Observed Resonance Raman Spectra

Abstract

Raman (R) spectra observed when the exciting frequency approaches or enters the region of electronic (vibronic) absorption of the molecule are called resonance Raman (RR) spectra. This condition is contrary to one of the conditions necessary for application of Placzek’s polarizability theory (cf., the work of Placzek,11 p. 270) so that the resonance Raman effect (RRE) requires special theoretical treatment. In considering the vibrational RRE (to which we shall restrict ourselves in this article), it is convenient to distinguish between the preresonance case and the rigorous resonance case. In the former, the exciting frequency still lies outside the observable vibrational structure of the electronic absorption band responsible for R scattering (either on the long- or short-wave side); in the latter, it falls into the interior of this structure, either coinciding or not coinciding with a vibrational band. The fact that the vibrational structure of the absorption band may be latent (e.g., because of line broadening due to intermolecular interactions) does not seriously impair the above distinction, since in this case the characteristic features of the rigorous RRE (differing much from those of the preresonance effect) disappear. The limit between preresonance and ordinary Raman effect (RE) is completely arbitrary; every RE may be conceived as a rudimentary RRE.