Tunnelling magnetic resonances

Lineshape studies identify the processes responsible for two resonant features in the low-temperature magnetic-field dependence of the recovery of proton magnetization following saturation in gamma -irradiated 4-methyl-2,6-ditertiarybutylphenol. One of the lines occurring at a field of 0.33 T exhibits 'hole burning' and other effects associated with inhomogeneously broadened lines and may be inverted. By contrast, the other at 0.15 T has always the same shape. The resonances are both due to processes which involve spin symmetry conversion of a methyl group with simultaneous flips of both nuclear and electronic spins, but they differ in that the 0.15 T resonance involves methyl groups strongly coupled by the hyperfine interaction with the unpaired electron spins while the 0.33 T resonance is due to the relatively weakly dipole-dipole coupled methyl groups fairly remote from the unpaired electron.