Vibrational line shifts in supercritical fluids

A microscopic statistical mechanical theory of solvent-induced vibrational line shifts of dilute solutes in supercritical fluids is presented. The the ory is based on a simple model of a spherical solute present at infinite di lution in a fluid of spherical solvent particles. A microscopic expression for the vibrational line shift is given, which involves the solute-solvent radial distribution function and interaction potentials. The distribution f unction is obtained from integral equations and from Monte Carlo simulation s. The theory is applied to study the experimentally observed anomalous den sity dependence of line shifts in supercritical fluids in the vicinity of t he critical point. Model calculations of spectral shifts are performed for a range of solvent densities and temperatures and model potential parameter s. In addition, a quantitative comparison of the theory with experimental d ata on vibrational spectral shifts is performed, and the agreement is satis factory.