Computer simulation of Raman scattering from molecular fluids

Abstract

We present the results of molecular dynamics (MD) simulations of the nonvibrational contributions to Raman spectra of dense fluids of linear molecules. We include orientational and collision induced (CI) contributions to depolarized scattering time correlation and the CI contribution to the isotropic scattering time correlation. The intermolecular potential is represented by a two site Lennard-Jones model. The bond length and potential parameters are chosen to correspond to those of O2, and CO2. We consider two pair polarizability models, one based on the dipole–induced dipole (DID) interactions between molecular centers, the other on the DID interactions between Lennard-Jones sites. For both models, we study the variations of effective polarizabilities, integrated intensities, spectral moments, and time correlations with density and temperature. We find that the orientational-CI cross correlations are the main interaction-induced contribution to depolarized spectra. Effective polarizabilities, as well as the intensity and the second moment, M2(2), of the depolarized spectrum are all found to have a similar dependence on the fluid structure, which results in the linear density dependence of these quantities. This finding is confirmed by experimental results for M2(2) of fluids structurally similar to the ones considered here. We demonstrate how Raman MD results may be used to evaluate the Rayleigh effective anisotropic polarizability in the ‘‘product’’ approximation. In the case of CO2, significantly different density dependence of the CI part of the LS spectra is predicted by the two DID models.