The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4-40 eV or electron pulses have been studied in ␣-Al 2 O 3 single crystals at 8 K. The intrinsic A ͑7.6 eV͒ and E emissions ͑3.77 eV͒ can be effectively excited in the region of long-wavelength ͑8.85-9.1 eV͒ and short-wavelength ͑9.1-9.3 eV͒ components of exciton absorption doublet, respectively. Fast ͑ϳ6 and ϳ20 ns͒ and slow ͑ϳ150 ns͒ components of the A emission correspond to the creation of singlet and triplet p 5 s 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 quadratically͒ with a rise of the excitation density by nanosecond electron pulses. In Al 2 O 3 :Sc, the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations ͑ϳ8.5 eV͒ as well as by the electron recombination with holes localized near Sc 3ϩ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of hϾ25 eV. One photon of 26-29 and 30-37 eV causes the ionization of the 2p 6 or 2s 2 shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of ␣-Al 2 O 3 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. ͓S0163-1829͑99͒01825-1͔

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Investigation of radiation defects induced by the irradiation of LiF crystals with 5-or 10-MeV Au ions ͑fluences of 10 11-2ϫ 10 14 ions/ cm 2 ; flux varies by 2 orders of magnitude͒ at room temperature has been performed using the methods of optical absorption and high-temperature ͑400-750 K͒ thermoactivation spectroscopy. The creation efficiency of color centers ͑F , F 2 , F 3 ,. . .͒ and colloids drastically depends on both the fluence and ion flux ͑beam current͒. Besides impurity ͑magnesium͒ colloids with the absorption band peaked at 4.4-4.6 eV, the broad absorption band at 2.3-3.3 eV related to intrinsic Li colloids is reliably distinguished. The creation efficiency of Li colloids by 5-MeV Au ions is lower than that by 10-MeV ions, which form ␦ electrons with higher energies sufficient for the creation of cation excitons ͑ϳ62 eV͒. The cation exciton decays, in turn, with the formation of a group of spatially close F centers. At a high ion flux, the next bombarding ions hit the same crystal region with a small time delay ͑10-100 s͒ and also form, after similar intermediate processes, the groups of F centers that participate in the formation of stable agglomerates of several F 3 or even more complex centers, which serve as stable ͑up to 620 K͒ seeds for nanosize Li colloids. The peculiarities of the formation, enlargement, and annealing of intrinsic colloids in LiF crystals are considered, invoking a formal analog with the processes in photographic materials based on silver halides.

Optical transitions associated with γ -radiation-induced defects in crystalline α-quartz were investigated by photoluminescence excited by both pulsed synchrotron radiation and steady-state light. After a 10 MGy γ -dose we observed two emissions at 4.9 eV (ultraviolet band) and 2.7 eV (blue band) excitable in the range of the induced absorption band at 7.6 eV. These two luminescence bands show a different temperature dependence: the ultraviolet band becomes bright below 80 K; the blue band increases below 180 K, but drops down below 80 K. Both emissions decay in a timescale of a few ns under pulsed excitation, however the blue band could also be observed in slow recombination processes and it afterglows in about 100 s at the end of steady-state excitation. The origin of the observed luminescence bands and the comparison with optical features of oxygen-deficient centres in silica glass are discussed in the framework of different models proposed in the literature.

The thermal quenching or plastic strain of RbCI, KC1, and KBr crystals leads to an increase in the number of associations of anion and cation vacancies. Using the methods of VUV spectroscopy, electronic excitations localized near divacancies are detected at 5 K. The process of radiative and nonradiative decay of these excitations is studied as well as the process of radiational creation of divacancies.