Saturation and spectral diffusion in electron spin resonance

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Abstract
The saturation behaviour of an electron spin resonance line depends on the rate at which energy, absorbed from the microwave field by a relatively small number of resonant spins, is shared with the remainder of the spin system through the process of spectral diffusion or cross relaxation. This rate, being dependent on the spin-spin interactions, offers a possible means for their study in dilute spin systems. With this object, a general theory of saturation has been developed. It is based on the theory of Provotorov which has been successful in accounting for saturation and related effects in nuclear magnetic resonance. Whereas Provotorov's theory regards the spectral diffusion process as instantaneous (the case of homogeneous broadening), our approach takes account of the finite rate and is therefore applicable to dilute spin systems and inhomogeneously broadened lines. As a result of the generality of our method, the results of Provotorov, Portis, Wolf and Bloembergen et al. occur as special cases. Measurements are described on a copper complex at 4 °k which support the thermodynamic model on which the theory is based. Where more than two levels are involved, flip-flop transitions connecting one of the saturated levels with a third level transfer local field energy away from the saturated levels. A consequence is that the theory of Bloembergen et al. becomes applicable. Absorption and dispersion saturation measurements made without field modulation on ruby at 4 °k confirm this.