The absorption and fluorescent spectra of "pure" CdS platelets (impurity concentrations ∼${10}^{17}$/cc) have been studied at low temperatures. In addition to intrinsic exciton lines, many sharp absorption and fluorescent lines were observed at slightly lower energies. Many of these lines are due to transitions involving bound excitons, in which the light creates (or destroys) an exciton bound to a neutral or charged donor or acceptor. Arguments from the known-band symmetries and electron $g$ values permit the identification of the generic centers (neutral donor, neutral acceptor, charged center) with which particular transitions are associated. Transitions in which excitons from the second valence band are bound were also observed, in some cases lifetime broadened by phonon transitions to excitons from the first valence band. States due to neutral donors and neutral acceptors are usually both observed in the same crystal. This nonequilibrium situation is caused by the trapping of holes made by the light which is being used to study the crystal. The "bleaching" of the trapped holes by the application of infrared light during ordinary transmission measurements near the band gap supports the generic transition assignments given. Centers of appropriate symmetries exhibit splittings due to electron-hole and hole-hole j-j coupling. The magnitudes of these splittings agree with crude theoretical estimates. The oscillator strength per center should be directly related to these splittings for the case of excitons bound to neutral impurities. Several different donors are discernible, but only a single acceptor is observed. The generic classification and energies of the observed centers should make possible the combined chemical and optical identification of the corresponding donors and acceptor in doped crystals. The arguments which permit the identification of the centers can easily be generalized to the case of cubic crystals.

• Received 29 June 1962

DOI:https://doi.org/10.1103/PhysRev.128.2135