Molecular dynamics simulations of D2O ice photodesorption

Abstract

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D2O in an amorphous D 2O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H 2O, the main processes after UV photodissociation are trapping and desorption of either fragments or D2O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. 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 calculated for photoexcitation of D2O in the top four monolayers and are compared quantitatively with those for H 2O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa, J. Chem. Phys. 132, 184510 (2010)10.1063/1.3422213]. 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 ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D2O photodesorption probability is larger than that of H2O (by about a factor of 1.4-2.3 depending on ice temperature), 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 D 2O ice, because the D atom formed after D2O photodissociation has a larger momentum than photogenerated H atoms from H 2O, and D transfers momentum more easily to D2O than H to H2O. The total (OD D2O) yield has been compared with experiments and the total (OH H2O) 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. © 2011 American Institute of Physics.