Structure of the liquid-vapor interface of a dilute alloy of Pb in Ga

We report the results of x-ray reflectivity and grazing-incidence x-ray diffraction studies of the liquid-vapor interfaces of three dilute lead-in-gallium alloys (0.16, 0.037, and 0.025 wt %) over the temperature range 23-76 degreesC. These experiments determine, respectively, the density distributions along the normal to the interface and the in-plane pair correlation functions. In this temperature range, at each of the alloy concentrations, the excess Pb in the interface forms a complete monolayer that is the outermost stratum of the interface. When the temperature is below 58 degreesC that Pb monolayer is in a two-dimensional hexagonal crystalline state, evidence for which is the appearance of four sharp peaks in the grazing-incidence x-ray diffraction pattern. The structural parameters of the hexagonal crystalline state are a = 0.342 nm and b = 0.592 nm, with a and b the basis vectors of a degenerate two-dimensional body-centered orthorhombic lattice. At about 58 degreesC the Pb monolayer undergoes a transition to a state with less order, evidence for which is the appearance of broadened diffraction peaks; the locations of the broadened and sharp diffraction peaks are the same. The nearly discontinuous change in the range of positional order at the transition temperature, coupled with the lack of change of the positions of the diffraction peaks, suggest that this transition is first order with either very small or zero density change. Our data cannot determine if the disordered phase is liquid or hexatic. An analogy between the character of the observed transition and the first-order melting transition in a one-component two-dimensional classical plasma is suggested.