Brillouin Scattering in Water: The Landau—Placzek Ratio

An experimental study of the ratio of intensities of the central line, in the molecular scattering of light, to the Brillouin components has been made in water over a temperature range from 0° to 50°C. A 75-mW He–Ne gas laser was used as a light source and the scattered radiation was analyzed by a pressure-scanned Fabry—Perot interferometer in conjunction with a dry-ice-cooled phototube detector with recorder output. The ultrahigh-frequency sound velocity is compared with values obtained by using ultrasonic techniques. There is no evidence of relaxation effects in the frequency range studied. An analysis of the Landau—Placzek ratio is made for water considered as an ideal fluid. It is shown that the usual approximations used in deriving the ratio (κT—κS)/κS are not valid for water. The actual behavior of the Landau—Placzek ratio for water as a function of temperature can be explained in part when an exact solution is used.