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Thermoluminescence properties of the Eu2+-, R3+-doped calcium aluminate materials, CaAl2O4:Eu2+,R3+, were studied above room temperature. The trap depths were estimated with the aid of the preheating and initial rise methods. The seemingly simple glow curve of CaAl2O4:Eu2+ peaking at ca. 80 degrees C was found to correspond to several traps. The Nd3+ and Tm3+ ions, which enhance most the intensity of the high-temperature TL peaks, form the most suitable traps for intense and long-lasting persistent luminescence, too. The location of the 4f and 5d ground levels of the R3+ and R2+ ions were deduced in relation to the band structure of CaAl2O4. No clear correlation was found between the trap depths and the R3+ or R2+ level locations. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions. A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu2+ to the conduction band followed by the electron trapping to an oxygen vacancy, which is aggregated with a calcium vacancy and a R3+ ion. The migration of the electron from one trap to another and also to the aggregated R3+ ion forming R2+ (or R3+-e-) is then occurring. The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence. The ability of the R3+ ions to trap electrons is probably based on the different reduction potentials and size of the R3+ ions. Hole trapping to a calcium vacancy and/or the R3+ ion may also occur. The mechanism presented can also explain why Na+, Sm3+, and Yb3+ suppress the persistent luminescence.

The properties of green thermoluminescence (TL) and persistent luminescence of Lu 2 O 3 :Tb 3+ and Lu 2 O 3 :Tb 3+ ,Ca 2+ materials sintered in vacuum at 1700 °C were investigated. The concentration of Tb varied in the range 0.1-3 mol %, and the Ca content was 1 mol %. Ca 2+ codoping enhanced the room temperature persistent luminescence intensity and its duration as well as reduced the number of TL bands for lightly doped materials from four components covering about 50-400 °C range of temperatures to only one peaking around 100 °C. The Tb 3+ ,Ca 2+ (0.1 and 1 mol %, respectively) codoped material showed the most efficient persistent luminescence and TL, originating mainly from the 5 D 4 f 7 F 5 transition at around 545 nm, among all the compositions investigated. For this material the persistent luminescence could be observed in the dark for about 15 h. There are indications that the efficient persistent luminescence of the codoped system is governed by tunneling mechanism, and the trapping centers are postulated to be [Tb Lu × -V O •• -2Ca Lu ' ] aggregates. Oxygen vacancies are supposed to serve as traps for free electrons giving F + (e O •• ) or F (V O × ) centers while holes are temporarily immobilized in the vicinity of Tb Lu × giving [Tb Lu × -h • ] entities. Alternatively, hole can be trapped in the Ca Lu ' site due to its negative net charge, giving [Ca Lu ' -h • ]. Air-sintered specimens did not show any significant persistent luminescence or TL, although they produce quite significant photoluminescence. Also, vacuum sintering at lower temperatures-1600 °C and below-was not sufficient to get efficient persistent luminescence.

A systematic spectroscopic study of the 4 f 7 energy levels of Gd 3ϩ in LiYF 4 in the vacuum-ultraviolet spectral region (50 000-70 000 cm Ϫ1) is reported. Using energy-level calculations, all observed spectral lines could be assigned to free-ion term symbols ͑including term symbols with unusually high L and J, e.g., a 2 Q 23/2 level around 67 000 cm Ϫ1 ͒. From the 6 G J levels around 50 000 cm Ϫ1 quantum cutting ͑or two-photon luminescence, photon-cascade emission͒ is observed: the emission of a red photon due to the 6 G J → 6 P J transition is followed by the emission of an ultraviolet photon due to the 6 P J → 8 S 7/2 transition.

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