Permalink : http://hdl.handle.net/2115/34705

水素分子は地球大気中間圏から宇宙まで広く存在している．これら水素分子の生成源は気相反応だけでなく，固体表面での化学反応も重要である．地球大気中間圏には極域中間圏雲と呼ばれる氷微粒子が存在し，宇宙空間においては星間塵と呼ばれる表面を氷層で覆われた粒子が存在する．氷表面上で 生成した水素分子の多くは振動回転励起した状態で気相に飛び出す．気相反応において，これらの励起水素分子は基底水素分子とは異なる役割をもつと考えられる．本稿では，氷表面上での光化学反応による水素分子生成機構について述べる．
The formation mechanisms of molecular hydrogen by ultraviolet photolysis of water ice and the importance of vibrationally excited H2 to atmospheric and astrophysical chemistry are reviewed. There are two important candidates for molecular hydrogen formation from the photodissociation of H2O + hv → H + OH, that is, H + H2O → H2 + OH (abstraction reaction) and H + H → H2 (recombination reaction). The abstraction reaction is 0.6 eV endothermic, and it can proceed for energetic H atoms above the threshold energy of 1 eV (c.m.). The recombination reaction can produce the H2 molecule on ice surface, and is 4.5 eV exothermic. Using quantum state-elective detection of the H2 photoproduct, two mechanisms were characterized by which molecular hydrogen is formed following 157 nm photodissociation of amorphous water ice at 100 K. Abstraction of a hydrogen atom from H2O by the photolytically produced H-atom is observed to lead to internally "cold" H2 products, whereas recombination of two hydrogen atoms is observed to produce internally "hot" H2. The formation of vibrationally excited H2 products following photodissociation of ice could thus lead to a significant acceleration of oxygen chemistry in the interstellar medium.