, N Watanabe
    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 144 71 - 74 2005年06月 [査読有り][通常論文]
     
    The relative photoion yield in the 4d photoionization of Eu+ has been measured with an energy resolution of E/Delta E similar or equal to 820. The obtained spectrum shows that the 4d 'giant resonance' of Eu+ is not a single peak but consists of many resonances. The result is compared with a recent theoretical calculation and our previous result which has been measured with lower resolution. The calculation well reproduces the experimental result in the region of E >= 140 eV, while some peaks are not corresponding in lower energy region. (c) 2004 Elsevier B.V. All rights reserved.
  • A Nagaoka, N Watanabe, A Kouchi
    ASTROPHYSICAL JOURNAL 624 1 L29 - L32 2005年05月 [査読有り][通常論文]
     
    Deuterium enrichment of interstellar methanol is reproduced experimentally for the first time via grain-surface H-D substitution in solid methanol at an atomic D/H ratio of 0.1. Although previous gas-grain models successfully reproduce the deuterium enrichments observed in interstellar methanol molecules (D/H of up to 0.4, compared to the cosmic ratio of similar to 10(-5)), the models exclusively focus on deuterium fractionation resulting from the successive addition of atomic hydrogen/deuterium on CO. The mechanism proposed here represents a key route for deuterium enrichment that reproduces the high observed abundances of deuterated methanol, including multiple deuterations.
  • A Nagaoka, N Watanabe, A Kouchi
    ASTROPHYSICAL JOURNAL 624 1 L29 - L32 2005年05月 [査読有り][通常論文]
     
    Deuterium enrichment of interstellar methanol is reproduced experimentally for the first time via grain-surface H-D substitution in solid methanol at an atomic D/H ratio of 0.1. Although previous gas-grain models successfully reproduce the deuterium enrichments observed in interstellar methanol molecules (D/H of up to 0.4, compared to the cosmic ratio of similar to 10(-5)), the models exclusively focus on deuterium fractionation resulting from the successive addition of atomic hydrogen/deuterium on CO. The mechanism proposed here represents a key route for deuterium enrichment that reproduces the high observed abundances of deuterated methanol, including multiple deuterations.
  • 渡部 直樹, 香内 晃
    日本物理学会誌 60 1 39 - 44 一般社団法人日本物理学会 2005年 [査読有り][通常論文]
     
    星間分子雲に漂うサブミクロンサイズの塵(星間塵)表面では10K程の低温にもかかわらず,活発な化学反応により簡単な分子がより複雑な分子へと進化していると考えられている.しかし,その進化過程の詳細は未解明な部分が多い.我々は最近,星間塵上に多量に存在するメタノール分子の生成過程を実験て明らかにした.メタノールは低温表面CO分子ヘの水素原子逐次付加反応によりホルムアルデヒトを経て効率よく生成される.
  • 星間塵表面反応による星間分子の重水素濃集メカニズム
    渡部直樹, 長岡明宏, 日高宏, 香内晃
    日本惑星科学会誌 14 175 - 182 2005年 [査読有り][通常論文]
  • Naoki Watanabe
    Proceedings of the International Astronomical Union 1 231 415 - 426 2005年 [査読有り][通常論文]
     
    A series of experiments on the surface reactions of hydrogen and deuterium atoms with solid CO, formaldehyde (H2CO), and methanol (CH 3OH) has been performed. Successive hydrogenation of CO on surfaces at ∼10 K was found to proceed efficiently via tunneling to produce H 2CO and CH3OH on dust grains under the typical conditions of molecular clouds. Formation rates are strongly dependent on the surface temperature and composition. The role of surface reactions in the formation of deuterated formaldehyde and methanol was investigated. The deuterium fractionation of methanol observed in molecular clouds was reproduced experimentally via H-D substitution in solid methanol at an accreting atomic D/H ratio of 0.05-0.1. This is the first evidence that grain-surface reactions can be responsible for fractionation. We have determined several effective rate constants for hydrogenation, deuteration, and H-D substitution to construct the surface reaction network for CO, H2CO, CH3OH, deuterated formaldehyde, and deuterated methanol. © 2006 International Astronomical Union.
  • 長岡 明宏, 渡部 直樹, 日高 宏, 香内 晃
    日本鉱物学会年会講演要旨集 2005 136 - 136 日本鉱物科学会 2005年 
    最近,星間分子雲ではメタノール(CH3OH)にDが異常に濃集していることが電波観測からわかってきた. CH3OHとその重水素置換体(CH3OH-d)の存在比はCH3OH-d / CH3OH?0.4であり,D/Hの宇宙存在比 (?1×10-5)の1万倍も大きい.このD異常濃集過程としてこれまでは気相反応が考えられたが,観測値を満足に説明できない.そこで私たちは,極低温での塵表面反応が重水素濃集過程としても重要な役割を担っていると考え,極低温の CO薄膜へHおよびD原子を同時に照射する実験を行った. 実験の結果,分子雲での106年間に相当するHおよびD原子を照射した時,実験から得られたCH3OH-d / CH3OH比は観測値を良く再現した.さらに,CH3OH-dの生成過程は,COへのH原子逐次付加によってまずCH3OHが生成された後,D原子がCH3OHと反応し,メチル基のHがDに置換される反応(H-D置換反応: CH3OH→CH2DOH→CHD2OH→CD3OH) が支配的であることが初めてわかった.
  • N Watanabe, A Nagaoka, T Shiraki, A Kouchi
    ASTROPHYSICAL JOURNAL 616 1 638 - 642 2004年11月 [査読有り][通常論文]
     
    Using a cold ( 30 K) atomic hydrogen beam, hydrogenation of CO on pure solid CO and CO-H2O mixed ice is investigated at temperatures below 20 K. Hydrogenation proceeds efficiently on both pure solid CO and CO-H2O mixed ice below 12 K, but the rate of reaction on pure CO decreases significantly at 15 K compared to CO-H2O mixed ice. Hydrogenation of CO at 12 K and above is found to be promoted by H2CO and H2O molecules on the CO surface.
  • N Watanabe, A Nagaoka, T Shiraki, A Kouchi
    ASTROPHYSICAL JOURNAL 616 1 638 - 642 2004年11月 [査読有り][通常論文]
     
    Using a cold ( 30 K) atomic hydrogen beam, hydrogenation of CO on pure solid CO and CO-H2O mixed ice is investigated at temperatures below 20 K. Hydrogenation proceeds efficiently on both pure solid CO and CO-H2O mixed ice below 12 K, but the rate of reaction on pure CO decreases significantly at 15 K compared to CO-H2O mixed ice. Hydrogenation of CO at 12 K and above is found to be promoted by H2CO and H2O molecules on the CO surface.
  • H Hidaka, N Watanabe, T Shiraki, A Nagaoka, A Kouchi
    ASTROPHYSICAL JOURNAL 614 2 1124 - 1131 2004年10月 [査読有り][通常論文]
     
    The conversion of formaldehyde (H2CO) to methanol (CH3OH) by successive hydrogenation on H2O ice was measured at 10, 15, and 20 K using atomic hydrogen beams of 30 and 300 K. The conversion rates and CH3OH yields under the 30 K beam are very similar to those under the 300 K beam at all ice temperatures, demonstrating that the reaction is independent of beam temperature. The dependence of the conversion rates on ice temperature is consistent with that for previous experiments on CO hydrogenation. The conversion rate for H2CO --> CH3OH at 15 K was found to be about half that for CO --> H2CO. The dependence of the reactions on the initial thickness of H2CO was also measured. More than 80% of H2CO was converted to CH3OH for H2CO layers of less than 1 monolayer in average thickness. Irradiation of CH3OH with H atoms did not produce H2CO, demonstrating that the reverse process, CH3OH --> H2CO (H abstraction), is minor compared to the forward process.
  • H Hidaka, N Watanabe, T Shiraki, A Nagaoka, A Kouchi
    ASTROPHYSICAL JOURNAL 614 2 1124 - 1131 2004年10月 [査読有り][通常論文]
     
    The conversion of formaldehyde (H2CO) to methanol (CH3OH) by successive hydrogenation on H2O ice was measured at 10, 15, and 20 K using atomic hydrogen beams of 30 and 300 K. The conversion rates and CH3OH yields under the 30 K beam are very similar to those under the 300 K beam at all ice temperatures, demonstrating that the reaction is independent of beam temperature. The dependence of the conversion rates on ice temperature is consistent with that for previous experiments on CO hydrogenation. The conversion rate for H2CO --> CH3OH at 15 K was found to be about half that for CO --> H2CO. The dependence of the reactions on the initial thickness of H2CO was also measured. More than 80% of H2CO was converted to CH3OH for H2CO layers of less than 1 monolayer in average thickness. Irradiation of CH3OH with H atoms did not produce H2CO, demonstrating that the reverse process, CH3OH --> H2CO (H abstraction), is minor compared to the forward process.
  • 星間塵表面反応による有機分子生成および重水素濃集機構
    渡部直樹, 香内晃, 白木隆裕, 長岡明宏, 日高宏
    日本惑星科学会誌 13 226 - 232 2004年 [査読有り][通常論文]
  • 渡部 直樹, 白木隆裕, 香内 晃
    天文月報 97 2 101 - 106 日本天文学会 2004年 [査読有り][通常論文]
  • Influence of ice composition on the formation on interstellar formaldehyde and methanol on dust grains
    A. Nagaoka, N. Watanabe, T. Shiraki, H. Hidaka, A. Kouchi
    Proceedings of the 37th isas luna and planetary symposium 4 91 - 94 2004年 [査読有り][通常論文]
  • N Watanabe, T Shiraki, A Kouchi
    ASTROPHYSICAL JOURNAL 588 2 L121 - L124 2003年05月 [査読有り][通常論文]
     
    The formation of formaldehyde (H(2)CO) and methanol (CH(3) OH) by the successive addition of hydrogen atoms to CO molecules in H(2)O-CO mixed ice was measured at 10, 15, and 20 K. The maximum yield of CH(3)OH was obtained at 15 K, with the next highest yield at 10 K. At 20 K, the reactions proceeded very slowly, and the resulting yield of CH(3)OH was significantly smaller than at 10 and 15 K and was considered to be due to a fall in the sticking probability of hydrogen at this temperature. The observed temperature dependence indicates that the abundances of H(2)CO and CH(3)OH in a molecular cloud are strongly dependent on the temperature of dust as well as the flux of atomic hydrogen. The diffusion of hydrogen atoms in ice is also discussed based on measurements of the dependence of CO hydrogenation on ice thickness in the range of less than 30 monolayers.
  • N Watanabe, T Shiraki, A Kouchi
    ASTROPHYSICAL JOURNAL 588 2 L121 - L124 2003年05月 [査読有り][通常論文]
     
