Thermo- and Elasto-optic Parameters of NaF and Their Implications for Light Scattering from Second Sound

The intensity of light scattered from second sound is estimated for the case of NaF, the only dielectric crystal in which second sound is presently known. For this purpose we have measured the thermo-optic coefficient ${\left(\frac{\partial n}{\partial T}\right)}_P$ of NaF between 300 and 10 °K as well as the elasto-optic tensor at room temperature. The scattered intensity is calculated to be less than the value predicted by the Landau-Placzek ratio, and in fact the Rayleigh component is expected to vanish entirely at one particular temperature (approximately 34 °K).