Abstract
The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4–40 eV or electron pulses have been studied in single crystals at 8 K. The intrinsic A (7.6 eV) and E emissions (3.77 eV) can be effectively excited in the region of long-wavelength (8.85–9.1 eV) and short-wavelength (9.1–9.3 eV) components of exciton absorption doublet, respectively. Fast (∼6 and ∼20 ns) and slow (∼150 ns) components of the A emission correspond to the creation of singlet and triplet excitons. The efficiency of the A emission in the region of band-to-band transitions is low. The intensity of A emission sharply increases (approximately quadratically) with a rise of the excitation density by nanosecond electron pulses. In the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations (∼8.5 eV) as well as by the electron recombination with holes localized near centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of One photon of 26–29 and 30–37 eV causes the ionization of the or shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excited at the direct photocreation of excitons or at the recombination of free electrons and free holes is interpreted by us as the radiative decay of self-shrunk excitons. The theoretical model of Sumi allows the existence of such immobile self-shrunk excitons even if an electron and a hole do not separately undergo the self-trapping.
- Received 30 November 1998
DOI:https://doi.org/10.1103/PhysRevB.60.502
©1999 American Physical Society