Paramagnetic Resonance Absorption in Neptunium Hexafluoride

The paramagnetic resonance absorption of crystalline NpF₆ has been studied at 3 cm wavelength and at the boiling point of liquid He. The data were fitted to the spin Hamiltonian, $\mathcal{H}\mathrm{=\beta }\mathbf{H}\mathrm{·g·}\mathbf{S}+\mathbf{I}\mathrm{·A·}\mathbf{S}$ . The parameters are isotropic within the errors of the measurements, with g = −0.604 and | A/hc | = 0.1100 cm⁻¹. These results are interpreted in terms of the configuration f¹ perturbed by spin‐orbit coupling and an octahedral molecular potential. The relationships of the present results to optical data and measurements of magnetic susceptibility for the same substance are discussed. Hyperfine structure due to F is observed and is anisotropic.