Higher-order interaction-induced effects on the allowed Raman spectra of liquid CS2

Abstract

Molecular dynamics (MD) computer simulation is used to study nonvibrational contributions to the ν1 (symmetric stretch) mode Raman spectra of liquid carbon disulfide in two thermodynamic states: 193 K, 1.42 g/cm3 and 293 K, 1.30 g/cm3. A computational method for evaluating the Raman polarizability to all orders of dipole-induced dipole (DID) interactions is developed. The exact DID model results for Raman intensities, effective polarizabilities, time correlation functions and line shapes are compared to their counterparts obtained using the same model and first order perturbation theory for the interaction-induced Raman polarizability. Inclusion of higher-order DID interaction terms causes a dramatic change in the relative importance of collision-induced (CI) and orientational components of the depolarized Raman spectrum. The first order model predicts that the CI contribution to the spectrum is considerably larger than the orientational one, while the exact DID model predicts the reverse. The presence of higher-order DID interactions results in appreciably faster relaxation of the CI components of both isotropic and depolarized spectra. It is shown that the depolarized CI spectral component in both models contains a significant slowly relaxing portion due to mutual diffusion of pairs of molecules. The results for Raman and Rayleigh spectra are compared and their high-frequency line shapes are discussed in terms of the Madden–Cox theory. MD Raman depolarization ratios and depolarized spectra are compared to experiment at 293 K. The inclusion of higher-order DID terms improves agreement with experiment, but the DID model does not reproduce accurately the high frequency line shape. The sensitivity of this line shape to the changes in potential parameters is tested by varying the atom–atom Lennard-Jones parameters within reasonable limits and by adding atom–atom Coulomb potential terms to represent quadrupolar interactions. It is found to be only slightly sensitive to these changes.