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

Observation, investigation, and characterization of vacuum ultraviolet (VUV) $(\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ luminescence due to interconfigurational $4{f}^{n\ensuremath{-}1}5d\text{\ensuremath{-}}4{f}^{n}$ transitions in ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ are reported. High-resolution $(\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ VUV emission and excitation spectra as well as decay kinetics of VUV luminescence have been measured for $\mathrm{Li}\mathrm{Gd}{\mathrm{F}}_{4}$, $\mathrm{Gd}{\mathrm{F}}_{3}$, $\mathrm{Li}\mathrm{Lu}{\mathrm{F}}_{4}$, and $\mathrm{Lu}{\mathrm{F}}_{3}$ crystals using synchrotron radiation. The obtained results are the direct proof that the VUV luminescence observed from $\mathrm{Li}\mathrm{Gd}{\mathrm{F}}_{4}$ or $\mathrm{Gd}{\mathrm{F}}_{3}$ crystals and $\mathrm{Li}\mathrm{Lu}{\mathrm{F}}_{4}$ or $\mathrm{Lu}{\mathrm{F}}_{3}$ originates from $4{f}^{6}5d\text{\ensuremath{-}}4{f}^{7}$ transitions in ${\mathrm{Gd}}^{3+}$ and $4{f}^{13}5d\text{\ensuremath{-}}4{f}^{14}$ transitions in ${\mathrm{Lu}}^{3+}$, respectively. The fine structure due to zero-phonon and vibronic lines was well resolved in VUV emission and excitation spectra of $\mathrm{Li}\mathrm{Gd}{\mathrm{F}}_{4}$, whereas the spectra of $\mathrm{Gd}{\mathrm{F}}_{3}$ and $\mathrm{Lu}{\mathrm{F}}_{3}$ have a smooth shape because of stronger electron-lattice coupling between the $4{f}^{n\ensuremath{-}1}5d$ electronic configuration of the ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ ions and the lattice vibrations.