Lattice-Vibration Effects in the Spectra of ZnO:Ni and ZnO:Co

The optical absorption spectra of ZnO:${\mathrm{Ni}}^{2+}$ and ZnO:${\mathrm{Co}}^{2+}$ have been studied in the visible region, at temperatures from 7°K up. From correlation with lattice phonon energies, evidence has been found for the assignment of much of the band structure to vibronic sidebands. For both crystals, some weak lines on the low-energy side of the bands have been shown by their concentration dependence to be due to transition-ion pairs. In ZnO:Ni, a level whose polarization shows it to be an $A_1$ appears 8 ${\mathrm{cm}}^{-1\mathrm{}}$ above the strong $E-T_2\left({}_{}{}^3{}_{}{}^{}P\right)$ transition. It is suggested that the $A_1$ level is produced by the Jahn-Teller interaction.