Fast Relaxational Dynamics in theo-Terphenyl Glass

The dynamic structure factor $S(q,\omega )$ of $o$-terphenyl, obtained by Brillouin light scattering ( $q\approx 0.04\mathrm{nm}{}^{-1\mathrm{}}$) in the 1 to 30 GHz range, shows signatures of relaxation phenomena at temperatures well within the glassy phase. Using a phenomenological ansatz for the density fluctuations memory function and $S(q,\omega )$ data obtained by inelastic x-ray scattering at high momentum transfer ( $q=1\mathrm{}–6\mathrm{nm}{}^{-1\mathrm{}}$), we describe this relaxational dynamics in terms of two distinct processes: (i) one in the ${10}^{-11\mathrm{}}\mathrm{s}$ range and slightly temperature dependent, and (ii) an “instantaneous” one, representing the microscopic transient and giving rise to a $T$-independent contribution to the Brillouin linewidth.