Effect of Non-Adiabatic Coupling on Vibrational Energy Transfer

3 Effect of Non-Adiabatic Coupling on Vibrational Energy Transfer 

The concepts treated above are based on the adiabatic approximation for electronic states implying the existence of a well-defined intermoleciilar potential and completely neglecting transitions between different electronic states. The necessary (but not sufficient) condition for adiabatic approximation is the large energy splitting of electronic states of free molecules. However, when one or both colliding partners are in a degenerate electronic state, the adiabatic approximation is invalid. 

Intermolecular interactions lift the electronic degeneracy so that, when molecules approach, there arises a whole set of adiabatic potential energy surfaces which are strongly coupled. 

Assume now that one of the partners is in a degenerate electronic state. Electronic and vibrational states form the so-called vibronic terms. Those correlating at R - oo with different vibrational states of BC can cross or pseudocross. In these regions (one shown in Fig. 18), non-adiabatic transitions are probable. They represent an additional path of changes in the vibrational quantum number of BC (dashed arrows near R2). There are several types of interactions 

For atoms of a medium atomic number the spin-orbital splitting is sufficiently weak to be treated by perturbation (see Section III.9). Then the transition probability between vibronic terms will be low. However, since these paths do not involve tunnelling, the probability of a combined electronic-vibrational transition might be higher than that of purely vibrational transition [339, 344]. 

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