Vibronic couplings through totally symmetric modes

A term. One excited electronic state e is included in this term. With vq approaching Vg the Raman line associated with a totally symmetric mode derives its intensity from this term through the Franck-Condon mechanism. This term does not depend on vibronic mixing of state e with odier electronic states of the molecule. If the two electronic states g and e lie far below the next electronic levels and, hence, the probability of vibronic coupling between state e and the other excited electronic states is very low, the A term dominates in magnitude over the other two terms in Eq. (8.68). 

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