Quasiresonant Raman Scattering from TlBr Crystals

Second-order Raman-scattering spectra have been measured in TlBr single crystals as a function of exciting-light frequency at low temperatures. The results obtained indicate that as the exciting frequency approaches the first exciton peak frequency, a new resonant-scattering mechanism becomes important, as evidenced by the appearance and rapid increase of a sharp peak at twice the LO phonon frequency ($q=0$), and by the absence of such a feature in analogous spectra taken for TlCl, where only off-resonant scattering is expected to occur.