The paramagnetic resonance spectrum of ${\mathrm{Yb}}^{3+}$ in Ca${\mathrm{F}}_2$ was observed at 20°K and 3 cm wavelength. The spectrum is described by a cubic spin Hamiltonian $\mathcal{H}=g\beta \mathrm{H}·\mathrm{S}+A\mathrm{S}·\mathrm{I}$ with $g=3.426\mathrm{}\pm 0.001$, $S=\frac{1}{2}$, $A^{171}=(886.5\mathrm{}\pm 1.5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $A^{173}=(243.2\mathrm{}\pm 0.4)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $I^{171}=\frac{1}{2}$, $I^{173}=\frac{5}{2}$. The ratio of magnetic moments is $\frac{{\mu }^{173}}{{\mu }^{171}}={1.374}_9\pm 0.005$. The optical spectrum shows lines of 9774A, 9770A, 9763 A, and more diffuse and unresolved bands at 9080 and 12730 A. The paramagnetic spectrum is explained as arising from the ${\Gamma }_7$ doublet. The other levels are removed by at least a few ${\mathrm{cm}}^{-1\mathrm{}}$ leading to an isotropic $g$ value of $3\Lambda$ or $\frac{24}{7}$ for the lowest ${\Gamma }_7$ level.

• Received 28 January 1960

DOI:https://doi.org/10.1103/PhysRev.118.1608