Totally Symmetric Modes and Franck-Condon Scattering

5 Totally Symmetric Modes and Franck-Condon Scattering 

The appropriate FC overlap integrals for Raman scattering are of the mixed type  

If e is non-degenerate, the matrix element of the force vanishes unless Q transforms under the totally symmetric irreducible representation of the point group appropriate to Q = 0. This is the reason for the rule shifts of equilibrium position only take place along totally symmetric coordinates. 

If e transforms under a degenerate irreducible representation, Pg (not a damping factor ), then the totally symmetric part of the direct product Pg Pg can still allow a shift of potential minimum, which will be equal for all degenerate components of e . However, if non-totally symmetric vibrational coordinates are available belonging to irreducible representations contained in the symmetric part, Pg Pg, of the direct product, these coordinates may cause a Jahn-Teller effect in the excited state. For example, if Pg = ( 4) then 

Returning now to the case of a non-degenerate resonant state, Tang waA Albrecht (15) have shown how Albrecht s A term. 

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