  Absorption of ultraviolet radiation by water ice coating interstellar grains
can lead to dissociation and desorption of the ice molecules. These processes
are thought to be important in the gas-grain chemistry in molecular clouds and
protoplanetary disks, but very few quantitative studies exist. We compute the
photodesorption efficiencies of amorphous water ice and elucidate the
mechanisms by which desorption occurs. Classical molecular dynamics
calculations were performed for a compact amorphous ice surface at 10 K thought
to be representative of interstellar ice. Dissociation and desorption of H2O
molecules in the top six monolayers are considered following absorption into
the first excited electronic state with photons in the 1300-1500 Angstrom
range. The trajectories of the H and OH photofragments are followed until they
escape or become trapped in the ice. The probability for H2O desorption per
absorbed UV photon is 0.5-1% in the top three monolayers, then decreases to
0.03% in the next two monolayers, and is negligible deeper into the ice. The
main H2O removal mechanism in the top two monolayers is through separate
desorption of H and OH fragments. Removal of H2O molecules from the ice, either
as H2O itself or its products, has a total probability of 2-3% per absorbed UV
photon in the top two monolayers. In the third monolayer the probability is
about 1% and deeper into the ice the probability of photodesorption falling to
insignificant numbers. The probability of any removal of H2O per incident
photon is estimated to be 3.7x10^-4, with the probability for photodesorption
of intact H2O molecules being 1.4x10^-4 per incident photon. When no desorption
occurs, the H and OH products can travel up to 70 and 60 Angstroms inside or on
top of the surface during which they can react with other species.
