The optical absorption band in single-crystal ${\mathrm{Al}}_2$${\mathrm{O}}_3$ at 6.1 eV associated with atomic-displacement-type damage has been investigated through its relationship with the 3.0-eV $V_{\mathrm{OH}}^{-}$ hole center whose identity had been previously established. Using lightly-neutron-irradiated [(1-4) × ${10}^{15}$ reactor neutrons/${\mathrm{cm}}^2$] thick crystals to facilitate measurement of the absorption spectra, it was shown that subsequent room-temperature ${}_{}{}^{137}{}_{}{}^{}\mathrm{Cs}$ $\gamma$ irradiation produced a further enhancement in the 6.1-eV band as well as developing a composite $V$ band near 3.0 eV whose intensity bears a linear relationship to that of the 6.1-eV band. Optical bleaching at 410 nm (3 eV) decreases both the $\gamma$ and 6.1-eV bands again with a linear relationship which indicates an oscillator-strength ratio of $f_{\frac{204}{f410\mathrm{}}}$ of ∼ 19. Irradiation in the 6.1-eV band likewise causes a decrease in the 410-nm region. These results indicate that (i) the 6.1-eV nm band is due to an electron trapped at some defect termed the $d$ center produced by atomic-displacement processes, and (ii) the 6.1-eV band reflects the concentration of trapped holes rather than the concentration of $d$ centers. Isochronal annealing causes thermal decay of the 6.1-eV band in the range 300 to 600° C in these reactor-irradiated specimens.

• Received 4 August 1975

DOI:https://doi.org/10.1103/PhysRevB.13.1735