Methyl Zeeman-tunnel resonance and nuclear spin relaxation in copper acetate

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Abstract
The methyl tunnel spectrum of copper acetate is studied at low temperatures by means of nuclear magnetic resonance level-crossing spectroscopy employing a field-cycling technique. The recovery of the proton magnetisation following saturation is measured as a function of magnetic field. Changing the magnetic field causes energy levels to cross when the methyl tunnel frequency is equal to one or two times the proton Larmor frequency, and abrupt changes in the recovered magnetisation, showing up as peaks or dips in a smooth background, reveal the positions of the level crossings. A variety of possible field cycles and sequences of cycles leads to a series of different experimental procedures which highlight different aspects of the thermodynamic properties of the system. In particular the dependence of the spectra on scanning schemes and on the thermodynamic preparation conditions is investigated. The experiments are understood in a semi-quantitative way in terms of a simple mathematical model for the thermal equilibration of three energy reservoirs, of which two are resonantly coupled at the level crossings. This simulation shows that, while the dominant processes have been clearly identified and incorporated in the model, some important factors remain to be explained. The methyl tunnel frequency in copper acetate is found to be (64+or-5) MHz.