    The formation of formaldehyde (H(2)CO) and methanol (CH(3) OH) by the successive addition of hydrogen atoms to CO molecules in H(2)O-CO mixed ice was measured at 10, 15, and 20 K. The maximum yield of CH(3)OH was obtained at 15 K, with the next highest yield at 10 K. At 20 K, the reactions proceeded very slowly, and the resulting yield of CH(3)OH was significantly smaller than at 10 and 15 K and was considered to be due to a fall in the sticking probability of hydrogen at this temperature. The observed temperature dependence indicates that the abundances of H(2)CO and CH(3)OH in a molecular cloud are strongly dependent on the temperature of dust as well as the flux of atomic hydrogen. The diffusion of hydrogen atoms in ice is also discussed based on measurements of the dependence of CO hydrogenation on ice thickness in the range of less than 30 monolayers.
  • JSA Perry, JM Gingell, KA Newson, J To, N Watanabe, SD Price
    MEASUREMENT SCIENCE AND TECHNOLOGY 13 9 1414 - 1424 2002年09月 [査読有り][通常論文]
     
    An experiment has been constructed to determine the rovibrational states populated in the formation of H-2 on the surface of cosmic dust under conditions approaching those of the interstellar medium (ISM). During the experiment, a beam of atomic hydrogen of controlled temperature is incident upon a target which is an analogue of cosmic dust. Molecular hydrogen desorbing from the target's surface is ionized using (2 + 1) resonance enhanced multiphoton ionization and detected using time-of-flight mass spectrometry. The experiment allows the rovibrational populations of the H-2 molecules desorbing from the cosmic dust targets to be determined providing information on the energy budget of the H-2 formation process in the ISM. Preliminary results from the experiment, to prove its viability, show that H-2 molecules formed on an highly oriented pyrolytic graphite surface have a measurable population of excited vibrational and rotational states.
  • JSA Perry, JM Gingell, KA Newson, J To, N Watanabe, SD Price
    MEASUREMENT SCIENCE AND TECHNOLOGY 13 9 1414 - 1424 2002年09月 [査読有り][通常論文]
     
    An experiment has been constructed to determine the rovibrational states populated in the formation of H-2 on the surface of cosmic dust under conditions approaching those of the interstellar medium (ISM). During the experiment, a beam of atomic hydrogen of controlled temperature is incident upon a target which is an analogue of cosmic dust. Molecular hydrogen desorbing from the target's surface is ionized using (2 + 1) resonance enhanced multiphoton ionization and detected using time-of-flight mass spectrometry. The experiment allows the rovibrational populations of the H-2 molecules desorbing from the cosmic dust targets to be determined providing information on the energy budget of the H-2 formation process in the ISM. Preliminary results from the experiment, to prove its viability, show that H-2 molecules formed on an highly oriented pyrolytic graphite surface have a measurable population of excited vibrational and rotational states.
  • N Watanabe, A Kouchi
    ASTROPHYSICAL JOURNAL 571 2 L173 - L176 2002年06月 [査読有り][通常論文]
     
    Formaldehyde and methanol were produced efficiently by the hydrogenation of CO in H(2)O-CO ice at 10 K in an atomic hydrogen beam experiment. The relative yields to the initial CO were of the order of 10%, 3 orders of magnitude larger than those reported previously. This reveals for the first time experimentally that successive hydrogenation of CO is most likely to produce formaldehyde and methanol on the surface of icy grains in molecular clouds as suggested by theoretical models.
  • Naoki Watanabe, Akira Kouchi
    Astrophysical Journal 571 2 L173 - L176 2002年06月01日 [査読有り][通常論文]
     
    Formaldehyde and methanol were produced efficiently by the hydrogenation of CO in H2O-CO ice at 10 K in an atomic hydrogen beam experiment. The relative yields to the initial CO were of the order of 10%, 3 orders of magnitude larger than those reported previously. This reveals for the first time experimentally that successive hydrogenation of CO is most likely to produce formaldehyde and methanol on the surface of icy grains in molecular clouds as suggested by theoretical models.
  • N Watanabe, A Kouchi
    ASTROPHYSICAL JOURNAL 567 1 651 - 655 2002年03月 [査読有り][通常論文]
     
    The formation of carbon dioxide molecules from D2O(H2O)/CO binary ice induced by UV irradiation was studied experimentally. The measured conversion rate of CO CO2 to is about one order higher than previously reported rates. Our results imply that CO2 formation by the UV process is efficient even in a quiescent dense cloud.
  • A Kouchi, T Kudo, H Nakano, M Arakawa, N Watanabe, SI Sirono, M Higa, N Maeno
    ASTROPHYSICAL JOURNAL 566 2 L121 - L124 2002年02月 [査読有り][通常論文]
     
    We experimentally found interstellar grains covered with organic matter in an asteroid belt, and more importantly, the organic matter played an essential role in the formation of the asteroids. The sticking threshold velocity of 5 m s(-1) of the millimeter-sized organic grains was several orders of magnitude higher than those of the coexisting silicate and ice grains. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. In contrast, there was no coagulation of the silicate and ice grains in the terrestrial and Jovian regions, respectively.
  • A Kouchi, T Kudo, H Nakano, M Arakawa, N Watanabe, SI Sirono, M Higa, N Maeno
    ASTROPHYSICAL JOURNAL 566 2 L121 - L124 2002年02月 [査読有り][通常論文]
     
    We experimentally found interstellar grains covered with organic matter in an asteroid belt, and more importantly, the organic matter played an essential role in the formation of the asteroids. The sticking threshold velocity of 5 m s(-1) of the millimeter-sized organic grains was several orders of magnitude higher than those of the coexisting silicate and ice grains. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. In contrast, there was no coagulation of the silicate and ice grains in the terrestrial and Jovian regions, respectively.
  • Naoki Watanabe, Akira Kouchi
    Astrophys. J. 567 1 651 - 655 2002年 [査読有り][通常論文]
  • Y Itoh, A Ito, M Kitajima, T Koizumi, TM Kojima, H Sakai, M Sano, N Watanabe
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 34 17 3493 - 3499 2001年09月 [査読有り][通常論文]
     
    Using the photon-ion merged-beam technique, we have measured the absolute photoionization cross sections for Xe2+ and Xe3+ formation from Xe+ at selected energies between 80 and 140 eV, and utilized the absolute data to normalize our previously reported relative cross sections. The structure and energy dependence of the cross sections for Xe+ deviate from those of neutral Xe; however, the maximum total cross section for Xe+ is, within the experimental error, the same as for Xe.
  • Y Itoh, A Ito, M Kitajima, T Koizumi, TM Kojima, H Sakai, M Sano, N Watanabe
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 34 17 3493 - 3499 2001年09月 [査読有り][通常論文]
     
    Using the photon-ion merged-beam technique, we have measured the absolute photoionization cross sections for Xe2+ and Xe3+ formation from Xe+ at selected energies between 80 and 140 eV, and utilized the absolute data to normalize our previously reported relative cross sections. The structure and energy dependence of the cross sections for Xe+ deviate from those of neutral Xe; however, the maximum total cross section for Xe+ is, within the experimental error, the same as for Xe.
  • Formation of CO2 molecules from CO doped D2O amorphous ice by uv irradiation at 12K
    Naoki Watanabe, Akira Kouchi
    Prog. Rep. Atom. Coll. Res. Jpn. 27 62 - 63 2001年 [査読有り][通常論文]
  • N Watanabe, T Horii, A Kouchi
    ASTROPHYSICAL JOURNAL 541 2 772 - 778 2000年10月 [査読有り][通常論文]
     
    We have carried out experiments on the formation of D-2 molecules from amorphous D2O ice (thickness: 40, 120 Angstrom) by UV irradiation (lambda: 126, 172 Ka) at 12 K. Our results show that most of the D-2 products remain in and on the amorphous ice at 12 K; directly photodesorbed D-2 was little observed at 12 K. The amount of D2O destruction and D-2 yields after a dose of 10(18) photons cm(-2) at 12 K were about 10%-20% and 1%-2% of the initial number of D2O molecules, respectively. The formation reaction of D-2 was found to be a single-photon process via the first excited state of D2O. The reconstruction of D2O at 12 K (reverse reaction) was not observed within our experimental timescale (5 hr). We estimated the cross sections for the D2O destruction and the D-2 formation reactions by the photon impact at 12 K for the first time.
  • N. Watanabe, T. Horii, A. Kouchi
    Astrophys. J. 541 772 - 779 2000年 [査読有り][通常論文]
  • A.A.Pavlychev, N.G. Forminykh, N. Watanabe, K. Soejima, E. Shigemasa, A. Yagishita
    Physical Review Letters 81 17 3623 - 3626 1998年10月26日 [査読有り][通常論文]
     
    An essential correlation between intramolecular interference of photoelectron waves and core-hole molecular relaxation is revealed by analyzing the partial wave composition of the calculated angular distribution of photoelectrons in the N and O 1s−1σu* shape resonances of aligned N2 and CO2. The dominance of coherent (N2) and incoherent (CO2) interference is revealed. pf and spd hybridization controls the main direction of photoelectron emission from the N and O K shell, respectively. © 1998 The American Physical Society.
  • A. A. Pavlychev, N. G. Fominykh, N. Watanabe, K. Soejima, E. Shigemasa, A. Yagishita
    Physical Review Letters 81 17 3623 - 3626 1998年10月26日 [査読有り][通常論文]
     
    An essential correlation between intramolecular interference of photoelectron waves and core-hole molecular relaxation is revealed by analyzing the partial wave composition of the calculated angular distribution of photoelectrons in the N and O 1s−1σu* shape resonances of aligned N2 and CO2. The dominance of coherent (N2) and incoherent (CO2) interference is revealed. pf and spd hybridization controls the main direction of photoelectron emission from the N and O K shell, respectively. © 1998 The American Physical Society.
  • N Watanabe, Y Awaya, A Fujino, Y Itoh, M Kitajima, TM Kojima, M Oura, R Okuma, M Sano, T Sekioka, T Koizumi
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 31 18 4137 - 4141 1998年09月 [査読有り][通常論文]
     
