  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 5 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 intra-molecular vibrations, and
therefore to exclude any relationship between it and the glass transition.
