I am trying to get a better understanding of resonant raman scattering as a probe of properties of excited states of organic molecules. The figure below [taken from a JACS paper] shows how in the Green Flourescent Protein one sees an enhanced coupling to vibrations [blue curve in lower panel].
There is a nice site on Resonant Raman Theory set up by Trevor Dines (University of Dundee). It emphasizes that in the Born-Oppenheimer approximation one only sees a significant signal when the excited state involves a displacement of the vibrational co-ordinate.
This raises a question about whether in a completely symmetric molecule one can have a resonant Raman signal due to effects that go beyond Born Oppenheimer.