New Phase-Transition Phenomena in Thin Argon Films

A density-functional theory capable of predicting free energies and spatially dependent number densities of simple classical fluids is applied to the case of argon films in the presence of a solid carbon dioxide substrate. For pressures less than the saturated vapor pressure, a new phase transition for which the order parameter is the film thickness is described. The phases are the usual unsaturated film and a one- or two-atomic-layer structure localized at the argon-C${\mathrm{O}}_2$ interface.