  Ultraviolet irradiation of ice is of great interest for understanding the
chemistry in both atmospheric and astrophysical environments. In interstellar
space, photodissociation of H2O molecules can be a driving force behind the
chemistry on icy dust grains in dense, cold molecular clouds even though the
flux of UV photons is extremely low. The mechanisms of such photoinduced
processes are poorly understood, however. In this work the photodissociation
dynamics of a water molecule in crystalline ice at 10 K is studied
computationally using classical molecular dynamics. Photodissociation in the
first bilayer leads mainly to H atoms desorbing (65%), while in the third
bilayer trapping of H and OH dominates (51%). The kinetic energy distribution
of the desorbing H atoms is much broader than that for the corresponding
gas-phase photodissociation. The H atoms on average move 11 Angstroms before
becoming trapped, while OH radicals typically move 2 Angstroms. In accordance
with experiments a blueshift of the absorption spectrum is obtained relative to
gas-phase water.
