A COMPARISON OF MODELS FOR DEPOLARIZED LIGHT-SCATTERING IN SUPERCRITICAL CO2

Depolarized Rayleigh scattering (DRS) in supercritical CO2 is studied using molecular dynamics computer simulation. Results are presented for three thermodynamic states at approximately 313 K and at the following densities; ρ/ρc = 1.38, 1.95, and 2.35, where ρc = 0.468 g/cm3 is the critical density. We study the effects of intermolecular potential and interaction-induced polarizability models on the DRS spectra. Results for three different atom-atom intermolecular potentials are compared: Two potentials with Lennard-Jones and Coulomb terms, one developed by Murthy, Singer, and McDonald (MSM) and the other by Steele and Posch, as well as a third potential with MSM Lennard-Jones parameters, but without Coulombic interactions. The interaction-induced polarizability is calculated using center-center and site-site dipole-induced dipole (DID) interaction models. First order perturbation theory results are obtained for both models. The exact solution of the center-center DID model is also obtained. We find that both polarizability delocalization and higher-order DID interactions have a significant impact on the DRS intensity, but a more modest one on the related time-dependent properties. The results for the three intermolecular potentials differ significantly, with those for the MSM potential being in closest agreement with the experimental data. The collision-induced contribution to the DRS spectrum is found to decrease with increasing density and to exhibit cancellation effects due to contributions from 3 and 4 molecule correlations. © 1990 American Institute of Physics.