Multiphonon processes in YbS

The resonance of the Raman scattering by the 1 $\mathrm{LO}\left(\Gamma \right)$ phonon and its overtones has been investigated in YbS. The results are interpreted in terms of a configuration-coordinate model which considers a strongly interacting exciton-LO-phonons system. The model is also able to account for multiphonon structure in the wavelength-modulated reflectance. The phonon energy observed in the reflectance spectrum is, however, somewhat smaller than that observed in Raman scattering, a fact which is attributed to a frequency renormalization in the excited state of the exciton.