 In his own research , Franck developed what became known as the Franck @-@ Condon principle , a rule in spectroscopy and quantum chemistry that explains the intensity of <unk> transitions , simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the appropriate energy . The principle states that during an electronic transition , a change from one vibrational energy level to another will be more likely to happen if the two vibrational wave functions overlap more significantly . The principle has since been applied to a wide variety of related phenomena . 
