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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

The reflection spectra of CaWO, and CaMoO, single crystals were measured in the region between 4 and 25 eV. The measurements were performed using the electron synchrotron DESY as a light source. Both crystals show very similar spectral features. A relatively sharp, well separated low energy peak is accompanied on the high energy side by two groups of maxima, each group containing three broad main reflection bands which strongly overlap. Besides this, a less pronounced structure is observed over the whole spectral region.The experimental results are discussed in terms of semiempirical MO calculations of the anionic transition metal complexes.Die Reflexionsspektren von CaW0,-und CaMo0,-Einkristallen wurden zwischen 4 und 25 eV gemessen. Das Elektronensynchrotron DESY diente als Lichtquelle. Die Spektren der beiden Kristalle sind sehr Sihnlich. Ein relativ scharfer, wohl getrennter, niederenergetischer Peak wird zu kiirzeren Wellenlangen hin jeweils von zwei Bandengruppen gefolgt, die im wesentlichen aus drei sich stark uberlappenden Maxima aufgebaut sind. Daneben ist iiber den ganzen Spektralbereich eine weniger ausgepragte Struktur zu beobachten. Die experimentellen Ergebnisse werden mit Hilfe semiempirischer MO-Berechnungen der Anionenkomplexe diskutiert.