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

Collective density fluctuations in ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ 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 $\overrightarrow{\mathrm{k}}\lesssim 0.1$ ${\mathrm{\AA }}^{-1\mathrm{}}$ 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 $k=0\mathrm{}$ 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.