    Photoion yields from doubly charged Xe ions were measured in the 4d-threshold energy region. The experiment was performed by a photon-ion merged-beam technique with synchrotron radiation as a light source. The measured spectrum mainly consists of strong 4d --> 5p transition lines at around 57 eV, a very broad peak at around 100 eV, and the preceding strong discrete peaks lying between 79 and 90 eV. The peak position of the broad peak changes very little from that in Xe+ photoion-yield spectra of Sane et al. The enhancement of discrete peaks in the Xe2+ spectrum implies a further collapse of nf-wavefunctions than that of Xe+.
  • N Watanabe, Y Awaya, A Fujino, Y Itoh, M Kitajima, TM Kojima, M Oura, R Okuma, M Sano, T Sekioka, T Koizumi
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 31 18 4137 - 4141 1998年09月 [査読有り][通常論文]
     
    Photoion yields from doubly charged Xe ions were measured in the 4d-threshold energy region. The experiment was performed by a photon-ion merged-beam technique with synchrotron radiation as a light source. The measured spectrum mainly consists of strong 4d --> 5p transition lines at around 57 eV, a very broad peak at around 100 eV, and the preceding strong discrete peaks lying between 79 and 90 eV. The peak position of the broad peak changes very little from that in Xe+ photoion-yield spectra of Sane et al. The enhancement of discrete peaks in the Xe2+ spectrum implies a further collapse of nf-wavefunctions than that of Xe+.
  • TM Kojima, M Oura, Y Itoh, T Koizumi, M Sano, T Sekioka, N Watanabe, H Yamaoka, Y Awaya
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 31 7 1463 - 1468 1998年04月 [査読有り][通常論文]
     
    Using an ion-photon merged-beam apparatus, photoion yield spectra of Eu2+ and Eu3+ from the 4d photoionization of Eu+ were measured as a function of photon energy in the range 110-160 eV. The measured spectra look very similar to those from a neutral Eu target. Both Eu2+ and Eu3+ yield spectra, a broad 'giant resonance' appears at 140 eV with some preceding small peaks at 131.5, 132.8 and 134.7 eV. The summit of the 'giant resonance' in the Eu2+ spectrum is split into two peaks. This is not observed in neutral Eu photoionization.
  • TM Kojima, M Oura, Y Itoh, T Koizumi, M Sano, T Sekioka, N Watanabe, H Yamaoka, Y Awaya
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 31 7 1463 - 1468 1998年04月 [査読有り][通常論文]
     
    Using an ion-photon merged-beam apparatus, photoion yield spectra of Eu2+ and Eu3+ from the 4d photoionization of Eu+ were measured as a function of photon energy in the range 110-160 eV. The measured spectra look very similar to those from a neutral Eu target. Both Eu2+ and Eu3+ yield spectra, a broad 'giant resonance' appears at 140 eV with some preceding small peaks at 131.5, 132.8 and 134.7 eV. The summit of the 'giant resonance' in the Eu2+ spectrum is split into two peaks. This is not observed in neutral Eu photoionization.
  • E. Shigemasa, J. Adachi, K. Soejima, N. Watanabe, A. Yagishita, N.A. Cherepkov
    Physical Review Letters 80 8 1622 - 1625 1998年02月 [査読有り][通常論文]
     
    The first complete experiment for the C K-shell of CO molecules in the region of the sigma* shape resonance has been performed by detecting photoelectrons in coincidence with fragment ions. Four ratios of dipole matrix elements and four phase shift differences have been extracted from the experi mental data. Their analyses show that, in the sigma* shape resonance due to the l mixing, d and f partial waves give the main contribution to the cross section, and f and g waves give the main contribution to a rapid increase of phase shift by pi.
  • E Shigemasa, J Adachi, K Soejima, N Watanabe, A Yagishita, NA Cherepkov
    PHYSICAL REVIEW LETTERS 80 8 1622 - 1625 1998年02月 [査読有り][通常論文]
     
    The first complete experiment for the C K-shell of CO molecules in the region of the sigma* shape resonance has been performed by detecting photoelectrons in coincidence with fragment ions. Four ratios of dipole matrix elements and four phase shift differences have been extracted from the experi mental data. Their analyses show that, in the sigma* shape resonance due to the l mixing, d and f partial waves give the main contribution to the cross section, and f and g waves give the main contribution to a rapid increase of phase shift by pi.
  • N. Watanabe, J. Adachi, K. Soejima, E. Shigemasa, A. Yagishita, N. G. Fominykh, A. A.Pavlychev
    Physical Review Letters 78 26 4910 - 4913 1997年06月 [査読有り][通常論文]
     
    Angular distributions of 1s sigma(g,u), photoelectrons from CO2 molecules, oriented parallel to the electric vector of incident light, have been measured. The angular distribution patters at sigma(g)* and sigma(u)* shape resonances are significantly different. From those expected in the virtual molecular orbitals concept. The analyses of the patterns have made it clear that the mixing of outgoing photoelectron partial waves and the interference between them are responsible for the observed patterns, unlike N-2 photoionization. Calculations based on a quasiatomic model bare reproduced the observed patterns fairly well.
  • N Watanabe, J Adachi, K Soejima, E Shigemasa, A Yagishita, NG Fominykh, AA Pavlychev
    PHYSICAL REVIEW LETTERS 78 26 4910 - 4913 1997年06月 [査読有り][通常論文]
     
    Angular distributions of 1s sigma(g,u), photoelectrons from CO2 molecules, oriented parallel to the electric vector of incident light, have been measured. The angular distribution patters at sigma(g)* and sigma(u)* shape resonances are significantly different. From those expected in the virtual molecular orbitals concept. The analyses of the patterns have made it clear that the mixing of outgoing photoelectron partial waves and the interference between them are responsible for the observed patterns, unlike N-2 photoionization. Calculations based on a quasiatomic model bare reproduced the observed patterns fairly well.
  • T Kambara, A Igarashi, N Watanabe, Y Nakai, TM Kojima, Y Awaya
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 30 5 1251 - 1260 1997年03月 [査読有り][通常論文]
     
    Using high resolution recoil-ion momentum spectroscopy we have measured differential cross sections of single-electron capture from He atoms by multiply charged boron ions, bare B5+ and hydrogen-like B4+, at 0.5-1.0 MeV/u. From the measurements of the longitudinal momentum distribution of the recoil ions, the final states of the captured electron in B4+(nl) and B3+(1snl) ions were analysed and the n = 1 ground states were clearly separated from the other excited states. Capture to the excited states gives the dominant contribution to the capture cross section. However, the fraction of captures to the ground state increases with the collision energy. Theoretical calculations based on the eikonal approximation well reproduces the experimental transverse recoil-ion momentum distribution of capture to both the ground and excited states.
  • T Kambara, A Igarashi, N Watanabe, Y Nakai, TM Kojima, Y Awaya
    JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 30 5 1251 - 1260 1997年03月 [査読有り][通常論文]
     
    Using high resolution recoil-ion momentum spectroscopy we have measured differential cross sections of single-electron capture from He atoms by multiply charged boron ions, bare B5+ and hydrogen-like B4+, at 0.5-1.0 MeV/u. From the measurements of the longitudinal momentum distribution of the recoil ions, the final states of the captured electron in B4+(nl) and B3+(1snl) ions were analysed and the n = 1 ground states were clearly separated from the other excited states. Capture to the excited states gives the dominant contribution to the capture cross section. However, the fraction of captures to the ground state increases with the collision energy. Theoretical calculations based on the eikonal approximation well reproduces the experimental transverse recoil-ion momentum distribution of capture to both the ground and excited states.
  • 暗黒星雲における物質進化
    香内晃, 渡部, 直樹
    宇宙空間原子分子過程研究会 47 - 50 1997年 [査読有り][通常論文]
  • S.D. Kravis, N. Watanabe, Y. Awaya, M. Kimura, T. Kojima, K. Okuno, M. Oura
    Physica Scripta T71 121 - 126 1997年 [査読有り][通常論文]
  • T Koizumi, Y Awaya, A Fujino, Y Itoh, M Kitajima, TM Kojima, M Oura, R Okuma, M Sano, T Seikioka, N Watanabe, F Koike
    PHYSICA SCRIPTA T73 T73 131 - 132 1997年 [査読有り][通常論文]
     
    Using a photon-ion merged-beam technique, we studied the photoionization of multiply charged Xeq+ (q = 1-3) ions in the 4d excitation region (50-140eV). Relative yield spectra of photoions created by single (q-->q+1) and double (q-->q+2) photoionization were measured as a function of photon energy. A broad ''4d giant resonance'' peak with some preceding discrete peaks appears in each total ion spectrum. The intensities of discrete peaks observed at around 4d ionization threshold strongly depend on the target charge state. Those for Xe3+ target are very strong, whereas those for Xe+ are weak. Peaks due to 4d-->5p transition, which cannot occur in the neutral Xe case, has also been observed for each ion-target. The experimental spectrum for Xe+ has been analyzed by a multiconfiguration Dirac-Fock calculation This calculation shows that results observed may be explained in terms of the correlation of valence and excited orbitals, and the effect of 4f-orbital collapse, of which the degree varies strongly with the change of the effective nuclear charge.
  • SD Kravis, N Watanabe, Y Awaya, M Kimura, T Kojima, K Okuno, M Oura
    PHYSICA SCRIPTA T71 121 - 126 1997年 [査読有り][通常論文]
     
    The importance, feasibility and method for measuring the photo-ionization cross-section of multi-charged ions using the RIKEN Electron Beam Ion Source (REBIS) is discussed. Specifically the cases for the 2 s and 1 s photo-ionization of Ar14+ is explored. It is concluded that such measurements are possible today and in future (3(rd) generation) synchrotron radiation facilities, and will be carried out upon modifications of the apparatus. Also described, is the first test of a PHOton Beam Ion Source using REBIS. It's potential advantage over other ion sources is that in the ion production mechanism photoionization the recoil ions obtain only near thermal energies to making ion beams with very low energy spread possible.
  • S.D. Kravis, N. Watanabe, Y. Awaya, M. Kimura, T. Kojima, K. Okuno, M. Oura
    Physica Scripta T71 121 - 126 1997年 [査読有り][通常論文]
  • M. Sano, Y. Itoh, T. Koizumi, T. M. Kojima, S. D. Kravis, M. Oura, T. Sekioka, N. Watanabe, Y. Awaya, F. Koike
    Journal of Physics B: Atomic, Molecular and Optical Physics 29 22 5305 - 5313 1996年11月28日 [査読有り][通常論文]
     
