Two-dimensional freezing in the liquid-vapor interface of a dilute Pb : Gaalloy

We report the results of x-ray reflectivity and grazing incidence x-ray dif fraction studies of the liquid-vapor interface of a dilute alloy of Ph in G a over the temperature range of 23-76 degrees C, Our data show that the liq uid-vapor interface of this alloy is stratified for several atomic diameter s into the bulk liquid and that a monolayer of Pb forms the outermost strat um of the interface. Over the temperature range of 23-56 degrees C, the mon olayer 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 rema ins stable to 76 degrees C. An analogy between the observed transition and the first-order melting transition in a one-component classical plasma is s uggested.