Term: Franck–Condon principle
https://doi.org/10.1351/goldbook.F02510

Definition:
Classically, the Franck–Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The resulting state is called a Franck–Condon state, and the transition involved, a vertical transition. The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition.

Related Terms:
1) vibronic transition (http://doi.org/10.1351/goldbook.V06618).
2) overlap integral (http://doi.org/10.1351/goldbook.O04357).

Source: PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2243 (https://doi.org/10.1351/pac199668122223)

Citation: 'Franck–Condon principle' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.F02510

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