Optical Detection of Spin-Lattice Relaxation and hfs in the ExcitedE¯(E2)State ofV2+andMn4+inAl2O3

The hyperfine structure (hfs) and spin-lattice relaxation in the excited $\overline{E}\left({}_{}{}^2{}_{}{}^{}E\right)$ states of ${\mathrm{V}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ have been studied by optical-detection techniques. This is an extension of an earlier study of EPR (electron paramagnetic resonance) in the excited $\overline{E}\left({}_{}{}^2{}_{}{}^{}E\right)$ state of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$. The experimental results for all three isoelectronic ions in the same host lattice allow us to make meaningful comparisons with theoretical ideas of hfs and spin-lattice relaxation. A well-resolved hfs is found for ${\mathrm{V}}^{2+}$ and ${\mathrm{Mn}}^{4+}$, which is in contrast to the absence of hfs in the case of the isoelectronic ${\mathrm{Cr}}^{53}$ ion studied earlier. The values of the hyperfine-splitting parameter for the three ions can be adequately explained by considering the combined effects of the corepolarization hyperfine field, orbital hyperfine field, and dipolar hyperfine field. The experimental data are fitted to a simple spin Hamiltonian for an effective spin $S=\frac{1}{2}$ with $\left|g_{\parallel }\right|=2.2198\mathrm{}\pm 0.001$, and $\left|A_{\parallel }\right|=(46.3\mathrm{}\pm 1.5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{V}}^{2+}$, and $\left|g_{\parallel }\right|=3.0959\mathrm{}\pm 0.0006$, and $\left|A_{\parallel }\right|=(123\mathrm{}\pm 3)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Mn}}^{4+}$. Since $g_{\perp }\sim 0$ for both ions, $A_{\perp }$ could not be determined. In the temperature range in which $T_1$ could be measured (1.4-2.15°K for