Optically InducedTm2+Ions in Trigonal Sites in BaF2

Abstract

Some $\gamma$-irradiated crystals of thulium-doped Ba${\mathrm{F}}_2$ exhibit spectra associated with ${\mathrm{Tm}}^{2+}$ ions in both cubic and trigonal sites. The centers with trigonal symmetry are generated by irradiating the crystal at or below 77°K with light around 4000 Å. Irradiating the crystal with 7000 Å radiation or cycling to room temperature eliminates the trigonal centers and increases the number of ${\mathrm{Tm}}^{2+}$ cubic centers. The noncubic spectrum can be fitted using $g_{\parallel }=2.23\mathrm{}\pm 0.01$, $g_{\perp }=3.98\mathrm{}\pm 0.02$, $A=0.0248\mathrm{}\pm 0.0003$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $B=0.0432\mathrm{}\pm 0.0004$ ${\mathrm{cm}}^{-1\mathrm{}}$. Two optical fluorescence lines were also identified with the noncubic centers. An approximate value for the second-degree crystal-field parameter ${A_2}^0〈r^2〉 \mathrm{is} +140\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$.