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Abstract

A model of one-electron states is devised to describe within the strong-field limit the chromium states Cr²⁺(d⁴) and Cr⁺(d⁵) in tetrahedral crystal fields. The proposed advancement is the inclusion of spin-correlation effects, i.e. a differentiation between the energies of spin-up and spin-down electron configurations. The method comprises the known spin-allowed ⁵E(D)⇆⁵T₂(D) transitions of Cr²⁺ and, above all, spin-forbidden transitions. The ⁴T₁(G)→⁶A₁(S) luminescence of ZnSe:Cr⁺ peaking near v=10 300cm^-1 is interpreted as a t_2↓→e_↑ transition of the d⁵ configuration. A weak emission band of ZnSe:Cr peaking near 6100 cm⁻ at cryo-temperatures and its analogues in ZnS and other II–VI materials are now assigned to the t_2↑t_2↓e_↑²→t_2↑²e_↑² spin-flip process of the Cr²⁺ (d⁴) configuration whose Jahn-Teller-stabilised ground state had earlier been proved to participate in this luminescence. The described emission bands provide for the first time direct means to derive from an experiment the spin-flip energy within the one-electron limit.