Dynamic level-crossing model of antiphase electron spin polarization in spin-correlated radical pairs

Abstract

The unusual antiphase electron spin polarizations, attributed to residual spin correlations in incompletely separated radical pairs, are treated by a dynamic model in which the polarized electron spin transitions occur as the radicals diffuse through the separations where the transitions are in resonance with the microwave field. This model shows that the inter-radical exchange interaction, and in some cases magnetic dipolar interactions as well, can produce the observed polarizations despite varying rapidly with diffusive motions of the radicals. However, the apparent fixed splitting of the emissive and absorptive components of the antiphase line is usually determined by the intrinsic width of the unpolarized electron spin resonance (ESR) line rather than by the inter-radical interactions. Surprisingly, a static model, in which the radicals are immobile during the short ESR observation period, yields the same polarization as the dynamic model under quite general conditions.