On band shapes of electronic transitions in the multimode weak coupling limit

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Abstract
The influence of multidimensional Franck–Condon (FC) active vibrational subspace on the band shapes of electronic transitions is explored in the linear weak coupling limit. Given a constant (weak) coupling coefficient, computer calculated band shapes are seen to fall into three classes depending on the number of active vibrational modes, N. For large N, the electronic band of a polyatomic molecule will have a relatively weak zero–zero absorption compared to the ’’side‐band’’ absorption, which arises from a rapidly increasing density of states (DOS). For small N, a normal, weak FC progression is seen in which the peak absorption is in the zero–zero transition. The implications of the DOS/FC model are further discussed in relation to previous work. Calculations are presented to emphasize the effect of the multidimensional subspace on the relationship between the absorption band shape and the resonance Raman excitation profile.