Indirect Accidental Predissociation and Interference Effects

In Section 7.8 the possibility of predissociation of isolated lines was mentioned. This is usually called accidental predissociation and can be interpreted as perturbation of a nominally bound rotational level by a predissociated level that lies nearby in energy for this value of J. This type of predissociation should more generally be called indirect predissociation, since the predissociation takes place through an intermediate state (or doorway state, see Section 9.2). 

Let H 2 be the coupling between level 1 and level 2, where level 2 is predissociated by the continuum of a third state and has a linewidth F2. Direct predissociation of level 1 by state 3 is assumed to be negligible. 

Kovacs and Budo (1947) have argued that one should first compute the mixing of the two discrete states. Then, the width of level v of state 1, Fi)Ul, is borrowed from state 2 in proportion to the mixing coefficient ai)t,i 2)t,2, which expresses the level 2 character mixed into the nominal level 1 eigenstate, 

If the energy separation between the two discrete states, A q2, is large compared to the interaction matrix element, Hi2, this mixing coefficient is given by the perturbation formula, 

However, if state 1 borrows its width from several vibrational levels of state 2, Eq. (7.13.2) must be replaced by 

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