Nuclear spin relaxation in free radicals as revealed in a stimulated electron spin echo experiment

Electron spin echo envelope modulation (ESEEM) in a three-pulse stimulated echo experiment, when the time interval between the first and second pulses τ is varied, is attributed to a spontaneous change of the electron spin Larmor frequency in the time intervalT between the second and third pulses, due to the longitudinal relaxation of nearby nuclei. It is observed for nitroxide radicals in glassy matrices in the temperature range of 130–240 K. Nuclear relaxation is assumed to arise from fluctuation of the proton hyperfine interaction, due to fast rotation of the methyl groups. This contribution to ESEEM and the conventional one that is induced by the simultaneous excitation of allowed and forbidden electron spin transitions were found to be multiplicative. As the latter does not depend on the timeT, both contributions can be easily separated. The rate of nuclear spin relaxation was determined, and correlation time of methyl group rotation was estimated by Redfield theory of spin relaxation. Arrhenius parameters of this motion were estimated on the basis of these data and those at 77 and 90 K, where the previously developed approach was used (L.V. Kulik, I.A. Grigor’ev, E.S. Salnikov, S.A. Dzuba, Yu.D. Tsvetkov, J. Phys. Chem. A 106, 12066–12071, 2003).