Quenching of stimulated Raman scattering by two-photon absorption in fuels

There is renewed interest in stimulated Raman scattering in organic liquids because of its potential for droplet sizing in fuel sprays. Contrary to the accepted theories, we observed gain on more than one line simultaneously. In all but the simplest aromatics multiple lines are observed and some expected lines are missing. Absorption spectra suggest that two-photon absorption could quench the expected Raman signal. A model using real values of two photon cross-sections shows that in polyaromatics, these are a source loss term for the Raman signal. The resulting disperion in the loss profile leads to gain on the CH stretch rather than the ring breathing mode predicted from the spontaneous spectrum.