Two-dimensional freezing in the liquid-vapor interface of a dilute Pb:Ga alloy.

We report the results of x-ray reflectivity and grazing incidence x-ray diffraction studies of the liquid-vapor interface of a dilute alloy of Pb in Ga over the temperature range of 23-76 degrees C. Our data show that the liquid-vapor interface of this alloy is stratified for several atomic diameters into the bulk liquid and that a monolayer of Pb forms the outermost stratum of the interface. Over the temperature range of 23-56 degrees C, the monolayer of Pb is in an ordered hexagonal phase. At about 58 degrees C, this monolayer undergoes a first-order transition to a hexatic phase, which remains stable to 76 degrees C. An analogy between the observed transition and the first-order melting transition in a one-component classical plasma is suggested.