Surface structure of liquid metals and the effect of capillary waves: X-ray studies on liquid indium

We report x-ray reflectivity (XR) and small-angle off-specular diffuse-scat tering (DS) measurements from the surface of liquid indium close to its mel ting point of 156 degrees C. From the XR measurements we extract the surfac e structure factor convolved with fluctuations in the height of the liquid surface. We present a model to describe DS that takes into account the surf ace structure factor, thermally excited capillary waves, and the experiment al resolution. The experimentally determined DS follows this model with no adjustable parameters, allowing the surface structure factor to be deconvol ved from the thermally excited height fluctuations. The resulting local ele ctron-density profile displays exponentially decaying surface-induced layer ing similar to that previously reported for Ga and Hg. We compare the detai ls of the local electron-density profiles of liquid In, which is a nearly f ree-electron metal, and liquid Ga, which is considerably more covalent and shows directional bonding in the melt. The oscillatory density profiles hav e comparable amplitudes in both metals, but surface layering decays over a length scale of 3.5+/-0.6 Angstrom for In and 5.5+/-0.4 Angstrom for Ga. Up on controlled exposure to oxygen, no oxide monolayer is formed on the liqui d In surface, unlike the passivating film formed on liquid gallium. [S0163- 1829(99)11701-6].