Anomalies of linewidth and intensity in the reorientation spectra of a critical mixture

The critical behavior of the depolarized Rayleigh spectrum in the system nitrobenzene-$n$-hexane has not been previously accurately assessed because in usual Fabry-Pérot experiments the multiple-scattering contribution completely overwhelms the spectrum within a few degrees of the critical temperature $T_c$. By using a high-performance double-pass Fabry-Pérot interferometer, we have been able to obtain accurate measurements of both linewidth and intensity, from 50°C to temperatures as close as 0.026°C to $T_c$. Two lines due to nitrobenzene molecules in solution are visible. The first line (1) is intense and narrow, while the second line (2) is weak and broad. The spectral intensity of (2) is about ${10}^{-2\mathrm{}}$ times that of (1). The linewidth of (1) is seen to behave as the inverse shear viscosity, with a critical divergence with exponent $Y\approx 0.04$. The intensity of (1) exhibits a power-law divergence with exponent $\Omega \approx 0.2$. These results are compared with data obtained in pure nitrobenzene and $n$-hexane, and in noncritical mixtures of nitrobenzene.