Temperature and pressure dependence of the atomic dynamics of liquid rubidium

The dynamic structure factor, S(Q,omega), of liquid rubidium was measured b y inelastic neutron scattering at pressures of 0.86, 42, 83.5 and 125.1 N/m (2) and at temperatures of 328, 398 and 473 K on an isothermal, isobaric an d isochoric grid in phase space [1]. From the maxima omega(m) of the longit udinal current-current correlation functions, omega(2) S(Q,omega), and from a fit of a viscoelastic model to the dynamic structure factors, dispersion relations, omega(m) and omega(0), respectively, have been obtained. In con trast to previous results, the temperature dependence of these dispersions at fixed density is rather weak for momentum transfers, (h) over bar Q, aro und and smaller than Q(P), the Q at which the static structure factor, S(Q) , has its first maximum. A considerable temperature dependence of the colle ctive atomic dynamics is only observed for Q > 20 nm(-1), while the density dependence is largest near Q(P) and at smaller Qs. These effects demonstra te that we may possibly distinguish between the potential and the free part icle part of the collective atomic dynamics in a simple liquid. (C) 1999 El sevier Science B.V. All rights reserved.