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

Photoionization spectra of the F 2 center in α-Al 2 O 3 have been determined in the temperature region from 77 to 400 K by using reversible photoinduced interconversion phenomena of the F - and F 2 -type centers. It has been shown that photoionization of the F 2 center is most effectively initiated by resonant photoexcitation of the F 2 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 F 2 -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 F 2 center is hardly dependent on temperature, indicating that the direct photoionization of the F 2 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 energy, which can be varied from ∼3.85 eV at 77 K to ∼3.60 eV at 400 K. This temperature dependence of the photoionization onset energy can be interpreted in terms of the strong coupling between the photoexited electron and the phonon modes of the crystal.