Bulk crystals with nominal V dopant concentrations of between 1000 and 7000ppm were studied. Three V states, V+, V2+ and V3+, were identified by means of low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5E(D)  5T2(D) transition of V+(d4) at 3401/cm (0.422eV), for the 4T2(F)  4T1(F) transition of V2+(d3) at 4056/cm (0.503eV) and for the 3T2(F)  3A2(F) transition of V3+(d2) at 4726/cm (0.586eV). The energies of the internal transitions were reduced with respect to the corresponding transitions in V-doped ZnS and ZnSe. The respective excitation spectra exhibited, in addition to broad charge-transfer bands, higher excited levels of the individual charge states. Crystal-field calculations of the detected transition energies were presented that were based upon the Tanabe-Sugano scheme. A 1-electron model was developed in which the donor level (V2+/V3+) was situated at 12500/cm (1.55eV) below the conduction-band edge and the acceptor level (V2+/V+) was situated at 9400/cm (1.17eV) above the valence-band edge. The dynamic behavior of the 3 infra-red luminescence bands was measured. Decay-time constants of 43s (V+), 120s (V2+) and 420s (V3+) were found. Electron paramagnetic resonance results for the same samples confirmed the optical detection of isolated substitutional V2+(d3) and V3+(d2) ions.

P.Peka, M.U.Lehr, H.J.Schulz, U.W.Pohl, J.Kreissl, K.Irmscher: Physical Review B, 1996, 53[4], 1907-16