Forbidden Transitions Intensity Borrowing by Mixing with a Remote Perturber

4 Forbidden Transitions Intensity Borrowing by Mixing with a Remote Perturber 

It is always possible to express the eigenfunction corresponding to a nominal i, Vi, J electronic-vibration-rotation level as a sum of rovibronic basis functions, 

This equation can be generalized to include a mixed 0, vo, J ) eigenfunction. Whenever more than one term in the Eq. (6.4.2) summation is significant, the simple factorization of the transition probability into electronic (f 2), Franck- 

Condon ( qv v = (v v )2, and Honl-London rotational factors (Sj n, j n ) breaks down. In Section 6.2.1 the case was discussed in which only the k = i term in the summation was nonzero. Interference effects arise when two or more terms in Eq. (6.4.2) are significant and have comparable magnitudes. This section deals with the situation where the k = i term is zero and hence the transition is nominally forbidden. One example of this type, the forbidden 3 — 1E+ transition, was discussed in Section 6.3.2. 

There has been a great deal of confusion about which Franck-Condon factors should be used to model the vibrational intensities in a -progression in fluorescence originating from a perturbed, nominally nonfluorescing, upper level. Three cases must be considered (1) the i,v ) vibronic level borrows all of its i — 0 oscillator strength from a single, remote, electronic-vibrational level, 

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