TRANSLATIONAL AND ROTATIONAL ENERGY MEASUREMENTS OF PHOTODESORBED WATER MOLECULES IN THEIR VIBRATIONAL GROUND STATE FROM AMORPHOUS SOLID WATER

For interstellar grains coated with water ice, the most important desorption mechanism at the edge of molecular clouds is photodesorption of water. To reveal details of the photodesorption mechanism, we have measured the translational and rotational energies of H2O (v = 0) molecules photodesorbed from amorphous solid water and polycrystalline ice following excitation within the first absorption band using a 157 nm laser. The measured translational and rotational temperatures are 1800 K and 300 K, respectively. These energies are in good accord with those predicted by classical molecular dynamics calculations for the "kick-out" of an H2O molecule in the ice by an energetic H atom. The statistical ortho: para ratio of g(OPR) = 3 is appropriate for the Boltzmann rotational distribution of the H2O molecules.