Theory of resonance Raman scattering by doubly degenerate modes

A model is presented that allows calculation of excitation profiles, depolarization ratios, and resonance Raman spectra of doubly degenerate (e-type) modes. It consists of a doubly degenerate excited (E-type) electronic state and a nondegenerate excited A- or B-type electronic state. The sources of Raman scattering by the e-type modes are pseudo-Jahn–Teller (Herzberg–Teller) interactions between these excited states and Jahn–Teller and/or Renner–Teller interaction within the E-type electronic state. Molecules of trigonal, pentagonal, and hexagonal symmetries are investigated for different coupling schemes and different energy separations between the two excited states. The results are presented in the form of three-dimensional graphs for the fundamental and the first two overtones together with the corresponding absorption spectrum. The model also allows one to follow changes in resonance Raman scattering produced by trigonal distortion of molecules possessing a triply degenerate electronic state in resonance with the incident light. The case of resonance scattering in the region of a forbidden nondegenerate state which is likely to occur in hexagonal molecules such as benzene is also discussed.