Effect of Uniaxial Stresses on theE4Level of a3d5Ion in Tetrahedral Symmetry: Study ofMn++in ZnS

The fine structure of the lines appearing in the 21 500-${\mathrm{cm}}^{-1\mathrm{}}$ absorption band of ${\mathrm{Mn}}^{++}$ in ZnS has been studied in great detail. We show that the three sharp zero-phonon lines at 21 242.5, 21 238, and 21 233.5 ${\mathrm{cm}}^{-1\mathrm{}}$ are due to the transitions ${}_{}{}^6{}_{}{}^{}A_1^{}{}_{}{}^{}\to {\Gamma }_6\left({}_{}{}^4{}_{}{}^{}E\right)$, ${}_{}{}^6{}_{}{}^{}A_1^{}{}_{}{}^{}\to {\Gamma }_8\left({}_{}{}^4{}_{}{}^{}E\right)$, and ${}_{}{}^6{}_{}{}^{}A_1^{}{}_{}{}^{}\to {\Gamma }_7\left({}_{}{}^4{}_{}{}^{}E\right)$ of ${\mathrm{Mn}}^{++}$ in $T_d$ symmetry, the degeneracy of the ${}_{}{}^4{}_{}{}^{}E$ level being lifted by the first-order effect of the spin-spin interaction and by the second-order effect of the spin-orbit interaction. To confirm these assignments, we performed a uniaxial-stress experiment on these levels by applying a pressure along the [$1\overline{1}0$] crystallographic axis. We show that the observed splitting of the degenerate ${\Gamma }_8$ level (23×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$/dyn) can be interpreted as being due to the combined action of the induced rhombic distortion and the spin-orbit interaction. Furthermore, we show that the other sharp phonon-assisted lines of the optical spectra behave like cubic ${\Gamma }_6$, ${\Gamma }_8$, and ${\Gamma }_7$ levels. Complementary studies regarding the polarization and intensities of these cubic lines have been performed in order to reinforce these interpretations. Finally, we studied the case of a pressure-induced trigonal distortion and the splitting of the ${}_{}{}^4{}_{}{}^{}E$ level of ${\mathrm{Mn}}^{++}$ in the hexagonal phase.