Dynamic Jahn-Teller effect in the EPR spectrum ofNi1+andNi3+in magnesium oxide

An electron-spin-resonance study of the dynamic Jahn-Teller effect in ${\mathrm{Ni}}^{1+}$ and ${\mathrm{Ni}}^{3+}$ in MgO at liquid-helium temperature is reported. For both ions the spectra arise from strain-split ground vibronic doublets, with a small admixture of an excited singlet. From the anisotropy of the linewidth it follows that the symmetry of this singlet is $A_2$ for ${\mathrm{Ni}}^{1+}$ and $A_1$ for ${\mathrm{Ni}}^{3+}$.