Energy Levels and Spectra of Ho2+ in CaF2, SrF2, BaF2, and SrCl2

Abstract

The absorption and fluorescent spectra of ${\mathrm{Ho}}^{2+}$ in single crystals of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Sr${\mathrm{Cl}}_2$ were measured. There are low-energy, intense, broad bands in the absorption spectra due to $f\to d$ electric dipole transitions and line absorption and emission spectra due to $f\to f$ magnetic dipole transitions. The Zeeman effect of some of the lines was studied, and the measured $g$ values agree with those calculated on the basis of Russell-Saunders coupling and a crystal field of $O_h$ symmetry. The spin-orbit parameters are nearly constant for the four hosts, but the crystal-field parameters $B_4$ decrease with increasing lattice constants. The ratios $\frac{B_6}{B_4}$ are nearly equal for the fluorides, but the ratio is greater for the chloride. As a result, the ground state for ${\mathrm{Ho}}^{2+}$ in the fluorides is $E_{\frac{1}{2}}$, whereas in the chloride it is $E_{\frac{5}{2}}$. The fluorescent linewidths are 1 ${\mathrm{cm}}^{-1\mathrm{}}$ or greater even at 4°K and are attributed to hyperfine splitting caused by the interaction of the ${\mathrm{Ho}}^{165}$ $I=\frac{7}{2}$ nuclear moment with the electronic states. The large linewidth contributes to the failure to obtain laser oscillation, and an analysis is given.