The elastic case Franck-Condon mapping

The photodissociation dynamics becomes particularly simple if the translational motion (associated with R) and the vibrational motion (described by r) exactly decouple, i.e., if the full PES separates as 

If the interaction potential V/ depends only on R, energy cannot flow between the translational and the vibrational modes the full or half collision is elastic (no final state interaction). The resulting quantum mechanical or classical equations of motion separate in two uncoupled blocks and the motions in R and r evolve independently of each other. 

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