Collective molecular motions in liquids from depolarized light scattering

Abstract

The basic hypotheses used by some of the different theories (viscoelastic and microscopic) that have been proposed to explain the lineshapes observed in quasi-elastic depolarized light scattering are summarized, and the results predicted by these theories compared and contrasted in different situations. These theoretical results are in turn compared with the experimental results obtained from different physical systems: liquids with low viscosity composed of relatively small and simple molecules; liquids composed of more cumbersome but rigid molecules; supercooled liquids; liquid crystals in the isotropic phase. The coupling parameters obtained from VH and HH spectra, i.e., the coupling between molecular orientation and transverse or longitudinal waves in the liquid are assessed, whereby it is shown that the transverse (shear) waves can be strongly propagative at low temperatures in a supercooled liquid but that as the viscosity is decreased these waves become progressively softer and for low viscosities they have a purely diffusive character. Although most theories are adequate to explain the diffusive shear waves observed for low viscosities, a theory using generalised hydrodynamics is necessary to explain the results observed in supercooled liquids.