X-Ray Reflectivity Measurements of Surface Layering in Liquid Mercury

The surface normal structure of the mercury liquid-vapor interface has been investigated by measuring the x-ray reflectivity out to a momentum transfer of ${q}_{z}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.5$ ${\mathrm{\AA{}}}^{--1}$. The results provide direct experimental proof of surface layering in liquid metals. The layer spacing is given by the atomic dimensions of the Hg atoms. The minimum layer width agrees well with the predictions of capillary wave theory; the layering amplitude decays into the bulk with a characteristic length of 3--3.5 \AA{}, which is close to the decay length of the bulk pair correlation function.