Resonance Raman cross sections are generally larger than normal or preresonance Raman cross sections. Thus, higher Raman intensities are expected for resonance excitation, especially for backscattering measurements. However, self absorption decreases the observed Raman intensities. In the work here we examine the effect of self absorption on the observed preresonance and resonance Raman intensities. For the simplest case where a single electronic transition dominates the Raman scattering, and where the resonance enhancement scales with the square of the molar absorptivity of the absorption band, theory predicts that for close to resonance excitation the observed Raman intensities monotonically increase as resonance is approached. In the case that an impurity absorbs, the observed Raman intensities may decrease as excitation moves close to resonance for particular conditions of impurity absorption band widths and frequency offsets. Impurity absorption also causes decreases in observed Raman intensities for the more slowly increasing preresonance excitation.
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