What is franck condon principle?

The Franck-Condon principle is a spectroscopic rule that describes the relative intensities of electronic transitions between two electronic states of a molecule. This principle states that the transition between two electronic states of a molecule occurs without any change in the internal energy of the molecule.

The Franck-Condon principle is applied in various spectroscopic techniques, such as UV-Vis, fluorescence, and Raman spectroscopy. It explains why some electronic transitions are more intense than others, and is useful in understanding the electronic structure of molecules.

The Franck-Condon principle is based on the assumption that electronic and nuclear motion in a molecule are uncoupled during the electronic transition, thus the transition is said to be instantaneous. This principle is named after James Franck and Edward Condon, who developed the concept in the 1920s.