    Using a photon-ion merged-beam technique, we measured the relative yield spectra of Xe2+ and Xe3+ ions created by photoionization of Xe+ ions as a function of the photon energy in the 4d ionization region. The Xe3+ ion production, i.e. the double-electron ionization process is found to be dominant throughout the energy range investigated. A broad structure of 4d excitation ionization around 100 eV photon energy was observed in the Xe3+ yield spectrum. A spectral shoulder was observed for Xe3+ at around 85 eV, which was absent in the photoionization of neutral Xe. Some discrete lines were observed for both the Xe2+- and Xe3+-yield spectra below 75 eV. The experimental spectra were analysed by a multiconfiguration Dirac-Fock calculation. The largest structure at around 100 eV is attributable to 4d105s25p5 → 4d94f5s25p4np (n = 6, 7) two-electron transitions. The discrete lines were found to be due to 4d → np, nf transitions. A serious 4f-orbital collapse is suspected in the 4fnp two-electron excited states, whereas the collapse is moderate in the singly excited 4f-orbitals.
  • M. Sano, Y. Itoh, T. Koizumi, T. M. Kojima, S. D. Kravis, M. Oura, T. Sekioka, N. Watanabe, Y. Awaya, F. Koike
    Journal of Physics B: Atomic, Molecular and Optical Physics 29 22 5305 - 5313 1996年11月28日 [査読有り][通常論文]
     
    Using a photon-ion merged-beam technique, we measured the relative yield spectra of Xe2+ and Xe3+ ions created by photoionization of Xe+ ions as a function of the photon energy in the 4d ionization region. The Xe3+ ion production, i.e. the double-electron ionization process is found to be dominant throughout the energy range investigated. A broad structure of 4d excitation ionization around 100 eV photon energy was observed in the Xe3+ yield spectrum. A spectral shoulder was observed for Xe3+ at around 85 eV, which was absent in the photoionization of neutral Xe. Some discrete lines were observed for both the Xe2+- and Xe3+-yield spectra below 75 eV. The experimental spectra were analysed by a multiconfiguration Dirac-Fock calculation. The largest structure at around 100 eV is attributable to 4d105s25p5 → 4d94f5s25p4np (n = 6, 7) two-electron transitions. The discrete lines were found to be due to 4d → np, nf transitions. A serious 4f-orbital collapse is suspected in the 4fnp two-electron excited states, whereas the collapse is moderate in the singly excited 4f-orbitals.
  • SD Kravis, M Abdallah, CL Cocke, CD Lin, M Stockli, B Walch, YD Wang, RE Olson, VD Rodriguez, W Wu, M Pieksma, N Watanabe
    PHYSICAL REVIEW A 54 2 1394 - 1403 1996年08月 [査読有り][通常論文]
     
    A technique for electron spectroscopy which yields full two-dimensional momentum distributions for continuum electrons has been used to study ejected electrons from single ionization of lie by C6+ and proton projectiles at low velocities. Projectile velocities of 1.63, 1.38, and 1.16 a.u, for C6+ and 2.39, 1.71, 1.15, .85, and 0.63 a.u. for protons were used. All spectra show much broader distributions along the beam than transverse to the beam. For the case of proton bombardment, the spectra are strongly influenced by both target and projectile potentials, maximizing near the velocity of the saddle in the potential between the two, receding ion cores for the lowest projectile velocities. For C6+ projectiles, the spectra appear to be dominated by the projectile potential and the center of the distribution is strongly shifted toward the projectile velocity. Theoretical results from the continuum-distorted-wave-eikonal-initial state and classical-trajectory-Monte Carlo methods are in rather good agreement with the proton data but do not agree well with the C6+ data.
  • T Koizumi, Y Awaya, M Gonno, Y Itoh, M Kimura, TM Kojima, S Kravis, M Oura, M Sano, T Sekioka, N Watanabe, H Yamaoka, F Koike
    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 79 289 - 292 1996年05月 [査読有り][通常論文]
     
    Using a photon-ion merged-beam apparatus, the relative photoion-yield spectra for doubly- and triply-charged ions from Xe+, Ba+ and Eu+ ions were measured in the 4d ionization region. The giant resonance peak appears for all target ions. The peak of Xe+ is broad and that of Eu+ is relatively narrow, similar to those observed in the neutral atoms. The spectrum of Xe3+ shows some structures around 87 and 93 eV. The summit of the giant resonance peak of Eu2+ is obviously split into two. These structures observed in the Xe3+ and Eu2+ spectra are not observed in the photoion spectra of the neutral targets. Some small peaks are observed in the Xe+ spectrum between 70 and 76 eV. A multi-configuration Dirac-Fock calculation reveals that these peaks are attributed to 4d-np and 4d-nf transitions. It was also found that the broad peak centered at 100 eV corresponds to shake-up resonance (4d(10)5s(2)5p(5)-->4d(9)4f5s(2)5p(4)np [n=6,7]) processes. Some prominent peaks that do not appear in the neutral Ba spectrum are observed clearly in the Ba+ one. These peaks are most probably due to the 4d-nf transition followed by Auger processes.
  • Y Negishi, N Watanabe, H Shiromaru, Y Achiba, N Kobayashi
    SURFACE REVIEW AND LETTERS 3 1 661 - 665 1996年02月 [査読有り][通常論文]
     
    Ion-molecule reactions of silicon-carbon binary cluster ions SinCm+ (n = 1-2, m = 1-2) with O-2 molecules were studied by using an rf ion trap. The rate constants and branching ratios were determined for these cluster ions. For SiC2+, the rate constants for two isomers were obtained.
  • Molecular shape resonances studied by Angle-resolved PEPICO
    E. Shigemasa, J. Adachi, N. Watanabe, K. Soejima, A. Yagishita
    Eds. A. Yagishita and T. Sasaki: Atomic and Molecular Photoionization, Universal Academy Press, INC. 69 - 78 1996年 [査読有り][通常論文]
  • Y. Negishi, N. Watanabe, H. Shiromaru, Y. Achiba, N. Kobayashi
    Surface Review and Letters 3 1 661 - 665 1996年 [査読有り][通常論文]
     
    Ion-molecule reactions of silicon-carbon binary cluster ions SinC+m (n = 1-2, m = 1-2) with O2 molecules were studied by using an rf ion trap. The rate constants and branching ratios were determined for these cluster ions. For SiC+2, the rate constants for two isomers were obtained.
  • Y NEGISHI, A KIMURA, N KOBAYASHI, H SHIROMARU, Y ACHIBA, N WATANABE
    JOURNAL OF CHEMICAL PHYSICS 103 23 9963 - 9967 1995年12月 [査読有り][通常論文]
     
    Ion-molecule reactions of silicon-carbon binary cluster ions SiCn+ (n=1-6) and SiCn- (n=2-4) with O-2 molecules were studied by using an rf ion trap. For all selected positive and negative parent ions, only SiCnO+ or SiCnO- were produced, respectively. The rate constants for the oxidation of these clusters were determined for the first time. The size dependence of the reactivities are discussed, and the presence of the ring form positive ions for n=3 and 5 are suggested. (C) 1995 American Institute of Physics.
  • Y NEGISHI, A KIMURA, N KOBAYASHI, H SHIROMARU, Y ACHIBA, N WATANABE
    JOURNAL OF CHEMICAL PHYSICS 103 23 9963 - 9967 1995年12月 [査読有り][通常論文]
     
    Ion-molecule reactions of silicon-carbon binary cluster ions SiCn+ (n=1-6) and SiCn- (n=2-4) with O-2 molecules were studied by using an rf ion trap. For all selected positive and negative parent ions, only SiCnO+ or SiCnO- were produced, respectively. The rate constants for the oxidation of these clusters were determined for the first time. The size dependence of the reactivities are discussed, and the presence of the ring form positive ions for n=3 and 5 are suggested. (C) 1995 American Institute of Physics.
  • N WATANABE, H SHIROMARU, Y NEGISHI, Y ACHIBA, N KOBAYASHI, Y KANEKO
    ZEITSCHRIFT FUR PHYSIK D-ATOMS MOLECULES AND CLUSTERS 26 1 S252 - S254 1993年05月 [査読有り][通常論文]
     
    Reactions of carbon cluster ions with O2 were studied by using an RF ion trap in which cluster ions of specific size produced by laser ablation could be stored selectively. Reaction rate constants for positive and negative carbon cluster ions were estimated. In the case of the positive cluster ions, these were consistent with the previous experimental results using FTMS. Negative carbon cluster ions C(n)- (n=4-8) were much less reactive than positive cluster ions. The C(n)O- products were seen only in n=4 and 6.
  • N WATANABE, H SHIROMARU, Y NEGISHI, Y ACHIBA, N KOBAYASHI, Y KANEKO
    CHEMICAL PHYSICS LETTERS 207 4-6 493 - 497 1993年05月 [査読有り][通常論文]
     
    The reactions of C(n)Sc+ (n = 2-6) with O2 have been studied by using an rf ion trap. C(n)Sc+ was produced by the laser ablation of a mixed sample of graphite and scandium. Observed reaction product ions were ScO+ for all C(n)Sc+, and ScO2+ only for C(n)Sc+ with odd n. In the case of C6Sc+, the loss of 2C neutrals, leading to C4Sc+ production, was also observed. The estimated rate constants of C(n)SC+ + O2 reactions were found to alternate in magnitude with the cluster size; C(n)Sc+ clusters with even n have rate constants much lower than those with odd n.
  • N WATANABE, H SHIROMARU, Y NEGISHI, Y ACHIBA, N KOBAYASHI, Y KANEKO
    CHEMICAL PHYSICS LETTERS 207 4-6 493 - 497 1993年05月 [査読有り][通常論文]
     
    The reactions of C(n)Sc+ (n = 2-6) with O2 have been studied by using an rf ion trap. C(n)Sc+ was produced by the laser ablation of a mixed sample of graphite and scandium. Observed reaction product ions were ScO+ for all C(n)Sc+, and ScO2+ only for C(n)Sc+ with odd n. In the case of C6Sc+, the loss of 2C neutrals, leading to C4Sc+ production, was also observed. The estimated rate constants of C(n)SC+ + O2 reactions were found to alternate in magnitude with the cluster size; C(n)Sc+ clusters with even n have rate constants much lower than those with odd n.
  • N WATANABE, H SHIROMARU, Y NEGISHI, Y ACHIBA, N KOBAYASHI, Y KANEKO
    ZEITSCHRIFT FUR PHYSIK D-ATOMS MOLECULES AND CLUSTERS 26 1 S252 - S254 1993年05月 [査読有り][通常論文]
     
    Reactions of carbon cluster ions with O2 were studied by using an RF ion trap in which cluster ions of specific size produced by laser ablation could be stored selectively. Reaction rate constants for positive and negative carbon cluster ions were estimated. In the case of the positive cluster ions, these were consistent with the previous experimental results using FTMS. Negative carbon cluster ions C(n)- (n=4-8) were much less reactive than positive cluster ions. The C(n)O- products were seen only in n=4 and 6.
  • N WATANABE, H SHIROMARU, N KURIHARA, Y ACHIBA, N KOBAYASHI, Y KANEKO, J YODA
    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS 69 2-3 385 - 388 1992年06月 [査読有り][通常論文]
     
    Positive and negative carbon cluster ions produced by laser ablation have been simultaneously stored in an RF ion trap. The size distributions of the stored cluster ions, which were measured by means of ion dumping followed by time-of-flight mass analysis, are consistent with that reported previously.
  • N WATANABE, H SHIROMARU, N KURIHARA, Y ACHIBA, N KOBAYASHI, Y KANEKO, J YODA
    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS 69 2-3 385 - 388 1992年06月 [査読有り][通常論文]
     
    Positive and negative carbon cluster ions produced by laser ablation have been simultaneously stored in an RF ion trap. The size distributions of the stored cluster ions, which were measured by means of ion dumping followed by time-of-flight mass analysis, are consistent with that reported previously.

