Brillouin scattering study of salol: exploring the effects of rotation-translation coupling.

Brillouin scattering in liquids composed of optically and mechanically anisotropic molecules is affected by coupling between rotational and translational dynamics. While this effect has been extensively studied in depolarized (VH) scattering where it produces the “Rytov dip,” recent theoretical analyses by Pick, Franosch et al. show that it should also produce observable effects in polarized (VV) scattering [R. M. Pick et al., Eur. Phys. J. B 31, 217 (2003); 31, 229 (2003)]. To test this theory, we carried out Brillouin scattering studies of the molecular glassformer salol in the temperature range 210‐ 380 K, including VH-backscattering, VH-90°, and VV-90° spectra. The data were analyzed consistently to determine the effects of rotation-translation coupling on both the polarized and depolarized spectra. A previously unanticipated feature predicted by these authors was observed: a narrow negative region in the q-dependent part of the 90° VV spectra, which we designate as the “VV dip.” It is an analog of the Rytov dip observed at high temperatures in the 90° VH spectra, which is also accurately described by this theory. Analysis of the 90° VV spectra was carried out both with and without inclusion of translation-rotation coupling in order to determine quantitatively the role this coupling plays.