A simple interpretation of Raman excitation spectra of metalloporphyrins

A simple treatment of vibronic coupling in metalloporphyrins is described. Electronic degeneracy in the excited states is treated to arrive at simple expressions for the Raman excitation spectra and the absorption spectrum. The expressions are simple analogs of the usual A and B term expressions for the scattering tensor and are easily programmed for a small computer. They are only applicable in the weak vibronic coupling limit, but are nevertheless suitable for interpretation of the spectra of most biologically relevant porphyrin systems, at least as a starting point. They allow a simple discussion of a number of interference phenomena that influence the Raman intensity distribution in the excitation profiles of metalloporphyrins and other molecular systems. Some of these phenomena are consequences of electronic degeneracy in the resonant excited state. Nonadiabatic coupling contributions are also included in the expressions. Experimental Raman excitation spectra are discussed in light of this simple model.