講演・口頭発表等

  • Experimental approach to fast ortho to para nuclear spin conversion of hydrogen molecule on dust grain analogues  [招待講演]
    Naoki Watanabe
    COSPAR 2021, Chemical Evolution and Origin of Life 口頭発表(招待・特別)
  • Clarifying a proton-hole transfer in ice by the new method of OH detection  [招待講演]
    Naoki Watanabe
    Frontier of structures and dynamics of water by advanced spectroscopic techniques 2020年10月 口頭発表(招待・特別)
  • Behavior of OH radical on ice  [招待講演]
    Naoki Watanabe
    Water in the Universe in the ACS National Meeting 口頭発表(招待・特別)
  • Ortho-to-Para Ratios in water and H2 desorbed from ice --Experimental view--  [招待講演]
    Naoki Watanabe
    Astrochemistry: From nanometers to megaparsecs - A symposium in honour of John H. Black 口頭発表(招待・特別)
  • Behavior of OH Radical on an Ice Surface at Low Temperatures  [招待講演]
    Naoki Watanabe
    35th Symposium on Chemical Kinetics and Dynamics 口頭発表(招待・特別)
  • Detection of OH radicals on amorphous solid water  [招待講演]
    Naoki Watanabe
    IAU Symposium 350 Laboratory Astrophysics: From Observations to Interpretation 口頭発表(招待・特別)
  • Infrared measurements on efficient chemical desorption of hydrogen sulfide from amorphous solid water  [招待講演]
    Naoki Watanabe
    Oxygen in Space 口頭発表(招待・特別)
  • 星間塵表面の化学:重水素濃集に果たす役割  [招待講演]
    渡部 直樹
    日本地球化学会年会 2018年09月 口頭発表(招待・特別)
  • Measurements of activation energies for diffusion of hydrogen atom on pure CO solid  [招待講演]
    N. Watanabe
    European Conference On Surface Science (ECOSS34) 2018年08月 口頭発表(招待・特別)
  • Infrared Measurements of Efficient Chemical Desorption of Hydrogen Sulfide from Amorphous Solid Water  [招待講演]
    N. Watanabe
    COSPAR 2018, The Evolving Chemical Universe: from proto-stars to the Origin of Life 2018年07月 口頭発表(招待・特別)
  • Surface Chemistry of the ISM: Experimental Approach to grain surface processes  [招待講演]
    N. Watanabe
    The Olympian Symposium 2018 Gas and stars from milli- to mega- parsecs 2018年05月 口頭発表(招待・特別)
  • Strong temperature dependence of H2 nuclear spin conversion on ice: what controls the rate?  [招待講演]
    Naoki Watanabe
    Dust and Ice Particles Spectroscopy and Scattering 口頭発表(招待・特別)
  • H 2ortho-para conversion on amorphous solid water  [招待講演]
    N. Watanabe
    International Conference on Photonic Electronic and Atomic Collisions(ICPEAC2017) 2017年07月 口頭発表(招待・特別)
  • 低温氷表面における水素の化学物理過程:宇宙の分子進化の鍵  [招待講演]
    渡部 直樹
    日本真空学会関西支部&日本表面科学会関西支部合同セミナー2017水素の挙動と物質科学 -最近の展開- 2017年07月 口頭発表(招待・特別)
  • Ortho-to-para nuclear spin conversion of H2 on ice: a key to chemical evolution in space  [招待講演]
    Naoki Watanabe
    XXXVI BIENNAIAL MEETING OF THE SPANISh ROYAL SOCIETY OF CHEMISTRY 2017年06月 口頭発表(招待・特別)
  • Physicochemical processes on Ice Dust towards Deuterium Enrichment”,  [招待講演]
    N. Watanabe
    International Symposium on Molecular Spectroscopy 2017年06月 口頭発表(招待・特別)
  • Physicohemical Processes on ice dust towards deuterium enrichment  [招待講演]
    N.Watanabe
    International Symposium on Molecular Spectroscopy 72ND MEETING 2017年06月 口頭発表(招待・特別)
  • Ortho-to-para ratios of hydrogen molecules desorbed from ice at around 10 K: What happens on cosmic ice dust?  [招待講演]
    N. Watanabe, H. Ueta, T. Hama, A. Kouchi
    The 253thAmerican Chemical Society National Meeting 2017年04月 口頭発表(招待・特別)
  • Strong temperature dependence of ortho-to-para conversion of H2 on amorphous solid water at around 10 K  [招待講演]
    N. Watanabe
    EUROPEAN CONFERNCE ON LABORATORY ASTROPHYSICS (ECLA2016)GAS ON THE ROCKS 2016年11月 口頭発表(一般) Madrid, Spain
  • Experimental approach to hydrogen chemistry on grain surfaces: molecular formation  [招待講演]
    N. Watanabe
    Center for Astrochemical Studies Seminar 2016年10月 口頭発表(招待・特別) Max-Planck-Institut für extraterrestrische Physik, Germany
  • Experimental approach to ortho-to-para ratio of hydrogen and water molecules desorbed from ice at around 10 K  [招待講演]
    Naoki Watanabe
    Workshop on Astrochemistry in Star and Planet Formation 2016年02月 口頭発表(招待・特別) Riken, Saitama, Japan
  • Deuterium chemistry and nuclear spin conversion on grain surfaces: implication to deuterium enrichment  [招待講演]
    N. Watanabe
    From clouds to protoplanetary disk: the astrochemical link 2015年10月 口頭発表(招待・特別) Hans Harnack Haus, Berlin, Germany
  • Grain Surface Chemistry: What Happens Without Photons and Ions  [招待講演]
    N. Watanabe
    A symposium to honor Lou Allamandola’s Contributions to the Molecular Universe 2015年09月 口頭発表(招待・特別) Annapolis, MD, USA
  • Role of Tunneling in the Formation and Deuterium Enrichment of Molecules on Dust Grains  [招待講演]
    Naoki Watanabe
    Astrobiology Science Conference 2015 口頭発表(招待・特別)
  • Experimental Approach to Chemistry of Cosmic Dust  [招待講演]
    N. Watanabe
    COST Action Our Astrochemical History CM1401 2015年05月 口頭発表(招待・特別) The Czech Republic
  • Experimental approach to nonenergetic physicochemical processes on icy grains  [招待講演]
    Naoki Watanabe
    Icy Grain Chemistry for Formation of Complex Organic Molecules: From Molecular Clouds to Protoplanetary Disks, Comets and Meteorites 2015年03月 口頭発表(招待・特別) Tokyo Institute of Technology, Japan
  • What controls the diffusion mechanism of hydrogen atom on ice?  [招待講演]
    Naoki Watanabe
    Second Workshop on Experimental Laboratory Astrophysics 口頭発表(招待・特別)
  • Nuclear spin temperatures of hydrogen and water molecules formed and trapped on ice  [招待講演]
    Naoki Watanabe
    Nuclear Spin Effects in Astrochemistry 2014 口頭発表(招待・特別)
  • Diffusion of hydrogen atom on amorphous solid water: Thermal or tunneling?  [招待講演]
    N. Watanabe
    247th ACS Chemistry & Materials for Energy 2014年03月 口頭発表(招待・特別) Dallas, USA
  • Physics and chemistry of hydrogen on cosmic dust: diffusion, spin temperatures, and water formation  [招待講演]
    N. Watanabe, T. Hama, H. Hidaka, Y. Kimura, A. Kouchi, Y. Oba, V. Pirronello
    First Workshop on Experimental Laboratory Astrophysics 2013年02月 口頭発表(招待・特別) Kauai, Hawaii
  • Physicochemical Processes of Hydrogen on Ice: A Key for Chemical Evolution in Space  [招待講演]
    N. Watanabe
    2nd International Symposium on Hierarchy and Holism 2013年02月 口頭発表(招待・特別) Tokyo, Japan
  • Surface diffusion mechanism of atomic hydrogen and the ortho/para ratio of nascent H2 molecule on amorphous solid water at around 10 K  [招待講演]
    N. Watanabe
    International Lorentz Center Workshop: Dynamical Phenomena at Surfaces: The Role of Complexity 2012年11月 Leiden, The Netherlands
  • Grain surface processes at very low temperatures: hydrogen diffusion and tunneling reactions  [招待講演]
    N. Watanabe
    Gordon Research Conference, Radiation Driven Processes in Physics, Chemistry, Biology, and Industry 2012年07月 New Hampshire, USA
  • Spin temperatures of hydrogen and water molecules on amorphous solid water  [招待講演]
    N.Watanabe
    ASTROCHEM 2012 2012年07月 Kolkata, India
  • Hydrogen processes on dust grains: H atom diffusion and water formation  [招待講演]
    N.Watanabe
    30th COSPAR Scientific Assembly 2012年07月 Mysore, India
  • Hydrogen chemistry on cold grain surfaces  [招待講演]
    N.Watanabe
    XVIIIth Symposium on Atomic, Cluster, and Surface Physics 2012年01月 Alpe d'Huez, France
  • Hydrogen tunneling reaction and its isotope effect at a low temperature ice surface: Origin of deuterium enrichments in interstellar formaldehyde and methanol  [招待講演]
    N. Watanabe, H. Hidaka, A. Kouchi, N. Watanabe, H. Hidaka, A. Kouchi
    International Conference on the Research of Chemistry and Physics of Matrix Isolated Species(MATRIX2011) 2011年07月 Vancouver, Canada
  • Ice surface chemistry of hydrogen atom in space  [招待講演]
    N. Watanabe, H. Hidaka, T. Hama, A. Kouchi, T. Chigai, Y. Kimura
    27th European Conference on Surface Science, ECOSS 27 2010年08月 Groningen, the Netherlands
  • Surface processes of atomic hydrogen at very low temperatures relevant to astrochemistry  [招待講演]
    N. Watanabe
    24th International Conference on Atomic Collisions in Solids ICACS- 24 2010年07月 Krak?w, Poland
  • Chemistry of atomic hydrogen on interstellar ice grains at very low temperatures  [招待講演]
    N. Watanabe, H. Hidaka, A. Kouchi, T. Hama, Y. Kimura
    Western Pacific Geophysics Meeting 2010年06月 Taipei, Taiwan
  • Physicochemical processes of atomic hydrogen on amorphous solid water at very low temperatures  [招待講演]
    N. Watanabe, A. Kouchi, H. Hidaka, Y. Kimura, T. Hama
    239th American Chemical Society National Meeting & Exposition 2010年03月 San Francisco, USA
  • Ice surface reactions: its role in chemical evolution in space  [招待講演]
    N.Watanabe
    ILTS International Symposium "Frontier of Low Temperature Science", Sapporo 2009年11月 Sapporo
  • Tunneling reactions of atomic hydrogen and molecules on amorphous solid water at very low temperatures  [招待講演]
    N. Watanabe
    Horiba-ISSP International Symposium on “Hydrogen and water in condensed matter physics” 2009年10月 Chiba
  • Nonenergetic reactions between hydrogen and molecules On Interstellar Grain Surfaces  [招待講演]
    N. Watanabe
    XXVI International Conference on Photonic, Electronic, and Atomic Collisions 2009年07月 Michigan, USA
  • Reactions of hydrogen atoms and molecules on amorphous solid water at very low temperatures  [招待講演]
    N. Watanabe
    Gordon Research Conferences, Physics & Chemistry of Matrix-Isolated Species 2009年07月 Oxford, UK
  • Experimental approach to grain surface reactions of hydrogen and deuterium atoms  [招待講演]
    N.Watanabe
    International Lorentz Center Workshop: Interstellar Surfaces: From Laboratory to Models 2008年10月 Leiden, Holland
  • Grain surface chemistry: fractionation routes  [招待講演]
    N. Watanabe
    The Molecular Univers: an International Meeting on Physics and Chemistry of the Intersteller Medium 2008年05月 Arcachon, France
  • Simultaneous irradiation experiments of H2O-CO binary ice with UV photons and cold H atoms  [招待講演]
    N. Watanabe, A. Nagaoka, O. MouriI, A. Kouchi, V. Pirronello
    Asia-Oceania Geosciences Society Annual Meeting 2007年08月 Bangkok, Thailand
  • Laboratory study of chemical processes on interstellar ice grains  [招待講演]
    N. Watanabe, A. Nagaoka, H. Hidaka, A. Kouchi
    XXIV International Union of Feodesy and Geophysics (IUGG) General Assembly 2007年07月 Perguia, Italy
  • Reactions of hydrogen and deuterium atoms with CO, formaldehyde, and methanol on ice grains  [招待講演]
    N.Watanabe
    International Astrophysics and Astrochemistry Meeting: Molecules in Space & Laboratory 2007年05月 Paris, France
  • Formation of simple organic molecules by Hydrogenation on grain surfaces  [招待講演]
    N. Watanabe
    International Astronomical Union Symposium: Astrochemistry, 2005年08月 California,USA
  • Formation of deuterated methanol by surface reactions at 10 K  [招待講演]
    N. Watanabe, A. Nagaoka, A. Kouchi
    Asia-Oceania Geosciences Society Annual Meeting 2005年06月 Singapore
  • Evolution of CO molecule on dusts in dense core: formation of H2CO and CH3OH  [招待講演]
    N. Watanabe
    The 4th International Symposium on New Trends of Physics, Recent Advances in Astrophysics and Planetary Science -from the early universe to the Solar system- 2005年03月 Sapporo

