Optical Study of Vacuum-ultraviolet Fluoride Phosphors

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The Xe dimer discharge fluorescent lamp is one of the leading candidates for alternative lighting devices to a conventional Hg discharge fluorescent lamp. In the Xe_2 discharge florescent lamps, phosphors are excited by vacuum ultraviolet (VUV) light peaking at 〜172nm. With the aim of developing practical Xe_2 discharge fluorescent lamps, new phosphors suitable for the conversion of VUV light into visible light have been extensively researched. In the Hg discharge fluorescent lamps, oxide phosphors have been employed so far. The oxide phosphors are unlikely to be suitable for the conversion of VUV light (〜172nm), because the VUV light is little absorbed by activators due to the absorption of oxide hosts. In contrast, most fluoride hosts are transparent down to 〜150nm, so that the VUV light directly excites activators in fluoride hosts. Metal ions with s^2 electronic configuration doped in insulating materials, called Tl^+-type ions, show several absorption bands originating from s^2→sp … More transitions. Since the s^2→sp transitions are dipole-allowed, the absorption is much stronger than that due to the f-f transitions. The strong absorption may give efficient conversion of the VUV light. However, electronic transitions in Tl^+-type ions doped in fluoride hosts have been less investigated so far.We have investigated luminescence and excitation spectra of Bi^<3+> ions doped in alkali yttrium fluorides (NaYF_4:Bi^<3+> and LiYF_4:Bi^<3+>), by mainly using synchrotron orbital radiation (UVSOR facility, Institute for Molecular Science, Okazaki, Japan). Bi^<3+> ions are trivalent Tl^+-type ions, so that can easily substitute for Y^<3+> ions. A broad and asymmetric luminescence band peaking at 〜430nm was observed in NaYF_4:Bi^<3+>, at room temperature. The radiative intra-ionic transition from a relaxed excited state (^3P_1) to a ground state (^1S_0) in Bi^<3+> ions is probably responsible for this luminescence band. In the excitation spectrum for the 430-nm luminescence, several excitation bands were observed below 300nm. These excitation bands originate likely from the transition from the ground state to some of the excited states given by the sp electronic configurations. The Tl^+-type ions doped in alkali halide crystals with NaCl-type structure show three characteristic absorption bands arising from the s^2→sp transitions, namely A, B and C bands. The A, B and C absorption bands are attributed to the transition to ^3P_1, ^3P_0, and ^1P_1 states, respectively. In Bi^<3+> ions doped in Cs_2NaYCl_6, The A and C bands have been observed at 〜320 and 220nm. In Bi^<3+> ions doped in fluorides hosts, the absorption bands would shift to the shorter wavelength side, as compared to those in chloride hosts. Less