A-term resonance enhancement

For A-term resonance enhancement, either the frequency, m, or the equilibrium normal coordinate, Q, of the vibration must change between the ground and dipole-allowed excited electronic states. For B-term resonance enhancement, the vibration must be vibronically active for coupling the dipole-allowed excited electronic state to other excited electronic states. For C-term resonance enhancement due to a vibronically coupled excited electronic state, both the totally symmetric vibrational modes and the overtones and combinations of the vibronically active vibrational modes are enhanced. 

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