Charge-Transfer Spectra of Transition-Metal Ions in Corundum

The ultraviolet absorption spectrum of corundum (${\mathrm{Al}}_2$ ${\mathrm{O}}_3$) containing dilute concentrations of the $3d$-series transition-metal ions ${\mathrm{Ti}}^{3+}$, ${\mathrm{V}}^{3+}$, ${\mathrm{Cr}}^{3+}$, ${\mathrm{Mn}}^{4+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Ni}}^{3+}$ was measured at room temperature and at liquid-nitrogen temperature for photon energies in the range 3-9 eV. The most important features of the observed spectra are (a) the peak positions are characteristic of the particular impurity ion; (b) the peak positions and widths (∼0.7 eV) are independent of temperature over the range studied; and (c) the integrated intensities are strong. The spectra are identified with the allowed transition whereby an electron is transferred from a nonbonding orbital, localized predominantly on the ${\mathrm{O}}^{=}$ ligands, to either the $t_{2g}\left({\pi }^{*}\right)$ or $e_g\left({\sigma }^{*}\right)$ antibonding orbital, localized predominantly on the metal ion. The position in energy of the first absorption peak for the various ions of the series relative to one another and to the position of the intrinsic absorption edge of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ is in good agreement with the charge-transfer model.