A theoretical investigation of the temperature dependence of the optical Kerr effect and Raman spectroscopy of liquid CS2

The ambient pressure, temperature dependent optical Kerr effect (OKE) spectral density of CS2 has been calculated by way of time correlation function (TCF) and instantaneous normal mode (INM) methods and compared with corresponding experimental OKE data [R. A. Farrer, B. J. Loughnane, L. A. Deschenes, and J. T. Fourkas, J. Chem. Phys. 106, 6901 (1997)]. Over this temperature range the viscosity of CS2 varies by more than a factor of 5, and the molecular dynamics (MD) spectroscopic methods employed do an excellent job in capturing the associated changes in molecular motions that lead to the observed spectroscopy. The resulting TCF spectra are also in very good agreement with experimental measurements at all temperatures, and this is remarkable considering the range of conditions considered. When compared in the reduced Raman spectrum form, where the INM spectral density is proportional to the squared polarizability derivative weighted density of states (DOS), the INM results agree very well with the TCF results, and the low frequency OKE feature corresponding to rotational reorientation is suppressed in this form. Interestingly, the INM signal includes a significant contribution from the imaginary INM's at all the temperatures considered, and these contributions are crucial to the agreement between INM and TCF results. Furthermore, the INM approximation to the signal (OKE or reduced Raman) demonstrates that the contribution (spectral density) of the real INM's remains nearly unchanged over the temperature range considered, while the imaginary contribution grows with increasing temperature. The signal from the imaginary INM's is therefore deduced to be responsible for a large part of the temperature dependence of the OKE spectral density. Finally, the molecular motions that contribute to the OKE signal are analyzed using INM methods. (C) 2000 American Institute of Physics. [S0021-9606(00)51043-5].