Electron spin relaxation and tunnelling methyl groups

The energy transfer has been studied between free radicals and nearby tunnelling methyl groups near the resonant condition that the electron Larmor frequency and the methyl tunnelling frequency are equal. The measurements were made in a gamma -irradiated single crystal of 4-methyl-2,6-ditertiarybutylphenol (MDBP) at 4.2K and consisted of saturating the ESR signal and observing its return to equilibrium. This has been done at eleven ESR frequencies in the range 8.97\u3c nu s\u3c0.63 GHz and for saturation times of 5, 50 and 200 s. A theoretical model has been developed for the non-exponential recovery which assumes that all the 4-methyl groups of the molecules surrounding a free radical have the same tunnelling rotation frequency nu t and that the only coupling between the electrons and the methyl groups is the intermolecular dipole-dipole interaction