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

Abstract

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 ${\mathrm{cm}}^{\ensuremath{-}1}$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$ correspond, respectively, to transitions from the $^{6}A_{1}$ ground state to the $|{\ensuremath{\Gamma}}_{8}(\frac{5}{2})〉$ and $|{\ensuremath{\Gamma}}_{6}〉$ 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.