Methyl tunnelling spectroscopy and level crossing phenomena in solid acetone

Reports the observation in acetone at low temperature of a number of new phenomena connected with methyl tunnelling motion and field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field-cycling nuclear magnetic resonance spectroscopy. The authors observe three strong anomalies in the field dependence of proton spin-lattice relaxation at 4K for fields B 0 , B 0 /3 and B 0 /3 and B 0 /3 and B 0 /3 with B 0 =2.25T. They are due to internal resonances (or level crossings) occurring for n nu 0 = nu t , (n=1, 2, 3) where nu 0 is the field-dependent nuclear Larmor frequency and nu t is the methyl tunnel frequency. This structure is a consequence of the field-cycling technique which involves rapid switches of the magnetic field to enable all measurements of proton magnetisation to be made at a single field 0.6 T. A simple thermodynamic model is described in which there is very rapid thermal equilibration between the nuclear Zeeman and methyl tunnel reservoirs at the fields corresponding to the n=1 and 2 anomalies and the nuclear magnetisation is otherwise unaffected by the rapid field changes. This model successfully simulates the observed spectrum and a variety of other field-cycling experiments.