Resonant multiphonon Raman scattering in AgBr

Resonant Raman scattering in AgBr single crystals is studied at low temperatures. Excitation in resonance with the indirectΓ — L exciton as intermediate state gives rise to selectively enhanced narrow two-phonon Raman scattering. The phonons involved are pairs ofLA andTO phonons of opposite wavevectors near theL-point. A simple model involving one discrete exciton level is developed to explain the resonance behaviour. The temperature dependence of the scattered intensity, that is studied for 1.8 K <T < 35 K, can consistently be interpreted within this model as being due to the lifetime of the intermediate state. Assuming that the excitons predominantly decay by one-phonon scattering with long wavelength acoustical phonons the predicted temperature dependence of the intensity is found in good agreement with the experimental result. Additional scattering peaks are believed to be due to third-order processes involving an acousticalX-phonon in addition to theL-phonons of the second-order scattering. Using an oriented sample the resonant Raman peaks are found to be polarized.