Hypersonic attenuation in simple cresols

The attenuation of thermally induced sound waves in m- and p-cresol has been measured as a function of temperature by using Brillouin light scattering. The spectra are analyzed in terms of an extended hydrodynamic light scattering theory of Lipeles and Kivelson. Relaxation mechanisms at hypersonic frequencies are observed for both m- and p-cresol. In both simple cresols the Brillouin line width increases below the melting point, indicating a relaxation frequency lower than we can experimentally observe in m-cresol and near our experimental limit in p-cresol. This relaxation mechanism is assigned to structural relaxation in the shear contribution. In m-cresol a maximum is observed in the bulk contribution to Brillouin line width around 323 K. No corresponding maximum in p-cresol is observed. The relaxation mechanism is attributed to the orientational contribution to the shear viscosity.