Observation of Coupled Acoustic-Phonon-Reorientation Modes in a Plastic Crystal: Succinonitrile

Abstract

Rayleigh-Brillouin measurements of depolarized light scattering in the plastic phase of succinonitrile are reported as a function of crystal orientation. The Rayleigh wing is characterized by two distinct molecular-reorientation modes and strongly angular-dependent shear-wave peaks are seen with typical resonance line shapes. These spectra are understood as arising from coupling between acoustic phonons and reorientation modes. Excellent agreement with a hydrodynamic description of the coupled modes is obtained.