Orthorhombic Electron-Spin-Resonance Spectra ofYb3+Ions in CaF2

Abstract

Four orthorhombic electron-spin-resonance spectra have been observed from Ca${\mathrm{F}}_2$ doped with small amounts of ${\mathrm{Yb}}_2$ ${\mathrm{O}}_3$. In each case the $z$ axis of the $g$ tensor lies along the [110] direction, while the $x$ and $y$ axes lie in the (110) plane and are tilted at an angle $\delta$ to the [$\overline{1}10$] and [001] axes, respectively. The $g$ factors of the two strongest spectra are: Spectrum $R_1:g_x=6.99\mathrm{}\pm 0.01$, $g_y=1.355\mathrm{}\pm 0.002$, $g_z=1.094\mathrm{}\pm 0.002$, $\delta =3.2\mathrm{}°\pm 0.2°$. Spectrum $R_2:g_x=7.24\mathrm{}\pm 0.01$, $g_y=0.992\mathrm{}\pm 0.002$, $g_z=0.957\mathrm{}\pm 0.002$, $\delta =26.3\mathrm{}°\pm 0.2°$. The values of $\delta$ for the two weaker spectra are 8.8° and 0°. These observations cast doubt on the pair model previously proposed to explain the orthorhombic spectra observed in $\gamma$-irradiated ${\mathrm{Tm}}_2$ ${\mathrm{O}}_3$: Ca${\mathrm{F}}_2$. Possible alternative models are discussed.