A MOLECULAR-DYNAMICS INVESTIGATION OF LONGITUDINAL COLLECTIVE MODES IN METAL SALT-SOLUTIONS

Results are reported of extensive molecular dynamics (MD) simulations of a simple model of the prototype metal-salt solution K(x)(KCl)1-x. T he Coulomb interactions between ions are assumed to be screened by the degenerate valence electrons, within the Thomas-Fermi approximation. The main emphasis is on the concentration (x) dependence of the densit y and charge fluctuation spectra. The MD data for these spectra are an alysed in terms of the extended mode concept, by fitting the data to s uperpositions of Lorentzians, with wavenumber-dependent coefficients. This analysis clearly shows the emergence, in addition to an extended acoustic (sound) mode, of a high-frequency optic mode at intermediate and low metal concentrations x. This mode may be considered as a remna nt of the well known propagating charge fluctuation (or plasmon) mode in molten salts; at finite x the optic mode is strongly affected by el ectron screening. The subtle interplay between acoustic and optic mode s is in qualitative agreement with the predictions of a recent general ized hydrodynamics analysis.