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 dif fraction studies of the liquid-vapor interfaces of three dilute lead-in-gal lium alloys (0.16, 0.037, and 0.025 wt %) over the temperature range 23-76 degreesC. These experiments determine, respectively, the density distributi ons along the normal to the interface and the in-plane pair correlation fun ctions. In this temperature range, at each of the alloy concentrations, the excess Pb in the interface forms a complete monolayer that is the outermos t 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 crystal line state are a = 0.342 nm and b = 0.592 nm, with a and b the basis vector s of a degenerate two-dimensional body-centered orthorhombic lattice. At ab out 58 degreesC the Pb monolayer undergoes a transition to a state with les s order, evidence for which is the appearance of broadened diffraction peak s; the locations of the broadened and sharp diffraction peaks are the same. The nearly discontinuous change in the range of positional order at the tr ansition temperature, coupled with the lack of change of the positions of t he diffraction peaks, suggest that this transition is first order with eith er very small or zero density change. Our data cannot determine if the diso rdered phase is liquid or hexatic. An analogy between the character of the observed transition and the first-order melting transition in a one-compone nt two-dimensional classical plasma is suggested.