Multiple scattering of light in near-critical methanol-cyclohexane

We have experimentally and theoretically investigated the polarized and depolarized double-scattered intensity in the critical system methanol-cyclohexane in the temperature range $0.003<T-T_c<1\mathrm{}$°C. We have used a scattering geometry typical of spectroscopic studies, in which the single-scattering volume is illuminated by a narrow, cylindrically symmetric beam, viewed through small circular pinholes by a detector at a 90° scattering angle. We have investigated the multiple-scattering effects for aperture sizes ranging from 5×${10}^{-3\mathrm{}}$ to 5×${10}^{-4\mathrm{}}$ rad, where coherence effects become important. Our results for the total polarized and depolarized intensity are well described by double-scattering theory with $\gamma =1.26\mathrm{}$, ${\xi }_0=2.6\mathrm{}$ Å, and $\nu =0.60\mathrm{}$. At our closest approach to $T_c$, we find that the contribution of polarized double scattering to the total scattered intensity is ≅40%. At this temperature the effects of attenuation on the incident and scattered light become important. We have observed the effects of triple- and higher-order scattering events as an additional attenuation of the singly and doubly scattered light, and we have employed this observation to estimate the intensity of these higher-order scattering events.