その他活動・業績

受賞

  • 2020年04月 令和2年度 科学技術分野の文部科学大臣表彰 科学技術賞 研究部門

共同研究・競争的資金等の研究課題

  • 日本学術振興会:科学研究費助成事業 特別推進研究
    研究期間 : 2017年04月 -2022年03月 
    代表者 : 渡部 直樹, 大場 康弘, 杉本 敏樹, 中井 陽一
     
    超高真空実験槽中に氷星間塵表面を擬似したアモルファス氷表面を作製し,紫外線照射で生成したのOHラジカルの表面拡散の活性化エネルギーを求めることに,世界で初めて成功した(渡部,宮崎,羽馬)。また,OHラジカルの吸着状態と脱離メカニズムを明らかにした。これらの成果には,新たに雇用した研究補助者(WMC Sameera)の量子化学計算によるところが大きい。この結果を発表すべく,論文の執筆を始めた。 星間塵表面物質のひとつである炭素質表面を,グラファイトディスクのレーザーアブレーションにより真空実験槽内でその場作製し,その構造と作製条件の依存性を,透過型電子顕微鏡を用いて調べ,いつでも最適な試料表面を作製するノウハウを確立した(柘植,木村)。 極低温超高真空原子間力顕微鏡の特性を調べた。観測する基板が9~10Kの極低温に到達すること,それを維持出来る熱負荷の確認を行った。極低温下でシリコン結晶を観測することができ基本的な運転を習熟した。(日高,香内)。 氷表面上に分子イオンが吸着した際の生成物を検出するイオンピックアップ装置の立ち上げを行った。冷凍基板および赤外分光計,イオン源を超高真空槽に設置し,極低温氷の作製など予備実験を行った(渡部,大場,中井) 杉本は京都大学から分子科学研究所への異動に伴い,装置の移設・再構築を行った。また,分光測定中の振動ノイズを軽減するため,希釈冷凍機から液体ヘリウム貯め込み式の冷却方式への転換作業を進めた。氷表面に対する和周波発生振動分光を行ったところ,表面特有の水素結合構造を有する液晶相が熱力学的な安定相として氷表面に存在することを発見した。
  • 星間分子雲初期に起こるケイ酸塩星間塵表面での分子進化
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2017年04月 -2018年03月 
    代表者 : 渡部 直樹, 大場 康弘
     
    本研究は,未解明になっている,星間塵表面に氷マントルが生成される以前の,ケイ酸塩星間塵表面での分子生成,重水素濃集に着目し,化学・物理素過程を系統的かつ定量的な実験で調べ,星間塵上での分子進化の解明を目指すものである。採択と同時に,研究の根幹をなす,新たなケイ酸塩試料作製装置開発に向け,真空ポンプ,真空部品などを購入した。また,表面を原子間力顕微鏡で評価するため,カンチレバーチップを購入した。同時に,プラズマ放電による鉱物サンプル試料の作製を業者に依頼した。ケイ酸塩試料作製装置の開発は引き続き行っているところだが,業者に作製を依頼したサンプル試料がかなり高品質であることが分かった。 また,採択直後に東京理科大学にて,本研究課題についての研究討論を行い,本課題の独創性や重要性をアピールすることができた。 その後,すぐに現在実施中の特別推進研究が採択されたため,本研究課題の実施は7月28日をもって終了した。しかし,上記の取り組みは特別推進研究に引き継がれるため,基盤研究Aで行った討論,整備した装置類は将来にわたって有効に活用される。
  • 日本学術振興会:科学研究費助成事業 基盤研究(S)
    研究期間 : 2012年05月 -2017年03月 
    代表者 : 渡部 直樹, 日高 宏, 大場 康弘, 羽馬 哲也, 香内 晃
     
    星間分子雲における分子生成・進化・同位体分別(化学進化)を明らかにすることは、太陽系の物質起源を研究するうえで重要である。本研究では、化学進化の鍵を握る、極低温下における氷星間塵表面でのトンネル反応による分子生成・重水素濃集プロセスを定量的な実験で調べた。氷表面での水素原子拡散係数と拡散メカニズム、水分子などの基本分子や始原的有機分子の生成・重水素濃集の反応経路やと化学進化に果たす役割を明らかにした。
  • 日本学術振興会:科学研究費助成事業 挑戦的萌芽研究
    研究期間 : 2014年04月 -2016年03月 
    代表者 : 渡部 直樹, 植田 寛和
     
    宇宙空間に存在する固体微粒子は星間物質の進化にきわめて重要な役割を果たす.固体微粒子の形成過程の詳細は未解明だが,一般に中性ガスの冷却過程で,気相中で形成されたと信じられている.本研究は従来考えられてこなかった,イオンを核とする微粒子形成過程(イオン誘起微粒子核生成)に実験的に迫る試みである.イオンを真空中の微小領域に長時間閉じ込めることができるイオントラップ装置を製作し,装置の評価を行うと共に,イオン誘起微粒子核生成の初期過程であるクラスターイオン生成を調べた.
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2013年10月 -2016年03月 
    代表者 : 香内 晃, 日高 宏, 大場 康弘, 羽馬 哲也, 千貝 健, 渡部 直樹, 木村 勇気, 都丸 隆行
     
    氷の構造(ミクロ~マクロ)を解析するための超高真空極低温透過型電子顕微鏡を開発した.鏡体内で制御された条件での氷の作製が可能であり,氷に紫外線を照射することもできる. 本装置を用いて以下の研究を行った.(1)アモルファス氷のミクロな欠陥構造を初めて直接観察した.(2)氷Ih,氷Icへ75-100Kで紫外線を照射することにより,氷XIが生成されることを発見した.一方,アモルファス氷では相転移は観察されなかった.(3)CO:H2O=10:1-50:1を10Kで蒸着しその後温度を上昇させCOを昇華させると,高密度アモルファス氷が生成されることを見出した.この方法をマトリックス昇華法と命名した.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2013年04月 -2016年03月 
    代表者 : 中井 陽一, 渡部 直樹, 小島 隆夫, 日高 宏
     
    イオン選別入射型移動菅を用いて、H3O+(H2O)nイオンの水分子逐次付加反応に関するギブスの自由エネルギー変化を従来の測定に比べて安定で精度良く測定できた。さらに、水蒸気中でのH3O+(H2O)nイオンの水分子逐次付加/脱離反応平衡に対して、反応平衡定数の経験的な有効温度を得ることができた。有効温度は電場の影響を受けた衝突エネルギーを代表する物理量である。その電場依存性を表すパラメータにH3O+(H2O)nイオンの構造が反映される可能性を見出した。RFイオントラップをクラスターイオン生成実験に利用するために、トラップを用いたイオン質量選別などのイオン操作法の試験研究を行った。
  • 日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2010年04月 -2013年03月 
    代表者 : 中井 陽一, 渡部 直樹, 小島 隆夫, 日高 宏
     
