A combined instantaneous normal mode and time correlation function description of the optical Kerr effect and Raman spectroscopy of liquid CS2

The depolarized reduced Raman and corresponding optical Kerr effect (OKE) s pectral density of ambient CS2 have been calculated by way of time correlat ion function (TCF) and instantaneous normal mode (INM) methods and compared with experimental OKE data. When compared in the reduced Raman spectrum fo rm, where the INM spectrum is proportional to the squared polarizability de rivative weighted density of states (DOS), the INM results agree nearly qua ntitatively (at all but the lowest frequencies) with the TCF results. Both are in excellent agreement with experimental measurements. The INM signal h as a significant contribution from the imaginary INMs. Within our INM theor y of spectroscopy the imaginary INMs contribute like the real modes, at the magnitude of their imaginary frequency. When only the real modes are allow ed to contribute, and the spectrum is rescaled to account for the missing d egrees of freedom, the results are much poorer, as has been observed previo usly. When the spectra are compared in their OKE form, the INM spectrum is found to lack the low-frequency spike which is associated with long time sc ale rotational diffusion, and it is not surprising that an INM theory would not capture such a feature. The results demonstrate that while the OKE and spontaneous depolarized Raman spectrum contain the same information, they clearly highlight different dynamical time scales. At higher frequencies (o mega > 25 cm(-1)) the INM OKE results are in excellent agreement with TCF a nd experimental results. The TCF results capture the low-frequency spike an d are in agreement with experiment everywhere within the precision of the p resent calculations. The molecular contributions to the OKE signal are anal yzed using INM methods. (C) 2000 American Institute of Physics. [S0021-9606 (00)50709-0].