The fundamental processes of H2 formation via H + H → H2 on the surfaces of icy mantles of interstellar dust have been investigated consistently within a single model based on a classical molecular dynamics (MD) computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab was generated at 10 and 70 K under periodic boundary conditions. The first and second incident H atoms were then ‘thrown’ on to the model surface. Two MD procedures were employed: (i) the H2)O molecules were treated as rigid (hard ice model); (ii) the intramolecular vibrational modes of H2O were included (soft ice model). The amorphous water ice slabs produced by our MD simulations are found to be good models for the surfaces of icy mantles of dust grains. For the various fundamental processes of H2 formation on the dust surface, the following results emerge. (1) For the sticking of an H atom on to the surface, a sticking probability that depends on the temperature of the incident H atom is obtained. (2) For the diffusion of an H atom on the surface, it is found that the first incident H atom diffuses via the thermal hopping mechanism at the first stage, and then it is always trapped in one of the stable sites on the amorphous ice. The migration length and time have been calculated for the mobility of the incident H atom before it is trapped. The time‐scales of thermal diffusion and desorption of H atoms after trapping have also been estimated. (3) For the reaction of two H atoms on the surface, the following three reaction patterns are observed: (i) H2 is produced via the Langmuir–Hinshelwood mechanism; (ii) H2 is produced via the Eley–Rideal mechanism; (iii) the almost elastic collision of two H atoms occurs without H2 being formed. The effective reactive cross‐section is estimated at about 40 Å2. The reaction probabilities are found to be near unity. (4) For the ejection of H2 from the ice surface, the product H2 is subsequently ejected after the reaction process, using part of the excess energy derived from the H2 formation. The average lifetime of ejection is about 400–600 fs. Most of the ejected H2 molecules are found to be in vibrationally and rotationally excited states.
The formation pumping mechanism of molecules formed on icy mantles of interstellar dust was H 2 investigated theoretically based on a classical molecular dynamics (MD) computational simulation. The slab-shaped amorphous water ice was prepared at 10 and 70 K, as a realistic model surface for icy mantles of dust, and the formation process of molecular hydrogen, was simulated on the H ] H ] H 2 , ice surface at 10 and 70 K, where two MD procedures were employed. Method A :molecules were H 2 O treated as rigid (hard ice model). Method B : intramolecular vibrational modes of were taken into H 2 O account (soft ice model). A numerical energy analysis was performed, and the product energy distribution was obtained forIt has become clear that molecules formed on the amorphous water ice are in H 2 . H 2 highly excited states not only vibrationally, but also rotationally and translationally. The vibrational energy levels with large populations are, respectively, v \ 6È10 and 6È10 for 10 and 70 K hard ice systems and v \ 6È9 and 5È9 for 10 and 70 K soft ice systems. The average vibrational energies correspond to v \ 8È9 and v \ 7È8 for the hard ice and the soft ice, respectively. The evaluated rotational and translational temperatures were 5500È6000 and 4000È5000 K, respectively, for the hard ice, whereas they were 6500È8000 and 5500È6500 K, respectively, for the soft ice. The largest portion of the formation H 2 energy resided in the vibrational energy of (70%È79%), and the second and third largest portions H 2 were the rotational (10%È15%) and translational energies (7%È12%), respectively. The energy absorbed by the ice was evaluated to be only about 4È5 kcal mol~1 (3%È5% of the formation energy, 109.5 H 2 kcal mol~1). The present results suggest that the vibrational emission might be detectable in regions H 2 without a source of UV pumping or dynamical excitation.
The diffusion process of a hydrogen atom on the amorphous water ice Ž . surface was investigated under very low temperature conditions 10 and 70 K using both classical and quantum approaches. The model amorphous water ice slab was Ž . prepared by the classical molecular dynamics MD simulation under the two-dimensional periodic boundary condition with 1000 water molecules in a unit cell. For a H atom thrown onto the surface of the amorphous ice, the sticking and diffusion processes were studied. In the sticking case, the incident H atom initially diffused for 1᎐3 ps and then became trapped in one of the stable sites on the amorphous ice surface. To estimate the quantum mechanical diffusion constant, a new formalism was developed using the differential diffusion constant. A rate calculation for a H atom diffusing from one trapped site to another on the amorphous water ice was performed. The numerical value was compared with the hopping rate constant for the classical thermal diffusion, and a large quantum effect was found.
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