Molecular Dynamics Study of Ice-Vapor Interactions via the Quasi-Liquid Layer

Molecular dynamics simulations of ice I-h in a slab geometry with a free basal (0001) surface are carried out at 250 K in order to study the structure and dynamics of the ice/vapor interface, focusing on processes associated with sublimation and deposition. Surface melting, which results in the formation of a quasi-liquid layer, causes about 8% of the molecules originally constituting the surface bilayer to leave their crystal lattice positions and form an outer, highly mobile sublayer. Molecules in this sublayer typically form two H bonds, predominantly in a dangling-O orientation, with preference for a dangling-H orientation also evident. The remaining 92% of the quasi-liquid layer molecules belong to the deeper, more crystalline sublayer, typically forming three H bonds in an orientational distribution that closely resembles bulk crystalline ice. Transitions between the quasi-liquid layer and the first underlying crystalline bilayer were also observed on the molecular dynamics simulation time scale, albeit with substantially longer characteristic times. Regarding deposition, a very high (>99%) probability of water vapor molecules sticking to the ice surface was found. A total of 70% of incident molecules adsorb to the outer sublayer, whereas 30% are accommodated directly to the inner sublayer of the quasi-liquid layer, with an orientational relaxation time of similar to 2 ps and a thermal relaxation time of similar to 10 ps for molecules adsorbing to the outermost sublayer. Regarding the mechanism of sublimation, we found that prior to sublimation, departing molecules are predominantly located in the outermost sublayer and show a distinct preference for a dangling-O orientation.