A principle governing the intensity of transitions in the vibrational structure during an electronic transition in a molecule

The principle states that since nuclei are much heavier and move much more slowly than electrons, an electronic transition occurs much more rapidly than the time required for the nuclei to respond to it.

It was named after James Franck (1882 -1964) who stated it in 1925, and Edward Condon, who formulated it mathematically in terms of quantum mechanics in 1928.