Product energy distribution of molecular hydrogen formed on icy mantles of interstellar dust

The formation pumping mechanism of H(2) molecules formed on icy mantles of interstellar dust was 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, H + H --> H(2), was simulated on the ice surface at 10 and 70 K, where two MD procedures were employed. Method A: H(2)O molecules were treated as rigid (hard ice model). Method B: intramolecular vibrational modes of H(2)O were taken into; account (soft ice model). A numerical energy analysis was performed, and the product energy distribution was obtained for H(2). It has become clear that H(2) molecules formed on the amorphous wafer ice are in highly excited states not only vibrationally, but also rotationally and translationally. The vibrational energy levels with large populations are, respectively, nu = 6-10 and 6-10 for 10 and 70 K hard ice systems and nu = 6-9 and 5-9 for 10 and 70 K soft ice systems. The average vibrational energies correspond to nu = 8-9 and nu = 7-8 for the hard ice and the soft ice, respectively. The evaluated rotational and translational temperatures were 5500-6000 and 4000-5000 K, respectively, for the hard ice, whereas they were 6500-8000 and 5500-6500 K, respectively, for the soft ice. The largest portion of the H(2) formation energy resided in the vibrational energy of H(2) (70%-79%), and the second and third largest portions were the rotational (10%-15%) and translational energies (7%-12%), respectively. The energy absorbed by the ice was evaluated to be only about 4-5 kcal mol(-1) (3%-5% of the H(2) formation energy, 109.5 kcal mol(-1)). The present results suggest that the H(2) vibrational emission might be detectable in regions without a source of UV pumping or dynamical excitation.