Predissociation and curve crossing

One of the most intuitive examples to understand curve crossing and potential energy curve interaction, which complements the discussion of predissociation, is that of the interaction of ionic and covalent potentials (e.g. + B and A - - B) exhibiting the 

Quite frequently, potential energy curves (surfaces) cross and these crossings have very important consequences. One of the simplest examples for potential curve interaction is predissociation of a diatomic molecule. The key points are  

The efficiency of predissociation (e.g. the formation of A+ B from AB ) depends upon the strength of coupling, which is proportional to the interaction Hamiltonian ffi2p between the two electronic states correlating with A + B and A - - B. 

If the coupling is very weak, then predissociation has little effect and fluorescence back to the ground state is the dominant decay process. 

If the two states have the same electronic symmetry and spin, then the interaction can be 

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