A comparison of models for depolarized light scattering in supercritical CO2

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Abstract
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.