Paramagnetic resonance absorption at 3-cm wavelengths is observed for tetravalent ${\mathrm{Pa}}^{231}$ in a single crystal of ${\mathrm{Cs}}_2$Zr${\mathrm{Cl}}_6$ at helium temperatures. The observed spectra correspond to the allowed transitions ($S_z, I_z\to S_z\pm 1, I_z$) and the forbidden transitions ($S_z$, $I_z\to S_z\pm 1$, $I_z\mp 1$) of a system described by the spin Hamiltonian, $H_s=g\beta \mathrm{H}·\mathrm{S}+A\mathrm{I}·\mathrm{S}-{g_n}^{\prime }\beta \mathrm{H}·\mathrm{I}$, with $S=\frac{1}{2}$, $I=\frac{3}{2}$, $\frac{\mathrm{}\left|A\right|\mathrm{}}{h}=1578.6\mathrm{}\pm 1.4$ Mc/sec, $\mathrm{}\left|g\right|=1.1423\mathrm{}\pm 0.0014$, and $\left|{g_n}^{\prime }\right|\approx 8\times {10}^{-4\mathrm{}}$. The errors indicate a small deviation from isotropy. It is further observed that $\frac{{g_n}^{\prime }}{g}<0\mathrm{}$, indicating that if ${g_n}^{\prime }$ is positive, as is strongly indicated by the nuclear shell model, then $g$ is negative. An additional electron-nuclear double-resonance experiment is used to determine directly the nuclear magnetic moment $\mu \left({\mathrm{Pa}}^{231}\right)=1.96\mathrm{}$ nuclear magnetons. This value includes a correction of 9% due to perturbations of an excited state about 1900 ${\mathrm{cm}}^{-1\mathrm{}}$ above the ground-state doublet of ${\mathrm{Pa}}^{4+}$ in its octahedral crystal field. A lower frequency double-resonance experiment is used to measure the weak hyperfine interaction of the ${\mathrm{Pa}}^{4+}$ ion with its ${\mathrm{Cs}}^{133}$ neighbors, of order $\frac{A^{\prime \prime }}{h}\approx 0.5$ Mc/sec.

• Received 7 November 1960

DOI:https://doi.org/10.1103/PhysRev.121.1630