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Scattering by entropy fluctuations in a liquid leads to a Rayleigh signal which may be analysed by photon correlation spectroscopy, when small scattering angles of only a few degrees are used. Brillouin scattering, that is scattering by pressure fluctuations, is usually studied at large scattering angles of around 90°; Fabry-Pérot spectroscopy is the appropriate method in this case. In the present paper the possible role of Brillouin scattering in photon correlation spectroscopy at small scattering angles is studied. An experimental example is given and a resolution limit is discussed.

In light scattering measurements near the gas-liquid critical point double scattering becomes a non-negligible effect. In a Rayleigh-Brillouin study on n-butane using photon correlation spectroscopy double scattering could be demonstrated. On account of the small scattering angles used the double scattering signal appeared as a short-time contribution to the autocorrelation function well separated from the single scattering Rayleigh signal which could thus be evaluated without regard to double scattering. Depolarized scattering including cross-correlation between both polarization components was also studied. Results of measurements along the critical isochore for reduced temperatures ε = (T - Tc)/Tc between 1.2 and 8.0 x 10 -3 are reported (Tc being the critical temperature). Whereas the results for the double scattering contribution fit well into existing theories, the origin of a depolarized single scattering Rayleigh component which was also present remains open.

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.

Photon correlation spectroscopy has frequently been used to analyse scattering in pure liquids at small angles. Under normal experimental conditions, the signal may be analysed approximately as being a pure Rayleigh signal without regard to the Brillouin components, which are also present in the scattered radiation. It is shown here that the opposite conditions may be imposed using a frequency-shifted coherent background. The Brillouin signal may then be almost completely resolved from the Rayleigh signal. The effect is illustrated, and the utility of the method is briefly discussed.

The high spectral resolution inherent in correlation spectroscopy may also be used for Brillouin spectroscopy. The heterodyne method described in this Letter is based on spontaneous scattering, in contrast to the recently developed high-resolution gain spectroscopy. A measurement of n-butane vapor is reported as an example. A Brillouin linewidth of ~200 kHz was resolved with an accuracy of a few percent.

The high spectral resolution inherent in correlation spectroscopy may also be used for Brillouin spectroscopy. The heterodyne method described in this Letter is based on spontaneous scattering, in contrast to the recently developed high-resolution gain spectroscopy. A measurement of n-butane vapor is reported as an example. A Brillouin linewidth of ~200 kHz was resolved with an accuracy of a few percent.