VH-Depolarized Light Scattering Spectra of Liquids at High k2η/ρΓ

Abstract

If the reorientational motion of liquids composed of nonspherical top molecules couples to the shear modes of the liquid, a k‐dependent VH‐depolarized light scattering spectrum results. At low values of (k²η/ρΓ) (where η is the coefficient of shear viscosity and ρ is the mass density), the spectrum exhibits a sharp Rayleigh wing line with half‐width Γ and a k‐dependent doublet near its peak. A simple theory is explored which shows that the doublet feature vanishes when (k²η/ρΓ) exceeds a critical value dependent upon both the angle of scatter and a dimensionless parameter (R) which measures the coupling between molecular reorientation and translational shear modes of the liquid. (k²η/ρΓ) increases with decreasing temperature and increasing molecular size; therefore, the theory indicates that the doublet should vanish at low temperature as observed.