Molecular Mechanism of Stimulated Raman Scattering

Abstract

The nature of the four‐photon molecular scattering process involved in stimulated Raman scattering is considered by means of collision theory. An accurate expression is obtained for the scattering amplitude which explicitly illustrates its partitioning into coupled Stokes and anti‐Stokes scattering processes. The dependence of the cross section for stimulated Raman scattering upon the lifetime of the excited vibrational state is derived. Such a dependence was assumed from phenomenological considerations by Bloembergen and Shen and is consistent with the observation that stimulated Raman scattering is observed only for vibrations having narrow linewidths. The lifetime dependence of the cross section is shown to be in agreement with the two‐step mechanism of stimulated Raman scattering involving subsequent Stokes and anti‐Stokes scattering processes, and contradictory to a single‐step mechanism proposed by Buckingham. The single‐step mechanism is shown to correspond to a physically unrealizable limit of the two‐step process.