Properties of collective modes in fluids: Experimental results.

Abstract

The memory function for the longitudinal current-current correlation function has been measured at low frequencies for fluid krypton, argon, and rubidium with use of the new methods previously described by us [Phys. Rev. A 31, 3802 (1985)]. The memory relaxation time is much shorter than the structural relaxation time for many wave numbers (Q) in the range observed, 0.4--4 A${\r{}}^{\mathrm{\ensuremath{-}}1}$. There is a marked variation of the memory function with Q, although its variation with frequency over the range studied, \ensuremath{\omega}3Q \ensuremath{\surd}kT/MS(Q) , is not large. The data for noble gases and an alkali metal are compared and the significance of differences considered. For the discussion of collective modes, the most significant quantity is the ratio of imaginary to real parts of the memory function which we have measured for the first time. Some evidence for propagating modes of high wave numbers is presented and their relationship to peaks in S(Q,\ensuremath{\omega}) and other properties is discussed.