Vibronic coupling is the universal mechanism that gives nature a way around forbidden processes. It is because of vibronic coupling we see many weak absorption bands, Raman bands and other types of phenomena. The molecule distorts away from its equilibrium geometry as it vibrates. Despite the fact that it spends a very short amount of time in the distorted geometry it has chance to pick up oscillator strength by mixing to other states. It is this transient state mixing that makes a process that is forbidden allowed. This is described in the presentation.
