Nuclear Geometry after Excitation

Fig. 1. The lowest singlet (So) and first excited singlet (Si) surfaces of two hypothetical molecules. Vibrational wavefunctions for one and three vibrational levels, respectively, are indicated. Top part of a indicates schematically the time development of the nuclear geometry probability distribution after initial excitation...

Of considerable significance to any discussion of electronic spectroscopy is the Franck-Condon principle the time taken for an electronic transition is short compared with the times taken for nuclear movements. This means that the Franck-Condon excited state of a molecule, i.e. that existing momentarily after excitation, has the same nuclear geometry as the ground state from which the transition occurred. Further, the solvent molecules around the excited molecules also remain in the same position during the transition. Only after the transition is completed will the excited molecule in the Franck-Condon excited state and its solvation sphere (if any) rearrange into their new equilibrium (excited state) positions. This has an important bearing on the interpretation of solvent effects. 

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