Density Dependent Atomic Motion in a Liquid Alkali Metal

Inelastic x-ray and neutron scattering results obtained from liquid sodium and rubidium are presented. They cover the entire liquid range between melting and liquid vapour critical point. At high densities the dynamics of the liquid metal is characterized by collective excitations. The corresponding dispersion relations indicate the existence of surprisingly stable next neighbouring shells leading to an increase of the propagation speed for the collective modes. Below 2ρcrit. the dynamics changes from collective to localized indicating the existence of molecular aggregates. This interpretation is in accord with a simple model where the properties of a Rb- and a Rb2- lattice are calculated using density functional theory.