    イオンが誘発する微粒子核生成の初期過程であるクラスターイオン生成について実験的研究を行った。反応経路を制限するために、クラスターイオン生成の種イオンの生成および選別を行う領域とクラスターイオンを生成・成長させる領域を分離した実験装置を開発した。H3O+(H2O)nクラスターイオンについて、結合水分子数が1個変化する際のギブスの自由エネルギー変化の測定を行い、我々の手法は従来の測定にくらべて安定な精度の良い測定結果を得られることがわかった。NO+イオンを種イオンとする場合、水素分子を緩衝ガスにして、種イオンに含まれる準安定状態のNO+イオンが及ぼす影響を大きく減少させた実験を行えるようになった。
  • 星間塵表面反応:分子進化と同位体分別機構の新展開
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2012年 -2012年 
    代表者 : 渡部 直樹, 日高 宏, 羽馬 哲也
     
    本研究は,極低温下での星間塵表面反応素過程の詳細を実験的に調べ,星間分子雲での分子進化と同位体分別における星間塵の役割を明らかにすることを主たる目的としている.本研究課題が採択されると同時に,始原天体に関する国際会議(新潟),原始太陽系における鉱物・水・有機物に関する国際シンポジウム(東京)および日本地球惑星科学連合大会(千葉幕張)に出席し,星間塵表面における水分子および始原有機分子生成に関する研究の現状報告を行ない,同位体濃集プロセスに関する情報収集・議論を通して本研究の重要性や独創性を充分アピールすることができた. 現有実験装置の改良も本研究課題の重要な目的である。まず気相の原子・分子を検出する装置を駆動するための電源ユニット(NIM標準ビン電源,クリアパス社製)を購入し,信号検出系の改善に取り組み始めた.改良は現時点でも引き続き行っているところだが,これにより,実験の際の信号/ノイズ比が向上し,さらに規則的に現れる電気的なノイズを除去することが可能になると予想され、実験値の精度向上等大きな効果が期待される. その後すぐに現在実施中の基盤研究Sが採択されたため,本研究課題の実施は5月31日をもって終了した.しかし,上記の取り組みは基盤研究Sに引き継がれるため,基盤研究Aで行った発表・研究議論,整備した物品は、将来にわたって有効に活用される.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2010年 -2012年 
    代表者 : 香内 晃, 渡部 直樹, 日高 宏
     
    超高真空原子間力顕微鏡の試料冷却装置ならびに水蒸気を導入するためのガス導入系を作製した.これにより,低温の金属基板上にアモルファス氷を蒸着法で作製し,表面構造を原子分解能で観察できるシステムが完成した.蒸着法で作製したアモルファス氷の表面構造の直接観察に初めて成功し,アモルファス氷が非常にポーラスであることが分かり,触媒的効果を与える一因になっていることが明らかになった.
  • 日本学術振興会:科学研究費助成事業 新学術領域研究(研究課題提案型)
    研究期間 : 2009年 -2011年 
    代表者 : 渡部 直樹, 日高 宏
     
    光などの外からのエネルギーを必要としない氷表面反応,特に原子トンネル反応素過程とその同位体効果を実験的に調べ,以下の知見を得た. (1)極低温アモルファス氷上での水素原子表面拡散の活性化エネルギー, (2)氷表面でH-H再結合により生成した水素分子の核スピン温度, (3)ホルムアルデヒドの原子トンネル反応による重水素濃集過程, (4)OHラジカルとCO表面反応によるCO_2生成過程.
  • 日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2009年 -2011年 
    代表者 : 渡部 直樹, 日高 宏
     
    星間分子雲における水分子生成およびその重水素濃集過程の鍵を握る星間塵表面反応を実験的に調べ,以下のような知見を得た.(1)OHラジカルとH_2分子のトンネル表面反応により,分子雲環境でもH_2Oが生成し得ることを初めて明らかにした.(2)上記反応の同位体効果を調べたところ,重水素濃集には水酸基がD化(OD)していることが本質的で,その後D化した水素分子(HD, D_2)が反応したとしても,それ以上の重水素濃集は起こりにくいことを明らかにした.
  • 日本学術振興会:科学研究費助成事業 基盤研究(S)
    研究期間 : 2005年 -2009年 
    代表者 : 香内 晃, 渡部 直樹
     
    星間分子雲における水素原子の係わる表面原子反応を新たに開発した実験装置を用いて包括的に調べ,水素分子,水,二酸化炭素,ホルムアルデヒド,メタノールなど重要な星間分子の生成機構を解明した.水素原子のアモルファス氷への付着係数の測定に成功し,反応速度定数を求めることが可能になった.種々の反応において,アモルファス氷が存在する時には高温での反応性が著しく増すことを定量的に示した.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2007年 -2008年 
    代表者 : 渡部 直樹, 日高 宏
     
    星間分子の重水素濃集プロセスとして研究代表者らが提唱している極低温星間塵表面反応を, 実験により定量的に評価した. 特に, 高度な重水素濃集が観測で確認されているホルムアルデヒド, メタノールの重水素化表面反応に焦点をあて, 重要な反応経路について実効的な反応速度定数を決定し, 上記分子種に関する重水素濃集の星間塵表面反応ネットワークを完成させた.
  • 日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2004年 -2008年 
    代表者 : 山本 哲生, 墻内 千尋, 荒川 政彦, 渡邊 誠一郎, 渡部 直樹
     
    惑星系円盤におけるダストの衝突進化と熱進化の素過程,観測結果を読み解くうえで重要な光学に関する研究,ダスト生成とその後続解析実験,ダスト衝突実験,氷表面における分子反応等,物質進化の総合的研究を展開した.加えて,この分野の研究基盤形成にも貢献した.研究グループの交流を促進し,国内の関連研究グループの組織化を図り,研究コミュニティー形成を積極的に推進した
  • 星間塵表面における重水素濃縮機構
    日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2004年 -2005年 
    代表者 : 渡部 直樹, 香内 晃
     
    本研究では,これまで全く情報の無かった星間塵表面における星間分子の重水素濃集過程を実験的に調べることを目的としている.本年度は以下のことを明らかにした. ホルムアルデヒド(H_2CO)における重水素濃集過程を調べるために,10KのH_2O氷(疑似星間塵表面)上でCO+DおよびH_2CO+D,D_2CO+Hの表面反応を定性的に調べた.その結果,すべての反応で重水素(D)原子を含む重水素体ホルムアルデヒドが生成された.実験条件から推測すると,これらの反応はすべて宇宙空間で起こりうることが解った.大雑把な見積もりでは重水素体を生成するのにもっとも速い反応はH_2CO+Dの系に見られるH-D置換反応である.しかし,一度重水素体が生成してもH原子との反応でD原子が再びH原子に入れ替わる反応(D_2CO+H→・・→H_2CO)も見られたことから,ホルムアルデヒドにおけるH-D置換反応が結果として重水素濃集過程になりうるかどうかを見極めるにはより定量的な実験が必要であることがわかった.
  • 星間塵表面における極低温原子反応と重水素濃集機構
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2004年 -2005年 
    代表者 : 渡部 直樹, 香内 晃
     
    星間塵表面における化学反応は,H_2,H_2Oや星間有機分子等の生成・進化に不可欠である.本研究の目的は,分子雲の極低温領域で特に重要な星間塵原子表面反応を定量的に実験で調べ,さらに原子表面反応による星間分子の重水素濃集過程を解明することである.期間内に明らかにした主要な事柄を以下にまとめる. (1)固体COにD原子が逐次付加する反応を調べた.D原子の付加速度はH原子の1/10以下となり,付加反応によっては重水素濃集が起こり得ないことが分かった. (2)固体CH_3OHとD原子が反応し,HとDが置換される反応について.CH_3OHにD原子を照射した時の赤外線吸収スペクトルの変化を調べたところ,CH_3OHの減少にともない,D体(CH_3OH-d_n:CH_2DOH,CHD_2OH,CD_3OH)が生成することが分かった.一方,CH_3OH-d_nにH原子を照射しても全く変化は見られなかった.つまり,一度重水素化したCH_3OH-d_nは安定で,もとに戻らないことが分かった.これらの結果は表面におけるH-Dの置換が重水素濃集機構になりうることを示した初めての証拠で,これまで議論されたことのない新しい濃集機構である. (3)星間塵表面組成として考えられるCO,H_2CO,H_2O固体表面におけるCO分子への水素原子付加反応(生成物としてH_2CO,CH_3OHが得られる)の反応速度の違いを様々な温度で実験的に調べた.その結果,CO固体表面では15K付近,H_2O表面では20K付近で反応速度が急激に落ち,H_2CO表面では20Kにおいてもそれほど反応は遅くならなかった.これらの結果は水素原子の各表面に対する吸着係数の温度依存性を反映していると考えられ,20K付近ではCO,H_2O,H_2COの順で吸着係数が高くなることを示唆している.このことは星間塵上における化学進化を考える際は,単に反応する原子・分子だけではなく,反応場を形成する表面分子も重要になることを示している.
  • 星間雲での光化学反応によるダイヤモンドの生成機構
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2003年 -2004年 
    代表者 : 香内 晃, 荒川 政彦, 渡部 直樹
     
    隕石中のプレソーラーダイヤモンドおよび星間雲で観測されているダイヤモンドの起源を解明するために,星間雲で氷に紫外線が照射されて有機物ができる過程,およびその有機物がさらに低密度雲で紫外線を照射される過程を再現する実験をおこなった.超高真空容器中の金属板をヘリウム冷凍機で12Kに冷却する.H20:CO:NH3:CH4=4:2:2:1に混合したガスを真空容器中に導入し,アモルファス氷の薄膜を作った.氷薄膜に紫外線を10Kで照射し,照射終了後,金属板の温度を室温まで上昇させた.アモルファス氷は200Kまでに蒸発し,室温でも蒸発しない有機物が残った.その有機物が低密度雲でさらに紫外線照射を受ける過程を再現する実験も行った. 生成された有機物の高分解能透過型電子顕微鏡観察の結果,分子雲で生成された有機物中に1nm程度のダイヤモンド微結晶(ダイヤモンド前駆体)が生成されたことが明らかになった.これはまったく新しいダイヤモンドの生成法であり,これまでの常識を覆す発見である.さらに,低密度雲でのさらなる紫外線照射によりダイヤモンドが5nm程度まで成長することがわかった. 以上の結果から,隕石中のいわゆるプレソーラーダイヤモンドは,炭素星や超新星起源ではなく,星間雲起源だと考えるとこれまで問題になっている事を無理なく説明できることが明らかになった.
  • 静電場を用いた氷微粒子加速装置の開発
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2001年 -2003年 
    代表者 : 荒川 政彦, 渡部 直樹, 前野 紀一
     
