  The ultraviolet (UV) photodissociation of amorphous water ice at different
ice temperatures is investigated using molecular dynamics (MD) simulations and
analytical potentials. Previous MD calculations of UV photodissociation of
amorphous and crystalline water ice at 10 K [S. Andersson et al., J. Chem.
Phys. 124, 064715 (2006)] revealed -for both types of ice- that H atom, OH, and
H2O desorption are the most important processes after photoexcitation in the
uppermost layers of the ice. Water desorption takes place either by direct
desorption of recombined water, or when, after dissociation, an H atom
transfers part of its kinetic energy to one of the surrounding water molecules
which is thereby kicked out from the ice. We present results of MD simulations
of UV photodissociation of amorphous ice at 10, 20, 30, and 90 K in order to
analyze the effect of ice temperature on UV photodissociation processes.
Desorption and trapping probabilities are calculated for photoexcitation of H2O
in the top four monolayers and the main conclusions are in agreement with the
10 K results: desorption dominates in the top layers, while trapping occurs
deeper in the ice. The hydrogen atom photodesorption probability does not
depend on ice temperature, but OH and H2O photodesorption probabilities tend to
increase slightly (~30%) with ice temperature. We have compared the total
photodesorption probability (OH+H2O) with the experimental total
photodesorption yield, and in both cases the probabilities rise smoothly with
ice temperature. The experimental yield is on average 3.8 times larger than our
theoretical results, which can be explained by the different time scales
studied and the approximations in our model.
