Brillouin scattering in glass-forming liquids: Q-dependent linewidths and the generalized viscosity

Recently, Gomperts, Variyar, and Kivelson [J. Chem. Phys. 98, 31 (1993)] analyzed Brillouin linewidth data for triphenylphosphite and found a striking wave vector dependence which varied strongly with temperature. We present a simple explanation based on a Debye relaxation model from which results similar to theirs are obtained even though the longitudinal viscosity has no intrinsic wave vector dependence. We also explore the quantitative differences between their results and those obtained with the Debye model, and show that these differences can be explained by the two-step structural realaxation dynamics predicted by mode coupling theory and observed in recent neutron- and light-scattering experiments.