Theory of Raman scattering and Raman optical activity: near resonance theory and levels of approximation

A new level of the theory of Raman scattering and Raman optical activity (ROA) is identified between the general, unrestricted (GU) theory and the far-from-resonance (FFR) theory called the near resonance (NR) theory. In the NR theory, the Raman tensor is not symmetric, and there is nonequivalence between the incident and scattered circular polarization (ICP and SCP) forms of ROA and non-zero intensity for out-of-phase dual circular polarization (DCPII) ROA. Several levels of theory are identified in passing from the GU theory to the FFR theory. The NR theory provides vibronic detail, present in the GU theory but absent from the FFR, by assuming that the vibrational levels of the excited electronic states are the same as those of the ground electronic state. The NR theory obeys time-reversal symmetry and can be expressed in a form that is computationally tractable, thereby providing an improved description of the frequency dependence of Raman and ROA intensities relative to that provided by the FFR theory.