Spin Relaxation and Differential Saturation of ESR Multiple Lines

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Abstract
The effect of various spin‐relaxation mechanisms on the differential saturation of ESR spectra in the solid state is discussed by taking a two‐spin system as an example. It is shown that the spin—orbit interaction, the spin—local‐field interaction, and the intermolecular dipole—dipole interaction give similar values for the differential saturation. But they depend in complex manner on the angles between the molecular and the laboratory frame differently. In the weak mixing limit where the intensities of the satellite lines are much weaker than those of the main lines the saturation factors for the main and satellite lines are not very different from each other. The relative intensities of these lines change only by a factor of two even in the limit of H1→∞. Therefore, these mechanisms cannot explain the anomalous saturation of the ESR spectrum observed by Kurita and Ohigashi. On the other hand, if the relaxation occurs through the intramolecular dipolar interaction modulated by the torsional oscillation of the molecule, the saturation factors for the main lines are much larger than those for the satellite lines and the anomalous saturation could be produced.