Intramolecular origin of the fast relaxations observed in the Brillouin light scattering spectra of molecular glass formers

The Brillouin light scattering spectra of the o-terphenyl single crystal are compared with those of the liquid and the glass phases. This shows (i) the direct evidence of a fast relaxation at frequencies nu approximate to5 GHz in both the single crystal and the glass; (ii) a similar temperature dependence for the attenuation of the longitudinal sound waves in the single crystal and the glass; and (iii) the absence of coupling between the fast relaxation and the transverse acoustic waves. These results allow us to assign such a relaxation to the coupling between the longitudinal acoustic waves and intramolecular vibrations, and therefore to exclude any relationship between it and the glass transition.