  We present results of classical dynamics calculations, performed to study the
photodissociation of water in crystalline and amorphous ice surfaces at a
surface temperature of 10 K. Dissociation in the top six monolayers is
considered. Desorption of H2O has a low probability (less than 0.5% yield per
absorbed photon) for both types of ice. The final outcome strongly depends on
the original position of the photodissociated molecule. For molecules in the
first bilayer of crystalline ice and the corresponding layers in amorphous ice,
desorption of H atoms dominates. In the second bilayer H atom desorption,
trapping of the H and OH fragments in the ice, and recombination of H and OH
are of roughly equal importance. Deeper into the ice H atom desorption becomes
less important and trapping and recombination dominate. The distribution of
distances traveled by H atoms in the ice peaks at 6 - 7 Angstroms with a tail
going to about 60 Angstroms for both types of ice. The mobility of OH radicals
is low within the ice with most probable distances traveled of 2 and 1
Angstroms for crystalline and amorphous ice, respectively. OH is however quite
mobile on top of the surface, where it has been found to travel more than 80
Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and
liquid water are found to be in very good agreement with experiments.
