High-resolution Rayleigh-Brillouin spectroscopy in the critical mixture nitroethane-isooctane

We report new measurements of the Rayleigh-Brillouin spectrum of a critical binary mixture and review the existing data in this field. We used a double-pass Fabry-P\'erot interferometer and a weakly scattering system (nitroethane-isooctane) so that the Brillouin lines were not affected by the wings of the Rayleigh line, even in the immediate vicinity of the critical point. No anomaly was detected in the shift or width or intensity of the Brillouin lines. From the ratio of the intensities of the Rayleigh and Brillouin lines we deduced the ratio of specific heats γ₀, the correlation length ξ₀, and the susceptibility amplitude (∂c∂μ)pT⁰, without any hypothesis about light-scattering amplitudes. These values lead to an amplitude ratio R_ξ⁺Rc^-1/3 in full agreement with the theoretical values.