Molecular Dynamics study of ice-vapor interactions via the quasi-liquid layer

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, alb...