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 water ice are in highly excited states not only vibrationally, but also rotationally and translationally. The vibrational energy levels with large populations are, respectively, v = 6-10 and 6-10 for 10 and 70 K hard ice systems and v = 6-9 and 5-9 for 10 and 70 K soft ice systems. The average vibrational energies correspond to v = 8-9 and v = 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.