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High-resolution (~1 Å) emission and excitation spectra as well as luminescence decay kinetics have been studied for CaF 2 doped with Gd 3+ (0.1%) and Ce 3+ (0.05%) under excitation by synchrotron radiation near and above the edge of interconfigurational 4f 7 -4f 6 5d transitions in Gd 3+ and 4f -5d transitions in Ce 3+ . The fast (τ~8.5 ns) VUV luminescence has been detected from the crystal the spectrum of which shows zero-phonon line at 77660 cm -1 and dominating vibronic line at energy interval ~370 cm -1 . The vibronic structure in VUV emission spectrum well correlates with that observed at the onset of the Gd 3+ 4f 7 -4f 6 5d excitation spectrum. The obtained results confirm that the VUV luminescence observed in CaF 2 :Gd 3+ ,Ce 3+ originates from 4f 6 5d -4f 7 radiative transitions in Gd 3+ . The vibronic structure in emission and excitation spectra of Gd 3+ d -f luminescence from the CaF 2 :Gd 3+ ,Ce 3+ crystal differs from that observed for other trivalent rare earth ions doped into CaF 2 indicating that Gd 3+ ions reside in sites of different type in CaF 2 than that of tetragonal (C 4v ) symmetry.1 Introduction It is well known [1] that some trivalent rare earth (RE) ions doped into wide bandgap matrices possess luminescence in the vacuum ultraviolet (VUV) spectral range. This luminescence originates from parity-allowed interconfigurational 4f n-1 5d -4f n transitions in the RE ions. Up to recently, RE 3+ luminescence in VUV has been detected only from three heavier RE ions, namely from Nd 3+ , Er 3+ , and Tm 3+ . However, recent studies [2,3] have revealed that Gd 3+ containing wide band-gap fluoride crystals with scheelite and YF 3 structure emit VUV luminescence (hν ~ 10 eV), which is due to the interconfigurational 4f 6 5d -4f 7 transitions in the Gd 3+ ion. The observation of 4f 6 5d -4f 7 emission from the Gd 3+ ion is a rather unexpected result because of the considerable number of closely spaced 4f 7 levels in the same energy region as the 4f 6 5d states, that can open an efficient non-radiative relaxation channel from the lowest Gd 3+ 4f 6 5d level. The possible reason for the observation of efficient radiative decay from the Gd 3+ 4f 6 5d level is that the competing non-radiative transitions are heavily spinforbidden because the multiplicity of the lowest 4f 6 5d level of Gd 3+ is eight, whereas the 4f 7 levels of Gd 3+ closest to this 4f 6 5d level are doublet or quartet [4]. In the present work the spectral and temporal characteristics of VUV, UV and visible luminescence from CaF 2 doped with Gd 3+ (0.1%) and Ce 3+ (0.05%) have been studied under excitation by synchrotron radiation near and above the edge of interconfigurational 4f 7 -4f 6 5d transitions in Gd 3+ and 4f -5d transitions in Ce 3+ .

-The first observation and characterization of Lu 3+ 4 f 13 5 d -4 f 14 luminescence from the CaF 2 : Lu 3+ crystal are reported, and the multisite structure in the spectra of Ce 3+ , Gd 3+ , and Lu 3+ ions in the CaF 2 host is analyzed with the high-resolution VUV spectroscopy technique using synchrotron radiation. It is shown that vibronic structure in the emission and excitation spectra of interconfigurational transitions in Gd 3+ and Lu 3+ ions doped into CaF 2 differs from that observed for Ce 3+ ions entering mainly at the tetragonal (C 4v ) sites. However, the exact types of sites in which the Gd 3+ and Lu 3+ ions reside in a CaF 2 lattice cannot be identified using only the obtained experimental spectroscopic data.

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