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Abstract

The luminescence of synthetic ZnS: V crystals is studied at low temperatures (T ≈ 4 K). The emission spectra comprise (i) a structured band centred around v = 5600 cm^-1, assigned to ³T₂(F) -> ³A₂(F) transitions of substitutional V³⁺ (3d²) ions in a tetrahedral field, and (ii) a band around 4800 cm^-1 assigned to ⁴T₂(F) -> ⁴T₁(F) transitions of V²⁺ (3d³) ions. In the range of the narrow no-phonon lines detected with both these emission bands, a temperature rise from 2 to 5 K entails a thermalisation in the population of the spin-orbit multiplets forming the initial states of the corresponding transitions. A third emission band near 3800 cm^-1 grows after annealing the crystals in Zn vapour. It is tentatively attributed to ⁵E(D) -> ⁵T₂(D) transitions of V⁺ (3d⁴). A model of one-electron states represents donor-type as well as acceptor-type changes of the vanadium oxidation states, commencing from V²⁺, the state with neutral effective charge. The model is substantially founded on the measured excitation spectra of the V³⁺ and V²⁺ emission bands, supplemented by transmission spectra. Besides the corresponding broad charge transfer bands, the spectra display a number of structures which are associated with excited states of the ions. These energy levels are approximated in a computation following the strong-field Tanabe-Sugano scheme but in addition allowing for different radial extensions of e- and t₂-type wave functions. The method used also includes the possibility of fitting the free-ion levels. Various sets of numerical values are eventually obtained for the crystal-field splitting and the Racah parameters. Some of the levels involved are found to be subject to Jahn-Teller interaction.