LOW-ANGLE BRILLOUIN-SCATTERING UNDER A TEMPERATURE-GRADIENT

The asymmetry appearing in the dynamic structure factor of a medium un der a non-equilibrium state of temperature gradient was experimentally verified. Brillouin scattering was carried out in water with the opti cal beating spectroscopy technique at frequencies lower than 60 MHz wh ere a large asymmetry was expected. The finite-cell-size effect to whi ch the disagreement between the theory and experiment had been attribu ted was excluded by a specially designed tall cell which suffers from the least effect of the finite size. The observed Stokes line was show n to have intensity 2% to 8% higher than the anti-Stokes line dependin g on the phonon frequency for a temperature gradient applied into the same direction as the scattering vector. Based on a concept of anisotr opic phonon propagation under a temperature gradient, a simple theory was proposed which succeeded in estimating the asymmetry for the tempe rature distribution of actual experimental condition including the eff ect of cell boundaries. The observed asymmetry was quantitatively in g ood agreement with tie theoretical prediction.