Observation of coupled acoustic-phonon-reorientation modes in a plastic crystal: Succinonitrile

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. © 1975 The American Physical Society.