Spectroscopy of $f^{13}$-lanthanides in fluoride crystals

The absorption and luminescence spectra of $4f$–$4f$-transitions of Yb$^{3+}$ ions in crystals of CaF$_2$, SrF$_2$ and BaF$_2$ fluorides have been studied. The crystal splitting of the $^2F_{7/2}$, $^2F_{5/2}$ levels in Tm$^{2+}$, Yb$^{3+}$ by the cubic field decreases linearly with increasing lattice constant. Both lines intersect at the energy of $^2F_{7/2}$ – $^2F_{5/2}$ transitions in unperturbed Tm$^{2+}$, Yb$^{3+}$ ions. The positions of the $f^{13}$-levels of Tm$^{2+}$ or Yb$^{3+}$ in fluoride crystals are well described by ab-initio relativistic calculations.