Dissociation via a single excited electronic state is the exception rather than the rule. The remarkable success with which all experimental results for the dissociation of H2O, for example, have been reproduced by rigorous calculations without any adjustable parameter rests mainly upon the fact that only one electronic state is involved (Engel et al. 1992). Many other photodissociation processes, however, proceed via two or even more electronic states with the possibility of transitions from one state to another. Figure 15.1 illustrates a common situation: the photon excites the molecule from the electronic ground state (index 0) to a dipole-allowed upper state (index 1) which further out in the exit channel interacts with a second electronic state (index 2). The latter may be dipole-forbidden and therefore not directly accessible by the photon. The corresponding diabatic potentials V1 and V2 cross at some internuclear distance Rc. In the proximity of this point the coupling between the two electronic states, which was ignored throughout all of the preceding chapters, can be large with the consequence that a transition from state 1 to state 2 and/or vice versa becomes possible (radiationless transition, electronic quenching). Electronic transitions manifest the break-down of the Born-Oppenheimer approximation, i.e., the motion of the electrons and the heavy particles can no longer be adiabatically separated.

Let us imagine a wavepacket starting in the Franck-Condon region on potential V1 When it reaches the crossing region it splits under the influence of the coupling into two parts.