Radiofrequency and Mössbauer Spectra of a Paramaghetic Ion in the Presence of Hyperfine Interactions and of Tunnel Effect

The influence of a coherent tunnel effect on the magnetic hyperfine structure of a diluted paramagnetic ion, with an electronic ground state $|S_z=\pm S>(S > 1/2)$, is studied under the assumptions of zero longitudinal magnetic field and slow electronic relaxation. It is shown that the radiofrequency and Mössbauer spectra should be altered, with a change in the number and intensities of the lines. When the tunnelling parameter, $\Gamma$, is large compared to the hyperfine structure, the magnetic hyperfine structure splitting should be strongly reduced and it should vanish for $\Gamma \rightarrow \infty$. Tunnelling effects have already been observed in EPR experiments on ${\rm Tb}^{3+}$ diluted in diamagnetic matrices. The cases of NMR with $I=3/2$ and of Mössbauer spectroscopy for a transition $I_{\rm e}=3/2\rightarrow I_{\rm g}=1/2$ are considered