Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Experimental results on the formation of molecular hydrogen on amorphoussilicate surfaces are presented for the first time and analyzed using a rateequation model. The energy barriers for the relevant diffusion and desorptionprocesses are obtained. They turn out to be significantly higher than thoseobtained earlier for polycrystalline silicates, demonstrating the importance ofgrain morphology. Using these barriers we evaluate the efficiency of molecularhydrogen formation on amorphous silicate grains under interstellar conditions.It is found that unlike polycrystalline silicates, amorphous silicate grainsare efficient catalysts of H$_{2}$ formation within a temperature range whichis relevant to diffuse interstellar clouds. The results also indicate that thehydrogen molecules are thermalized with the surface and desorb with low kineticenergy. Thus, they are unlikely to occupy highly excited states.Comment: 5 pages, 3 figures, 1 table. Accepted to ApJL. Shortened a bi