Interference effects in resonance raman spectroscopy

We show that resonance Raman excitation profiles are not generally expected to superimpose on the profile of the absorption spectrum. The discrepancy is due to interference between transition amplitudes of preresonant and more nearly resonant states. Two cases are outlined: in the first the amplitudes have similar magnitudes, and in the second they are quite different. Examples discussed include hemeproteins, visual pigments and resonant scattering from triplet states of molecules. A main point is that qualitative aspects of these interferences may be used to assign states and vibrations. Some new experiments are reported for ferrocytochrome-c scattering.