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 ${\mathrm{q}}_{\mathrm{z}}$∼2.3–2.5 ${\mathrm{Å}}^{\mathrm{-}1}$. These results are an unambiguous indication of atomic layering with an interlayer spacing d∼2π/${\mathrm{q}}_{\mathrm{z}}$=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 (<0.5 mm deep), which leads to a macroscopically curved surface due to the large surface tensions in liquid metals. The experimental challenges posed by measurements on curved surfaces and the techniques that were developed are discussed in detail.