The intensity of scattered Raman radiation increases greatly as the exciting frequency approaches an absorption band of the molecule. This phenomenon, termed "Resonance Raman Effect", permits new spectroscopic applications. For instance, a red cyanine dye absorbing strongly at 542 mμ was successfully investigated, using the 5461 line of Hg for irradiation, at dye concentrations as low as 10<sup>−6</sup> mole/liter. Shifts in absorption due to solvent or substituent effects show great influence upon line intensities, as demonstrated with p-nitroso-dimethyl-aniline. There is no evidence of n→π<sup>*</sup> bands also causing the effect. According to theoretical aspects by Behringer, selection rules based on the Franck-Condon principle are valid. This makes for relatively simple resonance Raman spectra.
Werner Maier and Friedrich Dörr, "Some Results of Resonance Raman Spectroscopy," Appl. Spectrosc. 14, 1-3 (1960)