Multiphonon Vibronic TransitionT2g1−A2g3ofNi2+in MgO

The ${\Gamma }_{5g}({}_{}{}^1{}_{}{}^{}T_{2g}^{}{}_{}{}^{},t_{2g}^5{e}_g^3)-{\Gamma }_{5g}({}_{}{}^3{}_{}{}^{}A_{2g}^{}{}_{}{}^{},t_{2g}^6{e}_g^2)$ vibronic spectrum is recorded in absorption and emission—under x-ray excitation—at crystal temperatures of 77 and 5 °K. The spectrum involves up to four phonon processes and a theoretical treatment of such a transition is presented. A possible explanation of the band is suggested and, by using imperfect-lattice Green's functions for the nearest-neighbor motion, it is constructed to give excellent agreement with experiment. The transition is forced (electric dipole) by one of the $T_{1u}$ vibrations of the nearest-neighbor complex. Additional $E_g$ vibrations couple in the higher phonon processes which suggest the presence of a Jahn- Teller distortion in the ${}_{}{}^1{}_{}{}^{}T_{2g}^{}{}_{}{}^{}$ excited state. However, it has not been possible to establish the presence of such a Jahn- Teller distortion from any other experimental data. A similar calculation is undertaken for the two-phonon band of the ${\Gamma }_{3g}({}_{}{}^3{}_{}{}^{}T_{2g}^{}{}_{}{}^{},t_{2g}^5,e_g^3)-{\Gamma }_{5g}({}_{}{}^3{}_{}{}^{}A_{2g}^{}{}_{}{}^{},t_{2g}^6{e}_g^2)$ transition of ${\mathrm{Ni}}^{2+}$ in MgO. Again there is an indication of a coupling to $E_g$ vibrations in the two-phonon process.