Tunnelling resonance in electron spin-lattice relaxation

The time for the recovery after prolonged saturation of the electron spin resonance signal of free radicals in gamma -irradiated 4-methyl-2,6 ditertiarybutylphenol exhibits a remarkable peak in its magnetic-field dependence. At a temperature of 4.2K it may be more than fifty times greater at a field of 0.34 T than at 0.27 T. At 0.34 T the electron Larmor frequency coincides with the frequency of tunnelling rotation of the 4-methyl groups of neighbouring undamaged molecules with a consequent exchange of energy between the magnetic energy of the electron and the mechanical rotational energy of the methyl groups. The effect is to couple the electron Zeeman energy reservoir to a thermal system whose heat capacity is large and which is itself uncoupled to the lattice. The coupling can be switched on and off simply by changing the magnetic field, providing a simple means of studying the rate constants involved. The effect provides a new way of investigating quantum motions in molecular solids.