Anomalous Relaxation of Hyperfine Components in Electron Spin Resonance

Abstract

It is shown that the different hyperfine components in the electron‐resonance spectra of free radicals exhibiting proton hyperfine splitting saturate at different rates, and different sets of relaxation times must be used to describe the behavior of each component. The variations in the relaxation times (T₁ and T₂) were found to be approximately symmetric about the center of the spectrum, and the central component is the narrowest and saturates more readily than the outer components. A qualitative discussion is given of the relaxation and line‐broadening mechanisms which may be responsible for the variations in T₁ and T₂. In solutions of the p‐benzosemiquinone ion at room temperature, the values of T₁ and T₂ were found to be approximately 10^—6 sec. An analysis is given of the experimental parameters which affect saturation measurements, including the effect of a nonuniform rf field in a rectangular cavity, and a study is made of the type of spectra to be expected when the individual hyperfine components have different values of T₁ and T₂. An appendix is included on the work of Jen et al. on deuterium atoms trapped at liquid helium temperature, which shows that the anomalous intensities obtained in these low‐temperature experiments as a function of rf power is not the same as the phenomenon observed for free radicals in solution and cannot be adequately interpreted in terms of any currently proposed mechanisms.