The translational and rotational energy of hydrogen molecules after recombination on interstellar grains

Trajectories based on classical mechanics are computed for recombining H2 molecules on rigid surfaces. Monte Carlo sampling of initial locations and velocities is utilized to obtain the average translational energy per molecule formed and the average rotational state of the molecules that escape. Various values for the adsorption energy are considered. In the limit of pure physical adsorption, the average translational energy is about 0.2 eV and the average rotational state is greater than J = 7. Since transfer of energy into the surface is not included, the calculations are expected to yield upper limits to the actual energies after recombination on interstellar dust grains. For adsorption energies regarded as likely for hydrogen on interstellar grains, the calculations indicate that heating of the gas in interstellar clouds by energy from H2 recombination is less significant than might be supposed. The calculations do support the proposal that recombination contributes to the observed abundances of H2 molecules in the states where J is at least about 4.