Moving Along the States

This was the case of halogens. On the other hand, in molecules such as oxygen and nitrogen, the equilibrium distance r did not change, and the dissociation energy was essentially independent on excitation (D = D Fig. 3.12, center). Finally, the equilibrium distance r may be smaller in the excited state, as in molecules (or rather complexes) that were stable only in the excited state, as it was the case for Na2 and Hg2 (D D Fig. 3.12, right side). 

In terms of classical mechanics, 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 [55]. This principle was readily accepted and from the beginning quoted with the names of the two scientists [56-58]. 

Stokes GG (1852) On the change of refrangibility of light. Philos Trans R Soc Lond 142 463-562 

Lewis GN, Kasha M (1944) Phosphorescence and the triplet state. J Am Chem Soc 66 2100-2116 

Lewis GN, Magel TT, Lipkin D (1942) Isomers of crystal violet ion. Their absorption and re-anission of light J Am Chem Soc 64 1774-1782 

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