Franck–Condon principle

The Franck-Condon principle is a concept in spectroscopy that explains the intensities of molecular spectral bands during electronic transitions. It assumes that because atomic nuclei are relatively massive, their positions do not change significantly during the rapid electronic transition.1

This means that an electronic transition is most likely to occur between molecular states that have similar nuclear configurations. In simpler terms, the molecule is more likely to transition to a vibrational state that is already close to its current nuclear arrangement. This principle helps predict which spectral lines will be strongest when a molecule absorbs or emits light, as the intensity of a spectral band is proportional to the overlap between the vibrational wave functions of the initial and final electronic states.123