The metal-non-metal transition and the dynamic structure factor of expanded fluid alkali metals

This paper discusses the electrical and optical properties and, in particular the neutron scattering measurements of the dynamic structure factor in the metal-non-metal transition range. This transition occurs when the dense liquid evaporates to the dilute vapour or when the fluid is expanded by heating to its liquid-vapour critical point. The shape of S(Q, omega ) changes considerably on approaching the transition from the high-density liquid side, indicating a change in the interparticle interaction and the molecular structure. S(Q, omega ) of rubidium in the density range between the melting point density and three times the critical density is characterized by the existence of well defined acoustic-phonon-like collective density excitations at high momentum transfer, whereas S(Q, omega ) at a density of about twice the critical density is consistent with excitations of an optic-type mode in which two species tend to move in opposite directions.