Intrinsic structure of the free liquid surface of an alkali metal

An operational procedure to obtain the intrinsic properties of free liquid surfaces, and which had been previously applied to computer simulations of dielectric liquid models, is used here to sample the intrinsic surface properties of liquid lithium, sodium, and potassium, with an ab initio orbital free description of the electrons. When properly scaled, the intrinsic profiles of the three alkali fluids turn to be virtually identical. As in simple fluid models these intrinsic density profiles show a strong layering structure, similar to that of the bulk liquid pair distribution function, but with specific surface aspects like the density of the first liquid layer, and the interlayering spacing. The capillary wave fluctuations of the surface partially damp the intrinsic layering and produce density profiles which depend strongly on the transverse sampled area, but which still show an oscillating structure. The assumptions of the capillary wave theory, which is used in the interpretation of x-ray reflectivity data, may be directly checked, and a direct comparison with the experimental data presented, with very good agreement between our results and the measured electronic surface structure factor for potassium.