Neutron scattering study of thermally excited density fluctuations in a dense classical fluid

Collective density fluctuations in 36Ar gas at 456 atm pressure and 295 K have been studied by thermal-neutron scattering using time-of-flight techniques. Well-defined collective modes are observed for wave-vector transfers k < or approx. = 0.1 A-1 which become strongly damped at larger k. The data have been compared in detail with predictions of the generalized Enskog equation. Although extrapolation of the predictions to k = 0 gives the correct transport coefficients to better than 5% accuracy, the generalized Enskog equation does not properly predict the wave-vector dependence of the scattering law