Temperature dependence of the collective atomic dynamics of liquid rubidium

The collective atomic dynamics of liquid Rb has been investigated at 393, 593 and 776 K using cold neutron inelastic scattering techniques. The dispersion curves of longitudinal collective density fluctuations have been determined for momentum transfers hQ with 9 less than or equal to Q less than or equal to 20 nm(-1) by fitting a damped oscillator model and of kinetic theory to the fully corrected dynamic structure factor, and also from the maxima in the longitudinal current-current correlation function J(1)(Q, omega). The resulting dispersion depends strongly on the choice of the dynamical variable and on the model used, even though the dispersion from the fit of the damped oscillator model and from the J(1)(Q, omega) agree fairly well. The dispersion obtained from the J(1)(Q, omega) measured at 393 K agrees very well with that determined in a computer experiment, and the dispersion from the fit of three Lorentzians to S(Q, omega) measured at 393 K continues to 20 nm(-1) the behaviour of liquid Rb measured by Copley and Rowe up to 10 nm(-1) al 320 K. In contrast to the results obtained for simple liquids with molecular binding forces, the dispersion in the liquid metal Rb does not show any dispersion gap near Q(1) the Q-value of the first peak of the static structure factor. The temperature dependence of the dispersion shows a cross-over from the region at lower Q-values where the pair potential dominates to a region at larger Q-values where free-particle dispersion is more dominant.