VIBRATIONAL-RELAXATION IN SUPERCRITICAL FLUIDS NEAR THE CRITICAL-POINT

We present a theory for the temperature and density dependence of the vibrational lifetime T-1 and the vibrational line position nu of a sol ute in a supercritical solvent, both close to and far from the critica l point. The theory is based on the relation between a classical force correlation function and T-1 and nu. The force correlation function i s determined from density functional theory, and can be expressed in t erms of the solvent structure factor and the solute-solvent direct cor relation function, thereby allowing physical properties in the region of large critical fluctuations to be described by various phenomenolog ical scaling laws. The theory has been used to investigate recent expe riments on the density dependance of the lifetimes and frequencies of the asymmetric CO stretching mode of W(CO)(6) in supercritical ethane. Near the critical point, the experimental data are essentially indepe ndent of the density over a fairly broad range of densities. This beha vior is ascribed to the existence of long-range correlations in the fl uid mixture near the critical point. Such correlations, manifested in the divergence or vanishing of thermodynamic quantities, are shown to essentially eliminate the density dependence in the static and dynamic correlation functions that enter the theory. Because it is the anomal ous thermodynamics near the critical point that ultimately governs cha nges in T-1 and nu, the results are not dependent on specific intermol ecular interactions. The lack of a theoretical dependence on specific intermolecular interactions is supported by experiments that display t he same behavior for Various solute/solvent systems. (C) 1997 American Institute of Physics.