Quantum, classical, and semiclassical approximations to spectroscopy: The INM and TCF methods, and beyond

This work focus on theoretical calculations of the infrared (IR) and Raman spectrum. A detailed derivation of the theoretical modeling of the experiment is shown. Next an INM theory of condensed phase absorption is applied to determine the IR spectrum of condensed phase carbon disulfide. State points with pressures from 100 kPa (near boiling) to 1.16 GPa (near freezing) are considered, and excellent agreement between TCF and experimental results is demonstrated. Then a formal connection is made between the vibrational density of states (DOS) of a liquid and its approximation by way of INM. This analysis leads to a quantum generalization of the INM method (QINM), and to the possibility of evaluating the classical DOS exactly. INM and QINM methods are then applied along with the traditional time correlation function methods to analyze the entire IR spectrum of ambient water. The INM and TCF approaches are found to offer complimentary information. The depolarized reduced Raman and corresponding Optical Kerr effect (OKE) spectral density of ambient CS₂ are then calculated by way of TCF and INM methods and compared with experimental data. Both are in excellent agreement with experimental measurements. The INM signal has a significant contribution from the imaginary INM's. The results demonstrate that while the OKE and spontaneous depolarized Raman spectrum contain the same information, they clearly highlight different dynamical time scales. The molecular contributions to the OKE signal are analyzed using INM methods. Finally, the ambient pressure, temperature dependent OKE spectral density of CS₂ has been calculated by way of TCF and INM methods and compared with corresponding experimental OKE data. Over this temperature range the viscosity of CS₂ varies by more than a factor of five, and the molecular dynamics spectroscopic methods employed do an excellent job in capturing the associated changes in molecular motions that lead to the observed spectroscopy.