On the Capabilities of Optical Diagnostics Methods to Monitor the State of Supercritical Fluids near the Widom Line

A fundamental problem of the optical diagnostics of the supercritical fluid (SF) structure and its statistical properties in the vicinity of the Widom line is considered. The solution of this problem requires approaches that allow us to bring the peculiarities of optical measurement data in line with the peculiarities of the state of the fluid. The results obtained in the past ten years on the problem of the Widom lines in nonpolar supercritical media are briefly reviewed. Particular attention is given to the optical measurement data in the Widom region, namely, measurements of the nonlinear contribution to the refractive index and measurements of the Rayleigh light scattering intensity. It is demonstrated that these data can serve as a base for mutually complementary methods for the optical diagnostics of an SF state. As an example, the data on small-angle light scattering by SF-CO 2 were used to restore its pair correlation function, the temperature dependence of which fundamentally differs from that of the Ornstein–Zernike pair correlation function. It is noted that the method based on measuring the Rayleigh scattering intensity is general in nature and can be applied to any random unordered molecular media, including the atmosphere.