Electronic Excitation: Nonadiabatic Transitions

Nonadiabatic transitions are, strictly, those arising from a breakdown of the Born-Oppenheimer approximation, the term adiabatic being reserved for the limit in which the electronic problem is solved with the nuclei fixed. This corresponds to a situation in which, in classical terms, the electrons would pass through many cycles of their periodic or quasiperiodic(1) motion in a time short compared with that required to achieve a significant perturbation to this motion by changing the nuclear geometry.