RADIOFREQUENCY AND MOSSBAUER-SPECTRA OF A PARAMAGNETIC ION IN THE PRESENCE OF HYPERFINE INTERACTIONS AND OF TUNNEL EFFECT

The influence of a coherent tunnel effect on the magnetic hyperfine st ructure of a diluted paramagnetic ion, with an electronic ground state \S-z = +/-S > (S > 1/2), is studied under the assumptions of zero lon gitudinal magnetic field and slow electronic relaxation. It is shown t hat the radiofrequency and Mossbauer spectra should be altered, with a change in the number and intensities of the lines. When the tunnellin g parameter, Gamma, is large compared to the hyperfine structure, the magnetic hyperfine structure splitting should be strongly reduced and it should vanish for Gamma --> infinity. Tunnelling effects have alrea dy been observed in EPR experiments on Tb3+ diluted in diamagnetic mat rices. The cases of NMR with I = 3/2 and of Mossbauer spectroscopy for a transition I-e = 3/2 --> I-g = 1/2 are considered.