    広い速度範囲で氷微粒子及びその集合体を加速することができる静電加速器の開発を行なった。飛翔用の試料となる氷微粒子は凝縮法により作成した。これは水蒸気に飽和した-10℃の低温空気中に液体窒素温度の銅版を入れて,水蒸気を凝縮させる方法である.凝縮した氷微粒子はほぼ球形をしており,直径は約10μmである.微粒子集合体は、この凝縮場中に100V/cmを超える電場を与えることにより作成した。この氷微粒子集合体は電荷をもっているため,ある程度サイズが大きくなると電極から分離し静電加速されるようになる.この時の電位と速度の関係は150Vから3kVの範囲で次のように表された。V_i=0.25(V_
      /d)^<0.9>(m/s),ここでV_i飛翔速度,V_
        電位差,d電極間距離(d=2.5mm)である. 大気中では空気抵抗による減速が働くため氷微粒子加速装置に真空チャンバーを取り付け、減圧下での微粒子加速効率を調べた。また、加速部を伸長することにより、微粒子集合体にかかる加速度を減らし、加速中に構造が壊れないように工夫した。実験の結果、大気圧(1bar)下では1kV/mmの電場で、サイズ約500μmの氷微粒子集合体を20m/sまで破砕することなく飛ばすことに成功した。しかしながら、目標とする1km/sを超えるためには二桁大きな速度が必要である。0.1気圧に減圧した中で氷微粒子集合体を加速した結果、加速時間が0.5msec程度に長くなるとともに終端速度が3〜4倍速くなることがわかった。この結果を外押すると1kV/mmの電場で100m/sを超える速度が可能になると思われる。
  • 原始太陽系星雲での有機物粒子の衝突付着成長機構
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2000年 -2001年 
    代表者 : 香内 晃, 渡邊 誠一郎, 渡部 直樹, 荒川 政彦
     
    本研究では、鉱物の微粒子表面に存在する有機物が粒子の成長過程に与える影響を実験的に評価し、それをもとに原始太陽系での有機物粒子の成長過程を議論することが目的である。2年間を通して以下のような研究を行なった。 直径1cmの銅球を銅板上の試薬を用いて調整したモデル始源有機物の層に200〜500Kの温度で付着が起きる臨界速度の温度依存性を調べた。 付着臨界速度h温度が下がるにつれて増加し、250K付近で最大5m/sに達する。さらに温度が下がると、付着臨界速度は減少する。したがって、mmサイズの有機質星間塵は、氷や鉱物より数桁もおおきな数m/sの衝突速度でも付着すると結論できる。この結果から、原始太陽系星雲で有機物が星間塵表面に存在した領域では、星雲が乱流状態になっている時でも、星間塵やそれらの集合体の成長が急速に進んだと結論された。以上の結果をもとに小惑星の起源を議論した。 小惑星の特徴は次のようにまとめられる:(a)現在、小惑星領域に存在する固体物質非常に少ない(b)通常の惑星に比べて小さな天体が多数ある。(a)の「固体物質が非常に少ない」のは、次の理由による。有機物の付着効果によって小惑星は、星雲が乱流状態にあるにもかかわらず、急速に成長した。小惑星に取り込まれなかった1mサイズの粒子集合体は、ガス抵抗によって急速に内側に落下した。それで、2-3.5AUでは小惑星の材料物質が急速に減ってしまった。したがって、小惑星領域に固体物質が非常に少ないのは「なくなった=内側に落下した」ためである。 (b)の「小さいものが多数」あるのは「大きな天体が壊れた」のではなく、上の理由(材料物質がなくなった)で「大きくなれずに成長が止まった」ためである。
  • 微小空間に閉じ込めた微粒子の成長観測と赤外吸収測定
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 1999年 -2000年 
    代表者 : 荒川 政彦, 渡部 直樹, 香内 晃, 渡部 直樹, 荒川 政彦
     
    本研究の目的は,星間空間における星間塵の成長過程、およびその光学特性を実験的に明らかにすることにある.最終的にはサブミクロンサイズ(0.1〜1μm)の微粒子(鉱物、有機物、氷)を真空中の微小空間に長時間閉じ込め、それらが衝突付着により時間とともにどう成長するかをその場観察し、可能であれば赤外線吸収による分析を行う.本年度は研究遂行に必要な閉じ込め装置の開発を完了し,さらに有機物微粒子の付着力を見積った.具体的には以下のとおりである. 本研究を進める上で,もっとも重要な「微粒子閉じ込め装置」である,RF型イオントラップ装置を高真空槽中に構築し,分子イオンの数時間に及ぶ閉じ込めに成功した.装置の特徴として,イオントラップ電極には内部の観測に必要な直径5mmの穴を4つあけ,下方電極にはイオン化に必要な電子銃を設置した.イオン分子による予備実験後,微粒子を閉じ込めるのに必要な実験条件を整えるべく,高周波電場のインピーダンス整合装置の開発を行い,イオントラップの電極を改良した. 有機物微粒子の閉じ込め実験の前段階として,噴霧器を用いてエチレングリコール微粒子を作製し,200K付近における微粒子の付着力の測定を,流動法を用いて行った.その結果,鉱物微粒子のそれよりも数桁大きな値であることが分った.
  • 彗星核再現実験装置の開発
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 1998年 -1999年 
    代表者 : 香内 晃, 渡部 直樹, 荒川 政彦
     
    彗星の進化を明らかにするために,実際の彗星を構成している氷微粒子と同じ大きさ,同じ化学組成のアモルファス氷微粒子を用いた彗星核再現実験装置を開発した. (1)これまでのアモルファス氷微粒子の作製法と全く異なる新しい考えによるアモルファス氷微粒子の作製法を考案・実用化した: (i)10Kで厚いCO薄膜を蒸着し,その上に薄いアモルファス氷薄膜を蒸着する.蒸着終了後,基板の温度を上げると,COの蒸発温度付近でアモルファス氷薄膜の破壊が起こり,アモルファス氷微粒子が生成される. (ii)CO_2とH20の混合ガスを10Kで同時に基板に蒸着する.蒸着速度が大きい時に,薄膜の連続的な破壊が蒸着中でも進行し,CO_2とH_2Oを含むアモルファス氷微粒子が連続的に生成されることを見いだした. (2)以上の方法を取り入れた「彗星核再現実験装置」を開発し,純粋なアモルファス氷微粒子または不純物を含むアモルファス氷微粒子を作製・回収し,大量に堆積させることに成功した.堆積したアモルファス氷微粒子表面に紫外線レーザーを照射することによって,彗星核からのジェットの再現に成功した.この装置では,さらに,高速度ビデオカメラを用いて,模擬彗星核表面から飛び出してくる氷微粒子の運動の解析が可能になった. (3)完成した「彗星核再現実験装置」を用いて,シミュレーション実験を行った.模擬彗星核の熱伝導率,破壊強度の測定に成功した.
  • 不純物を含むアモルファス氷星間塵の構造と物性
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 1997年 -1999年 
    代表者 : 香内 晃, 渡部 直樹, 荒川 政彦
     
    開発した超高真空極低温原子間力顕微鏡および既存の実験装置を駆使して,アモルファス氷微粒子の生成・変成と彗星の起源・進化について以下の研究を遂行した. 1)アモルファス氷薄膜に,暗黒星雲と同じ条件(フラックス)の紫外線を照射して,水素分子の生成される反応を定量的に解析した.さらに,反応断面積を測定し,反応機構を決定した. 2)不純物を含むアモルファス氷の物性測定(熱伝導率および示差熱分析)を行った.熱伝導率は欠陥を含まないアモルファス氷より,2-3桁小さな値となった.示差熱分析の結果,不純物を含まないアモルファス氷では結晶化は発熱反応であるのに対して,不純物を含むアモルファス氷では吸熱反応となった.これはアモルファス氷の欠陥に含まれていた不純物分子が,アモルファス氷の結晶化に際して蒸発し,蒸発熱を奪うためである. 3)今回新たに研究を行ったアモルファス氷への紫外線照射による水素分子の生成反応や熱物性の測定の結果はアモルファス氷の欠陥構造(ミクロ〜マクロ)によっている事が明らかになった. 4)以上の結果をもとに,彗星核の熱史を計算し,不純物を含むアモルファス氷(実際の彗星に近い)では,これまで考えられていたような暴走的な結晶化は起こり得ないことが明らかになった.
  • アモルファス氷星間塵上での水素分子生成機構
    日本学術振興会:科学研究費助成事業 奨励研究(A)
    研究期間 : 1997年 -1998年 
    代表者 : 渡部 直樹
     
    本研究の目的は、アモルファス氷星間塵上での水素分子生成機構の解明である。そのうち、期間内にアモルファス氷への紫外線照射による水素分子生成を定性的、かつ定量的に調べ、その生成メカニズムを明らかにした。具体的には以下の通りである。 1. 紫外線照射による水素分子生成の確認:10K基板に蒸着法により生成されたアモルファス重水氷に126nm、172nmの紫外線を照射し、昇温脱離法により光反応生成物を分析した。その結果、重水素分子の生成が確認された。 2. 重水素分子の生成メカニズム:生成量の波長依存性、光量依存性を調べたところ、重水素分子は、重水分子の第一電子励起状態を経由した、1光子過程の光反応により生成されることが強く示唆された。 3. 重水素分子生成量の定量化:氷厚、紫外線照射時間を変えて生成量の変化を調べた。生成量は50〜150Åの範囲で膜厚に比例し、照射時間40分付近で飽和を始め、120分(星間空間10^6yに相当)で生成量は重水分子の数%になった。この照射時間の範囲では逆反応は観測されなかったことから、生成反応は氷固体内部よりも、むしろ氷内部の裂け目や孔の表面で起きているものと考えられる。以上のことから、紫外線による水素分子生成は星間空間で支配的とは言えないまでも無視できない過程であり、星間空間で観測された「氷内水素分子」の生成過程として有力な生成反応であることが分かった。
  • 原子・イオン・分子の光電離過程の研究
  • アモルファス氷表面上における原子・分子素過程
  • Study on Atomic,ionic,and Molecular Photoionization processes
  • Atomic and Molecular Processes on Amorphous ice surface

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