Forbidden luminescence and resonance Raman scattering of bound exciton states in CdS

A series of six energy levels is found at energy distances 2.5, 3.8, 4.7, 6.1, 7.0, and 8.3 meV on the high energy side of the neutral-acceptor bound exciton I1 in CdS. They appear in the emission and absorption spectra of some freshly grown virgin crystals and of some crystals damaged by laser-irradiation. Resonance Raman scattering (RRS) establishes the new levels as intermediate states in every high quality crystal. These levels are interpreted as excited states of the bound exciton I1. The direct radiative recombination is forbidden due to parity selection rules. These selection rules are removed in symmetry disturbed crystals, at high excitation intensities, or by recombination with phonon participation. In consequence they are detected as forbidden luminescence, as LO phonon replica, or as 1-LO- and 2-LO-Raman lines. The 2-LO scattering cross section of the energy region of bound excitons in CdS is determined. (author)