Atomic-Scale Structure of a Liquid Metal-Insulator Interface

The structure of the liquid Hg/sapphire interface was measured with angstrom-scale resolution by high-energy X-ray reflectivity. The atomic Hg layering found at the interface is less pronounced than at the Hg/vapor interface, showing a twice-shorter decay length with depth, and a weaker peak/valley density contrast. We also find a near-interface, 8 +/- 3 angstrom thick layer, the density of which, although depth-varying, is enhanced, on average, by 10 +/- 5% relative to the bulk. The enhancement is assigned to a 0.13 +/- 0.05 e/atom charge transfer from the Hg to the substrate, somewhat less than theory. The unexplained anomalous temperature dependence previously reported for the mercury/vapor density profile is absent here, implying a nonstructural origin for the anomaly.