Temperature and Excitation Energy Dependence of the Photoionization of the F2 Center in α-Al2O3

Photoionization spectra of the F2 center in α-Al2O3 have been determined in the temperature region from 77 to 400 K by using reversible photoinduced interconversion phenomena of the F- and F2-type centers. It has been shown that photoionization of the F2 center is most effectively initiated by resonant photoexcitation of the F2 center or by excitation with a ∼4.1 eV photon. The efficiency of photoionization increases with increasing temperature, demonstrating that a thermal process is involved for the electron to be promoted from the F2-center excited states to the conduction band under the resonant (or near-resonant) excitation condition. Under excitation in the energy region higher than 4.8 eV, however, the photoionization efficiency of the F2 center is hardly dependent on temperature, indicating that the direct photoionization of the F2 center is possible in this excitation energy region. It has also been demonstrated that the temperature has a substantial effect on lowering the photoionization onset e...