Structure of aT24level ofMn++in tetrahedral symmetry, dynamical Jahn-Teller effect and selective intensity transfer

An experimental study under uniaxial stresses and a theoretical analysis of the zero-phonon lines of the lowest ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ level of ${\mathrm{Mn}}^{++}$ in ZnSe are reported. We first show that a classical model in which vibronic interactions are neglected must be rejected since it predicts a zero-stress spectrum of the zero-phonon lines consisting of three absorption lines with relative dipole strengths of 14, 7, and 9 although only two absorption lines at 19597 and 19 607 ${\mathrm{cm}}^{-1\mathrm{}}$ are observed. Then, in order to interpret this unusual structure we consider Ham's effect corresponding to Jahn-Teller interactions of the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ electronic state with $E$ vibrational modes. It is shown that the hypothesis of strong Jahn-Teller interactions is consistent with the zero-stress spectrum but inconsistent with pressure-induced splittings and polarizations. Finally a model permitting a correct interpretation of all experimental results has been elaborated by considering a medium coupling to $E$ vibrational modes. We show that the observed lines at 19 597 and 19 607 ${\mathrm{cm}}^{-1\mathrm{}}$ correspond, respectively, to transitions from the ${}_{}{}^6{}_{}{}^{}A_1^{}{}_{}{}^{}$ ground state to the $\left|{\Gamma }_8\right(\frac{5}{2})〉$ and $|{\Gamma }_6〉$ fundamental vibronic levels of the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ state. Furthermore, a careful analysis of the dipole strengths of the zero-phonon lines and phonon-assisted lines shows the importance of intensity transfer from the zero-phonon lines to the phonon-assisted lines. The case of ${\mathrm{Mn}}^{++}$ in ZnS is also briefly considered.