Anomalous abundance ratios of nuclear-spin isomers of water induced by para hydrogen

2020 Fiscal Year Final Research Report

• Project/Area Number: 18H01262
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 17010:Space and planetary sciences-related
• Research Institution: The University of Tokyo (2019-2020); Hokkaido University (2018)
• Principal Investigator: Hama Tetsuya 東京大学, 大学院総合文化研究科, 准教授 (20579172)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 核スピン温度 / パラ水素 / 星間化学 / 彗星 / 原始惑星系円盤 / 赤外多角入射分解分光法 / 分子配向

Outline of Final Research Achievements

We measure the ortho-to-para ratios (OPRs) of water (H2O) photodesorbed from ice made from para-H2O monomers at 11 K. The photodesorbed H2O molecules from the ice have the statistical OPR value of three. This result clearly shows 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. These 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. In addition, we developed an experimental setup for in situ infrared multiple-angle incidence resolution spectrometry in low temperature, ultrahigh-vacuum conditions. The intensity ratio between the in-plane and out-of-plane vibration spectra of amorphous N2O at 6 K reveals an average molecular orientation angle of 65° from the surface normal.

Free Research Field

宇宙惑星科学

Academic Significance and Societal Importance of the Research Achievements

本研究によって，氷内でH2Oのオルソ-パラ転換は速やかにおきていることが初めて実験的に明らかになった．そのため「H2O のオルソ／パラ比は過去にH2Oが星間塵で氷として生成したときから変わらず，星間塵の温度を反映している」という仮説は間違いであることが明確となった．そのため，これまでのH2Oのオルソ／パラ比の観測研究の結果はすべて再解釈する必要がある．さらに本研究で開発した多角入射分解赤外分光法の実験装置によって，これまで不明であった星間塵を覆うアモルファス氷の構造を分子レベルで解明できる可能性が新たに開かれたことは特筆すべき成果である．