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. 70% of incident molecules
adsorb to the outer sublayer, while 30% are accommodated directly to the inner
sublayer of the quasi-liquid layer, with an orientational relaxation time of ~2
picoseconds and a thermal relaxation time of ~10 picoseconds. 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.