Photoluminescence of CdS: V, Cu crystals

In CdS, vanadium on a cadmium site of $C_{3\nu }$ symmetry represents a center at which various luminescence processes take place at low temperatures. In the present case, copper acceptors also participate. Two emission bands with zero-phonon structures at $\overline{\nu }=8681\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\overline{\nu }=4921\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ are ascribed to internal transitions within the ${\mathrm{V}}^{3+}$ ion, i.e., to transitions between levels which are produced in the crystal field by splitting of the degenerate ${}_{}{}^3{}_{}{}^{}F$ term of the free ion: ${}_{}{}^3{}_{}{}^{}T_1^{}{}_{}{}^{}\mathrm{}\left(F\right)\mathrm{}\to {}_{}{}^3{}_{}{}^{}A_2^{}{}_{}{}^{}\mathrm{}\left(F\right)\mathrm{}$ and ${}_{}{}^3{}_{}{}^{}T_2^{}{}_{}{}^{}\mathrm{}\left(F\right)\mathrm{}\to {}_{}{}^3{}_{}{}^{}A_2^{}{}_{}{}^{}\mathrm{}\left(F\right)\mathrm{}$, respectively. A further broad emission band at $\overline{\nu }=14\mathrm{}500$ ${\mathrm{cm}}^{-1\mathrm{}}$ seems to involve a vanadium donor which is part of an associated center. The calculated Tanabe-Sugano diagrams allow an interpretation of the measured excitation spectra and yield fitted Racah and crystal-field parameters. Charge-transfer processes for vanadium and copper ions are discussed in the band model.