We have observed resonant Brillouin scattering by TA phonons near the $A$ exciton of CdS in a usually forbidden backscattering configuration. This effect, attributed to electron-phonon piezoelectric coupling, is equivalent to the Fröhlich-interaction-induced forbidden LO scattering. A numerical estimate of the scattering strength of the forbidden TA scattering accounts for the intensity of the observed lines.

• Received 1 September 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.1286