Ab initio study of intrinsic profiles of liquid metals and their reflectivity

Abstract

Abstract. The free surfaces of liquid metals are known to exhibit a strati ed pro le that, in favourable cases, shows up in experiments as a peak in the ratio between the re ectivity function and that of an ideal step-like pro le. This peak is located at a wave-vector related to the distance between the layers of the pro le. In fact the surface roughness produced by thermally induced capillary waves causes a depletion of the previous so called intrinsic re ectivity by a damping factor that may hinder the observation of the peak. The behaviour of the intrinsic re ectivity below the layering peak is however far from being universal, with systems as Ga or In where the re ectiviy falls uniformly towards the q ! 0 value, others like Sn or Bi where a shoulder appears at intermediate wavevectors, and others like Hg which show a minimum. We have performed extensive ab initio simulations of the free liquid surfaces of Bi, Pb and Hg, that yield direct information on the structure of the pro les and found that the macroscopic capillary wave theory usually employed in order to remove the capillary wave components fails badly in some cases for the typical sample sizes a ordable in ab initio simulations. However, a microscopic method for the determination of the intrinsic pro le is shown to be succesful in obtaining meaningful intrinsic pro les and corresponding re ectivities which reproduce correctly the qualitative behaviour observed experimentally.