In ZnTe:V bulk crystals with nominal vanadium concentrations between 1000 and 7000 ppm three vanadium-ion states V ϩ , V 2ϩ , and V 3ϩ were found in low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5 E(D)→ 5 T 2 (D) transition of V ϩ ͑d 4 ͒ at 3401 cm Ϫ1 ͑0.422 eV͒, for 4 T 2 (F)→ 4 T 1 (F) of V 2ϩ ͑d 3 ͒ at 4056 cm Ϫ1 ͑0.503 eV͒, and for 3 T 2 (F)→ 3 A 2 (F) of V 3ϩ ͑d 2 ͒ at 4726 cm Ϫ1 ͑0.586 eV͒. The energies of the internal transitions are reduced with respect to the corresponding transitions in ZnS:V and ZnSe:V. The respective excitation spectra display, in addition to broad charge-transfer bands, higher excited levels of the individual charge states. Crystal-field calculations of the detected transition energies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in which the donor level ͑V 2ϩ /V 3ϩ ͒ is situated 12 500 cm Ϫ1 ͑1.55 eV͒ below the conduction-band edge and the acceptor level ͑V 2ϩ /V ϩ ͒ 9400 cm Ϫ1 ͑1.17 eV͒ above the valence-band edge. The dynamical behavior of the three infrared luminescence bands was measured. Decay time constants of 43 s ͑V ϩ ͒, 120 s ͑V 2ϩ ͒, and 420 s ͑V 3ϩ ͒ were found. Electron-paramagnetic-resonance ͑EPR͒ results measured on the same samples are presented in an accompanying paper and confirm the optical detection of isolated substitutional V 2ϩ ͑d 3 ͒ and V 3ϩ ͑d 2 ͒ ions. Relations between the EPR and optical results are discussed.

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Four V-related electron-paramagnetic-resonance ͑EPR͒ spectra are observed in Bridgman-grown ZnTe doped with vanadium. Two of them are attributed to the charge states V Zn 3ϩ ͑A ϩ ͒ and V Zn 2ϩ ͑A 0 ͒ of the isolated V impurity. For the ionized donor, V Zn 3ϩ ͑A ϩ ͒, the spectrum reveals the typical behavior of the expected 3 A 2 (F) ground state in tetrahedral symmetry. The incorporation on a cation lattice site could be proved by the resolved superhyperfine interaction with four Te ions. The second spectrum showing triclinic symmetry and Sϭ 3 2 is interpreted as the neutral donor state V Zn 2ϩ ͑A 0 ͒. The origin of the triclinic distortion of the cubic (T d ) crystal field could be a static Jahn-Teller effect. The two additionally observed EPR spectra are attributed to nearest-neighbor V-related defect pairs. The spectrum of the first one, V Zn 2ϩ

In ZnSe crystals grown by different techniques, titanium ions are incorporated as Ti +(d ) andTi +(d') centers on Zn sites. A strong Jahn-Teller (JT) effect acts on the doubly degenerate E ground state of Ti3+. The electron paramagnetic resonance (EPR) at T =3 K indicates a quasistatic JT effect of the ground state and a quasidynamic one of the first excited state for the strain-split vibronic E-A2 manifold. These effects are distinguished by their angular variations and the g values. Ti + causes an isotropic EPR signal. Excitation and sensitization spectra of Ti + and Ti'+ luminescence transitions are explained within a one-electron mode1 connecting internal (d-d) with charge-transfer transitions involving the valence and conduction bands. Both charge states are sensitive to illumination with nearinfrared light. The ions can be mutually converted, as shown by photo-EPR and sensitization experiments. The Ti +/Ti'+ donor level is situated approximately 8500 cm ' below the edge of the conduction band.