Microscopic Surface Structure of Liquid Alkali Metals

We report an x-ray scattering study of the microscopic structure of the surface of a liquid alkali metal. The bulk liquid structure factor of the eutectic ${\mathrm{K}}_{67}{\mathrm{Na}}_{33}$ alloy is characteristic of an ideal mixture, and so shares the properties of an elemental liquid alkali metal. Analysis of off-specular diffuse scattering and specular x-ray reflectivity shows that the surface roughness of the K-Na alloy follows simple capillary wave behavior with a surface structure factor indicative of surface-induced layering. Comparison of the low-angle tail of the ${\mathrm{K}}_{67}{\mathrm{Na}}_{33}$ surface structure factor with the one measured for liquid Ga and In previously suggests that layering is less pronounced in alkali metals. Controlled exposure of the liquid to ${\mathrm{H}}_{2}$ and ${\mathrm{O}}_{2}$ gas does not affect the surface structure, indicating that oxide and hydride are not stable at the liquid surface under these experimental conditions.