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 ar e compared with those of the liquid and the glass phases. This shows (i) th e direct evidence of a fast relaxation at frequencies nu approximate to5 GH z in both the single crystal and the glass; (ii) a similar temperature depe ndence for the attenuation of the longitudinal sound waves in the single cr ystal and the glass; and (iii) the absence of coupling between the fast rel axation 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 be tween it and the glass transition.