Franck-Condon principle

Article By:

Schulman, James H. Formerly, U.S. Naval Research Laboratory, Washington, DC.

Klick, Clifford C. Solid State Division, U.S. Naval Research Laboratory, Washington, DC.

Last reviewed:June 2020

DOI:https://doi.org/10.1036/1097-8542.271100

The generalization that the transition from one energy state of a molecular system to another occurs so nearly instantaneously that the nuclei of the atoms involved can be considered as stationary during the process. The Franck-Condon principle is closely related to the Born-Oppenheimer approximation, in which the various motions (electronic, nuclear vibrations and rotations) are considered to be separable, and in which the electrons respond to the instantaneous vibrations of the system whereas the system responds only to the average position of the electrons. The principle, proposed by J. Franck in 1925 and developed quantum-mechanically by E. U. Condon in 1928, is important in discussing systems of more than one atom. It is therefore valuable in molecular spectroscopy and in the interpretation of the optical properties of liquids and solids.

The content above is only an excerpt.