X-ray Reflectivity Studies of Liquid Metal and Alloy Surfaces

Surface-induced atomic layering at the liquid/vapor interface in liquid metals has been observed using x-ray reflectivity on sputtered clean surfaces under ultrahigh vacuum conditions. A well-defined quasi-Bragg peak is obtained for surfaces of elemental Ga and a Ga-In alloy at large wave vectors qz∼2.3–2.5 Å-1. These results are an unambiguous indication of atomic layering with an interlayer spacing d∼2π/qz=2.5–2.7 Å. For liquid Ga, the amplitude of the electron-density oscillations, which is significantly underestimated by existing theory and molecular simulation, decays with a characteristic length of 6 Å, which is twice that of Hg. Results on the alloy show a clear enrichment of indium at the topmost surface layer, consistent with the Gibbs adsorption rule. The enrichment consists of a single monolayer, with subsequent layers at the bulk eutectic composition. In order to suppress mechanically excited surface waves, the measurements were performed on thin liquid metal films (