Observation, investigation, and characterization of vacuum ultraviolet (VUV) ͑ϳ10 eV͒ luminescence due to interconfigurational 4f n−1 5d-4f n transitions in Gd 3+ and Lu 3+ are reported. High-resolution ͑ϳ0.5 Å͒ VUV emission and excitation spectra as well as decay kinetics of VUV luminescence have been measured for LiGdF 4 , GdF 3 , LiLuF 4 , and LuF 3 crystals using synchrotron radiation. The obtained results are the direct proof that the VUV luminescence observed from LiGdF 4 or GdF 3 crystals and LiLuF 4 or LuF 3 originates from 4f 6 5d-4f 7 transitions in Gd 3+ and 4f 13 5d-4f 14 transitions in Lu 3+ , respectively. The fine structure due to zero-phonon and vibronic lines was well resolved in VUV emission and excitation spectra of LiGdF 4 , whereas the spectra of GdF 3 and LuF 3 have a smooth shape because of stronger electron-lattice coupling between the 4f n−1 5d electronic configuration of the Gd 3+ and Lu 3+ ions and the lattice vibrations.

The luminescence of two newly developed blue-emitting long afterglow phosphors, Sr 2Àx Ca x MgSi 2 O 7 :Eu 2+ , Dy 3+ (x ¼ 0, 1), has been studied. The emission under VUV-UV excitation from a synchrotron radiation source, along with the long afterglow spectra have been measured at different temperatures ranging from 7 to 300 K. While the emission spectra under 170 nm excitation exhibit both the 5d-4f transitions of Eu 2+ and 4f-4f transitions of Dy 3+ , only the blue band corresponding to the 5d-4f transitions of Eu 2+ is observed in the long afterglow spectra. Emission of Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ excited by an intense VUV laser source (157.6 nm) is also reported. The rich line structures in the laser-excited emission spectra partly result from 4f-4f transitions of Eu 3+ , indicating an efficient photon-induced process, which promotes Eu 2+ to Eu 3+ . The thermoluminescence covering the temperature range from 50 to 450 K for both samples has been measured as well and the depth of the traps responsible for the long afterglow emission was estimated. The mechanism of the persistent luminescence and the origin of the traps are discussed in the light of these results. r

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The luminescent properties of Gd 2 SiO 5 powder crystals doped with Eu 3+ were investigated using synchrotron radiation and a VUV laser (157.6 nm) as excitation sources. The excitation spectra in the range of 160-330 nm monitoring the red emission from Eu 3+ ions reveal bands corresponding to intraconfigurational 4f-4f transitions of Gd 3+ and charge transfer states (CTS) of Eu 3+ -O 2− , as well as interband transitions of the Gd 2 SiO 5 host, indicating an efficient energy transfer process from the host or directly from Gd 3+ to Eu 3+ ions. The inspection of 5 D 0 → 7 F 0 emission lines from Eu 3+ ions suggests that there are three inequivalent sites for Eu 3+ in this host. The excitation band around 215 nm and the unique emission features associated with this excitation band were attributed to one of the inequivalent Eu 3+ sites, which exhibits an unusually weak coupling with host lattice. The energy levels of Eu 3+ in the Gd 2 SiO 5 host were tentatively assigned according to the laser excited emission spectra.

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