The Superposition of Rayleigh and Brillouin Radiation in Photon-correlation Spectroscopy of Liquids

The radiation scattered in a liquid at small angles may be analysed by photon-correlation spectroscopy. The hydrodynamic Rayleigh signal is then the dominating contribution from which the thermal diffusivity DT may be taken. It is necessarily mixed with the Brillouin signal and contributions from molecular scattering. This must be kept in mind even if nothing but a critical DT measurement is intended. However, under favourable conditions, the whole 'Rayleigh-Brillouin triplet' including the Landau-Placzek ratio may be determined. An experimental procedure based on some simple relations is presented. A measurement on n-pentane at 143°C is given as an example.