Conversion and Intersystem Crossing Theory

In Chapters 4 and 5 we made use of the theory of radiationless transitions developed by Robinson and Frosch. In this theory the transition is considered to be due to a time-dependent intramolecular perturbation on non-stationary Born-Oppenheimer states. Henry and Kasha and Jortner and co-workers have pointed out that the Born-Oppenheimer (BO) approximation is only valid if the energy difference between the BO states is large relative to the vibronic matrix element connecting these states. When there are near-degenerate or degenerate zeroth-order vibronic states belonging to different configurations the BO approximation fails. 

In the more recent theory the BO zeroth-order states are not considered to be pure, because of configuration interaction between the 

However, it has been pointed for large organic molecules 

Radiationless Transitions Phosphorescence Microwave Double Resonance  

In addition, it can be shown that second-order vibronic perturbation will make possible some intersystem crossing to the n ) state. However, 

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