High-resolution vacuum ultraviolet spectroscopy of 5d-4f transitions in Gd and Lu fluorides

Observation, investigation, and characterization of vacuum ultraviolet (VUV) (∼10 eV) luminescence due to interconfigurational 4fⁿ⁻¹5d-4fⁿ transitions in Gd³⁺ and Lu³⁺ are reported. High-resolution (∼0.5 Å) VUV emission and excitation spectra as well as decay kinetics of VUV luminescence have been measured for LiGdF₄, GdF₃, LiLuF₄, and LuF₃ crystals using synchrotron radiation. The obtained results are the direct proof that the VUV luminescence observed from LiGdF₄ or GdF₃ crystals and LiLuF₄ or LuF₃ originates from 4f⁶5d-4f⁷ transitions in Gd³⁺ and 4f¹³5d-4f¹⁴ transitions in Lu³⁺, respectively. The fine structure due to zero-phonon and vibronic lines was well resolved in VUV emission and excitation spectra of LiGdF₄, whereas the spectra of GdF₃ and LuF₃ have a smooth shape because of stronger electron-lattice coupling between the 4fⁿ⁻¹5d electronic configuration of the Gd³⁺ and Lu³⁺ ions and the lattice vibrations.