Brillouin Scattering in Liquid Benzene Under High Pressure

Brillouin spectra have been measured in liquid Benzene at high pressures up to 1300 bar and at the temperatures 298.15 K, 313.15 K, 323.15 K. and 343.15 K. From the experimental spectra the hypersound velocities and the energy relaxation times were determined. The total velocity dispersion v \ j v \ due to the relaxation of the vibrational specific heat c, is found to be nearly density independent in the pressure interval under study. It follows further that the experimentally determined specific heat corresponds to the theoretical value calculated with the vibrational modes of the Benzene molecule. The energy relaxation time t, connected with all vibrational modes but the lowest decreases with increasing density at constant temperature, at constant density r, decreases with growing temperature. The relative relaxation rates were used to test the cell model (CM) with movable walls and the collision theory (HS) of Einwohner and Alder. Using a hard sphere diameter derived from experimental transport coefficients the experimental results were predicted better by HS than by CM.