Light-Scattering Experiments and Generalized Transport Coefficients

We discuss the relation between the observed spectra in the Rayleigh—Brillouin light-scattering experiments for molecular fluids and the generalized transport coefficients. For a two-level molecular fluid undergoing thermal relaxation, we derive the generalized hydrodynamic equations for the conditions when translational motion is hydrodynamic. For two explicit examples of parahydrogen and sulphur hexafluoride, we numerically evaluate the wave-number- and frequency-dependent transport coefficients implied by the generalized hydrodynamic equations. We then critically examine the implication of these results for the phenomenological analysis of the polarized-light-scattering spectra, which is commonly employed by most workers in the field. We find that the most important quantity for the description of such experiments on intermediate-density fluids is a frequency- and wave-number-dependent bulk viscosity rather than a frequency-dependent thermal conductivity.