Approximate Selection Rules for Resonance Raman Spectroscopy

The resonance Raman scattering tensor is developed and discussed. We contrast the resonance conditions that result in the enhancement of combinations and overtones of nontotally symmetric modes with those for the corresponding Raman fundamentals. By assuming a simplified model in the description of the resonant adiabatic state we are able to discuss the symmetry of the scattering tensor based on equilibrium molecular configurations. Within the framework of this model we show by symmetry that in certain commonly encountered limits, the resonance enhancement of antisymmetric combinations is very small whereas for others (equally allowed by rigorous symmetry rules) the enhancement is large and can be predicted from the intensities of the corresponding Raman fundamentals. Similar arguments are applied to the analysis of resonance Raman scattering involving Jahn-Teller active modes. Dispersion and interference effects are also considered in relation to the Raman excitation profile of nontotally symmetric vibrations. Finally we illustrate with results from ferrocytochrome-c how these principles can provide the information to make new vibrational assignments.