New Paradigm of Spin Exchange and its Manifestations in EPR Spectroscopy

Abstract

Theoretical results are presented, which have been confirmed experimentally and became basis for a new paradigm of spin exchange between paramagnetic particles in solutions and measurement of the spin exchange rate by EPR spectroscopy. Fundamental changes in the basics of spin exchange in solutions are associated with the transfer of coherence back from the interaction partner. It was shown that the transfer of spin coherence in random bimolecular collisions creates collective modes of evolution of quantum coherence of electron spins of paramagnetic particles. The EPR spectrum is the sum of the resonance lines. In the case of a slow spin coherence transfer, each spectral line has a mixed shape, it is the sum of the symmetric absorption line and the antisymmetric dispersion line. In the case of fast spin coherence transfer, only one collective mode is manifested in the EPR spectrum, the other modes are not manifested, since they are very weakly excited by the external microwave field, and they can be considered as forbidden lines in the spectrum or “dark” modes. The establishment of the mixed shape of the spectrum individual lines has fundamentally changed the protocol for finding the rate of bimolecular spin exchange caused by the exchange interaction. In the current existing paradigm, the contribution of the dipole–dipole interaction to the spin coherence transfer was mistakenly neglected.