Vacuum ultraviolet 5d$^1$4f$^9$-4f$^{10}$ emission of Ho$^{3+}$ ions in   alkaline-earth fluorides
  Time-resolved emission, excitation as well as emission decay curves of CaF$_2$, SrF$_2$, BaF$_2$ doped with HoF$_3$ were investigated. Most intensive emission bands near 168 nm, having long decay time, belong to spin-forbidden transitions from 5d$^1$4f$^9$ high spin (HS) states to ground $^5$I$_8$ states of Ho$^{3+}$ ions. Weak spin allowed 5d$^1$4f$^9$(LS)-4f$^{10}$ emission band at 158 nm was observed only in CaF$_2$-Ho crystals. Spin allowed and spin-forbidden excitation bands were observed in all crystals near 166 and 155 nm respectively. Fast component of spin-forbidden emissions due to multiphonon relaxation to low-lying 4f$^{10}$ Ho$^{3+}$ level was observed for all crystals. 
E. A. Radzhabov and V. Nagirnyi and A. I. Nepomnyashchikh
cond-mat.mtrl-sci