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

Abstract

Molecular dynamics simulations of ice Ih 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 ∼2 ps and a thermal relaxation time of ∼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.