Franck-Condon active normal modes

As is ISC, IC is very slow for electronic states with similarly shaped potential energy surfaces. When the potential surfaces have very different shapes, there will be a small number of vibrational doorway states that are especially effective in coupling to the bright state. Conical intersections are a special class of potential surfaces of very different shapes. But even when potential surfaces have very different shapes, many normal coordinate displacements and the associated vibrational normal modes will have nearly identical forms on both surfaces. These normal modes are Franck-Condon inactive and do not contribute to IC. The normal coordinate displacements that express the differences in shapes of the potential surfaces are embodied in vibrational normal modes that are Franck-Condon active. These modes are called promoting modes because, when such a mode on one potential surface is plucked from an eigenstate on the other surface, intramolecular dynamics is promoted or initiated. 

If A and A differ, the coupling mode g is non-totally symmetric (coined g ). It can thus not coincide with (first-order) Franck-Condon active modes which are characterized by the decomposition of A <8> A or A <8 A and are totally symmetric (in Abelian point groups). The latter are also called tuning modes in the LVC scheme which forms the body of our early work in the field. The conical intersection is usually termed symmetry-allowed in such a case since it normally occurs (for a single coupling mode) in the subspace g = 0 where A 7 A and a free crossing is possible. 

Excitation into electronic absorption results in intensity enhancement of normal modes of vibration which are coupled to electronic transition by either Franck-Condon or Herzberg-Teller coupling allows for study of chromophoric active sites in biological molecules at low concentrations allows assignment of CT (and in some cases LF) transitions based on nature of excited state distortion can provide information on metal-ligand bonding as described above for vibrational spectroscopy... 

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