Multiple-Phonon Resonant Raman Scattering in CdS

We have observed nth order ($n=1,2,\ensuremath{\cdots},9$) Raman scattering in CdS under conditions of resonance between the laser frequency and the band gap or the associated exciton states. The frequency spectrum of these higher order processes demonstrates that the phonons involved are localized within a small volume of $k$ space centered at $\ensuremath{\Gamma}$. We believe that such spectral features are characteristic of direct-gap resonant scattering, and the kinematics of their production is discussed.