Self-consistent Monte Carlo simulations of the electron and ion distributions of inhomogeneous liquid alkali metals. I. Longitudinal and transverse density distributions in the liquid–vapor interface of a one-component system

Abstract

Previous Monte Carlo simulations of the liquid–vapor interfaces of simple metals, which predict stratification of the liquid–vapor transition zone extending three atomic diameters into the bulk, suffer from a limitation arising from the use of a local electroneutrality approximation. This approximation is embodied in the assumption that, for the purpose of calculating the change in potential energy accompanying a change in configuration, there is exact coincidence of the electronic and ionic density profiles. In this paper we describe a generalized Monte Carlo simulation which avoids the local electroneutrality approximation by directly incorporating the Lang–Kohn treatment of the jellium-vacuum interface into the simulations. We report the results of generalized Monte Carlo simulations of slabs of sodium at 100 and 200 °C and cesium at 100 °C. The new simulations predict structure in the liquid–vapor interface very similar to that predicted using the local-electroneutrality approximation. The geometry of the slabs used in the new simulations allows us to make more careful studies of the transverse pair correlation functions in the interface than does the geometry of the clusters used in previous simulations of simple alkali metals. We find that in the peaks of the oscillations of the longitudinal density profile, the transverse pair correlation function resembles that of a fluid which is less dense than that of the corresponding homogeneous liquid, except for a slight amplification of the height of the innermost peak. Thus the longitudinal density oscillations predicted by the model do not lead to transverse structure factors resembling those of a higher density liquid, as had previously been expected. Despite the stratification, outer layers of the interface are not packed in a crystalline lattice, as is indicated by the isotropy of the transverse structure factor.