Theory of the resonance Raman effect in crystals

Abstract

The dependence of the Raman-scattered intensity on exciting frequency is investigated theoretically for the case where this frequency is close to an absorption line or absorption band of the scattering crystal. Both spontaneous and stimulated Raman scattering are discussed. For scattering by electronic levels of impurity atoms in crystals, it is found that the Raman intensity achieves a constant maximum value when the absorption coefficient at the exciting frequency is much larger than the inverse sample length. It is assumed that the absorption in this case is produced by allowed transitions of the impurity atoms themselves. For Raman scattering by lattice vibrations, the effects of proximity of the exciting frequency to the intrinsic electronic absorption edge are investigated. It is found that the Raman efficiency increases steeply as the exciting frequency approaches the absorption edge from below. The efficiency falls to a small constant value for frequencies above the edge, while the absorption coefficient increases rapidly in this region. The optimum exciting frequency to use is the frequency at which the absorption coefficient begins to rise significantly from its zero value below the edge.