Electron centers in single-crystalAl2O3

The two prominent absorption bands centered at 6.1 and 4.8 eV in neutron-bombarded ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ single crystals have been investigated. Optical excitation into the 6.1 eV causes growth of bands at 5.4, 4.8, and 4.1 eV, whereas excitation into these latter bands causes growth of the 6.1-eV band. The oscillator strength of the 4.8-eV band was found to be in the range 0.5-1 as compared to a value nearly twice that for the 6.1-eV band. Excitation with 4.8-eV light produces an emission band at 3.75 eV and the yield and polarization of this emission as a function of the polarization of the exciting light was studied. This polarization behavior together with previous theoretical calculations is consistent with an assignment of the 4.8-eV band to the transition of the $F^{+}$ center. The oscillator strength ratio and the reciprocal relationship between the 4.8-eV band and the 6.1-eV band strongly indicate that the latter is associated with the $F$ center.