Collective Modes and Many Body Forces in Fluids: An Experimental Study

In recent years a number of high quality measurements, at different state parameters, of the dynamic structure factor have been made. These measurements justify two new interpretive approaches. In the first we deduce the magnitude and frequency dependence of the memory function of the current-current correlation function without performing the theoretically correct, but errorful, Fourier-Laplace double transform procedure. This can be achieved1,2 at a number of special points, and is essentially a direct measurement of this quantity in momentum-frequency space. For the first time the properties of collective modes in fluids can be discussed in a systematic way and this is done for dense krypton gas and for liquid argon. The same data on krypton and argon may be used to assess the role played by many body forces in dynamic properties because the true pair interaction potentials for these cases are accurately known. Thus in the second approach a high quality computer simulation for krypton, using the true pair potential, is compared to the experimental spectra over an appropriate range of momentum transfer3-4. Anomalous effects are observed over the same ranges as novel memory function effects were observed, and are related to the onset of collective mode behaviour.