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Astron. Astrophys. 330, 773-781 (1998)


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Sticking probability and mobility of a hydrogen atom
on icy mantle of dust grains

Koichi Masuda 1, Junko Takahashi 2 and Tadashi Mukai 1

1 The Graduate School of Science and Technology, Kobe University, Nada, Kobe 657, Japan
2 Institute for Fundamental Chemistry, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606, Japan

Received 18 April 1997 / Accepted 12 September 1997

Abstract

The sticking and the diffusion processes of a hydrogen atom on the surface of icy mantle of dust grains have been investigated based on the classical molecular dynamics (MD) simulation. As the model for the icy mantle, a slab-shaped amorphous water ice with infinite area was generated by MD simulation under the periodic boundary condition. It was found that the densities and the oxygen-oxygen distance radial distribution function of our resulting amorphous water ice at 10 K and 70 K were in good agreement with those of the experimental high- and low- density amorphous water ice, respectively. By using the potential energy field for an incident H atom on our amorphous water ice, the dynamical behaviors of an impinging H atom on its surface were examined. From a statistic study for about 60 simulations in each cases, we have deduced that the sticking probability [FORMULA] of an incident H atom with a kinetic temperature [FORMULA] of 10 K, 100 K, and 350 K on 10 K ice were 1.0, 0.98, and 0.53, respectively. On 70 K ice, the values of [FORMULA] became 0.98, 0.86, and 0.52 for [FORMULA] = 70 K, 100 K, and 350 K, respectively. Our simulations have revealed that the impinging H atoms in the sticking cases diffused on the surface of ice by thermal hopping, and then they were trapped in one of potential wells on the icy surface. It was found, furthermore, that the mobility of the H atom before it became trapped nearly depends on the ice temperature. No ejection of the trapped H atom thermally occurred at least in our simulations.

Key words: diffusion – dust – molecular processes – ISM: molecules

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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