The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4–40 eV or electron pulses have been studied in $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ 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 $p^{5}s$ 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 quadratically) with a rise of the excitation density by nanosecond electron pulses. In ${\mathrm{Al}}_2{\mathrm{O}}_3:\mathrm{S}\mathrm{c},$ 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 ${\mathrm{Sc}}^{3+}$ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of $h\nu >25\mathrm{}\mathrm{eV}.$ 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 $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ 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.

The peculiarities of multiplication of electronic excitations have been investigated in wide-gap alkali halide crystals where, in contrast to semiconductors, free and self-trapped excitons as well as free and self-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes of secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two different emissions from the same crystal as a function of the energy of exciting photons. The excitation spectra of triplet ($\pi$) and singlet ($\sigma$) emissions of self-trapped excitons as well as of impurity luminescence have been measured, using synchrotron radiation of 12 to 32 eV, for high-purity KI, KBr, and doped KI:Na, KI:Tl, KBr:I, KBr:Tl crystals at 8 K. The analysis of the intensity ratio spectra for two emissions allowed us to separate the regions of secondary triplet exciton formation in KI (13-15 and 26.5-29 eV) and KBr (15-17 and 28-30 eV). The threshold photon energy for the formation of a secondary exciton has been experimentally determined to be equal to 13.1±0.2 eV in KI and 15.2±0.2 eV in KBr. The probability of secondary exciton formation by hot photoelectrons reaches the main maximum at 0.5-0.7 eV above the value of threshold energy. The energy transport to impurity centers by free triplet secondary excitons has been revealed in KI:Na and KBr:I crystals. The decay of $3p^{5}4s$ cation excitons in KI and KBr, generated by 20-eV photons, leads to the formation of a double amount of electron-hole pairs.