Brillouin Scattering and Dispersion and Attenuation of Hypersonic Thermal Waves in Liquid Carbon Tetrachloride

Brillouin spectra of liquid CCl4 (at 20.0°C) have been investigated 20 scattering angles, from 40° to 145°. The spectra were excited by 6328-Å radiation from a He-Ne laser and analyzed with a pressure-scanned Fabry-Perot spectrometer. The measured frequency shifts and linewidths of the Brillouin component were used to determine velocities and attenuations of thermal waves in the frequency range 1.6–4.7 Gc/sec. A large dispersion (>10%) and corresponding change in attenuation were found, characteristic of a thermal relaxation process in this region. A broad new component in the spectrum of isotropic scattering was observed, which is also characteristic of a relaxation process. Analyses of these data were carried out in two ways: one is based on a simple extension of acoustic equations to light scattering, and the other, on Mountain's theory of light scattering in the region of a thermal relaxation. It is shown that the observed spectra and derived data agree with Mountain's theory in all respects. Also, from these data it was possible to evaluate the total internal specific heat CI = 11.6±0.3 cal/mole°C. Within experimental error, this relaxes in a single relaxation time τ = (6.5±0.5) × 10−11 sec.