Electron paramagnetic resonance related to optical charge-transfer processes in ZnSe:Ti

In ZnSe crystals grown by different techniques, titanium ions are incorporated as ${\mathrm{Ti}}^{2+}$(${\mathit{d}}^2$) and ${\mathrm{Ti}}^{3+}$(${\mathit{d}}^1$) centers on Zn sites. A strong Jahn-Teller (JT) effect acts on the doubly degenerate ${}_{}{}^2{}_{}{}^{}$E ground state of ${\mathrm{Ti}}^{3+}$. 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 ${}_{}{}^2{}_{}{}^{}$E${\mathrm{-}}^2$ ${\mathit{A}}_2$ manifold. These effects are distinguished by their angular variations and the g values. ${\mathrm{Ti}}^{2+}$ causes an isotropic EPR signal. Excitation and sensitization spectra of ${\mathrm{Ti}}^{2+}$ and ${\mathrm{Ti}}^{3+}$ luminescence transitions are explained within a one-electron model connecting internal (d-d) with charge-transfer transitions involving the valence and conduction bands. Both charge states are sensitive to illumination with near-infrared light. The ions can be mutually converted, as shown by photo-EPR and sensitization experiments. The ${\mathrm{Ti}}^{2+}$/${\mathrm{Ti}}^{3+}$ donor level is situated approximately 8500 ${\mathrm{cm}}^{\mathrm{-}1}$ below the edge of the conduction band.