Dynamical Correlations in Simple Liquids and Cold-Neutron Scattering by Argon

Abstract

Recent inelastic neutron-scattering measurements on liquid argon at 94.4°K are quantitatively analyzed in terms of velocity autocorrelation functions obtained from computer molecular-dynamics experiments and from a simple model which has no adjustable parameters. The model is derived using dispersion-relation and sum-rule arguments, and a single, operationally well-defined approximation. Physically it corresponds to treating the motions as if the atoms were moving in a relaxing cage. The model description is tested against computer results, and both are employed in the cross-section calculation, in which two recent convolution-type approximations are considered. Over-all agreement between present analysis and experiments is quite satisfactory. Moreover, detailed comparisons show that the results are not very sensitive to the differences in the model and computer velocity correlation functions, and that the observed discrepancies arise mainly from the existing methods of relating coherent and incoherent scattering functions.