Electron Spin Resonance of the ExcitedE2(3d3)Level ofCr3+andV2+in MgO

The electron spin resonance of the ${}_{}{}^2{}_{}{}^{}E$ level of the isoelectronic $3d^3$ ions, ${\mathrm{V}}^{2+}$ and ${\mathrm{Cr}}^{3+}$, in MgO is examined using an optical-detection technique. Sizeable, anisotropic $g$ shifts are observed, which are quantitatively accounted for by the orbital moment admixed into ${}_{}{}^2{}_{}{}^{}E$ by configuration mixing and spin-orbit coupling. The hfs observed for ${\mathrm{V}}^{51}$ in is agreement with quantitative estimates based on the observed $g$ shift. The resonances observed optically are anisotropically broadened and show sample-dependent asymmetries. The broadening is found to result from the influence of random local strain on the orbital degeneracy of ${}_{}{}^2{}_{}{}^{}E$. Asymmetric resonances are expected on the basis of the selection rules for the circular-polarized fluorescence used to detect the resonances. These effects are modified considerably by the details of thermal and cross relaxation among the four sublevels of ${}_{}{}^2{}_{}{}^{}E$. On this basis, the sample dependence of the resonance shapes is attributed to a type of cross relaxation due to the presence of ${\mathrm{Fe}}^{2+}$ in varying quantities in the samples studied. Samples having spin-lattice relaxation rates for ${}_{}{}^2{}_{}{}^{}E$ which agree with those calculated from static-strain-splitting data have asymmetric resonances of the type expected from the fluorescence selection rules.