  Molecular dynamics calculations have been performed to study the ultraviolet
photodissociation of D2O in an amorphous D2O ice surface at 10-90 K, in order
to investigate the influence of isotope effects on the photodesorption
processes. As for H2O, the main processes after UV photodissociation are
trapping and desorption. There are three desorption processes: D atom, OD
radical, and D2O molecule photodesorption. D2O desorption takes places either
by direct desorption of a recombined D2O molecule, or when an energetic D atom
produced by photodissociation kicks a surrounding D2O molecule out of the
surface by transferring part of its momentum. Desorption probabilities are
compared quantitatively with those for H2O obtained from previous MD
simulations of UV photodissociation of amorphous water ice. The main
conclusions are the same, but the average D atom photodesorption probability is
smaller than that of the H atom (by about a factor of 0.9) because D has lower
kinetic energy than H, whereas the average OD radical photodesorption
probability is larger than that of OH (by about a factor of 2.5-2.9 depending
on Tice) because OD has higher translational energy than OH for every Tice
studied. The average D2O photodesorption probability is larger than that of H2O
(by about a factor of 1.4-2.3 depending on Tice), and this is entirely due to a
larger contribution of the D2O kick-out mechanism. This is an isotope effect:
the kick-out mechanism is more efficient for D2O ice, because the D atom formed
after D2O photodissociation has a larger momentum than photogenerated H atoms
from H2O, and D transfers momentum more easily to D2O than H to H2O. The total
yield has been compared with experiments and the total yield from previous
simulations. We find better agreement when we compare experimental yields with
calculated yields for D2O ice than when we compare with calculated yields for
H2O ice.
