Franck-Condon Principle

The Franck-Condon principle explains how molecules absorb and emit light. It states that electronic transitions happen so quickly that the positions of atomic nuclei don’t change during the process. Because of this, the transition is most likely to occur where the nuclei are in a configuration that overlaps best between the initial and final states. This overlap determines the intensity of spectral lines, explaining why some transitions are stronger. Essentially, the principle links the likelihood of electronic events to the vibrational configurations of nuclei, shaping the detailed structure seen in molecular spectra.