Vibrational Lifetimes and Spectral Shifts in Supercritical Fluids as a Function of Density: Experiments and Theory

Abstract

Vibrational lifetime and spectral shift data for the asymmetric CO stretching mode of $W(CO)_6$ in supercritical ethane, carbon dioxide, and fluoroform as a function of density at two temperatures are presented, and the lifetime $(T_1)$ measurements are compared to theory. The data and theory are refinements of previous work. Measurements at zero density allow the contribution from solute-solvent interactions to be separated from strictly intramolecular contributions to $T_1$. The results in the polyatomic SCFs are compared to data in Ar. The density functional/thermodynamic $theory^1$ has been extended to include contributions at large wavevector (k). Very good, quantitative agreement between theory and data taken in ethane and fluoroform is achieved, but the agreement for data taken in carbon dioxide, while reasonable, is not as good. The theory uses a variety of input information on the SCF properties obtained from the fluids' equations of state and other tabulated thermodynamic data. The solute-solvent spatial distribution is described in terms of hard spheres. The theory is able to reproduce the data without solute-solvent attractive interactions or clustering in the calculation of the spatial distribution.