Question

What is the Frank-Condon principle?​

Answer 1

Franck-condon principle describes the intensities of vibronic transitions,or the absorption or emission of a photon.It states that when a molecule is undergoing an electronic transition such as ionization,the nuclear configuration of the molecule experiences no significant change.This is due infact that nuclei are much more massive than electron ,and the electronic transition takes place faster than the nuclei can respond.When the nucleus re-aligns itself with the new electronic configuration,the theory states that ,that it must undergo a vibration.

The Franck-condon principle has both classical and quantum application.Clasically ,the Franck-condon principle is the approximation that an electronic transition is most likely to occur with out changes in the position of the nuclei in the molecular entity and its environment .The resulting state is called 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 wave functions of the two states that are involved in the transition.

Franck's main idea that was developed mathematically and later given a wave mechanical basis by condon.The electron jump in a molecule takes place so rapidly in comparision to the vibrational motion that immediately afterwards the nucli still have very nearly the same relative positon and velocity as before the jump.