Manifestation of Exchange and Dipole-Dipole Interaction in the Form of EPR Spectra of Paramagnetic Particles in Solutions in Linear Response Case

Abstract

A general formula is given for the EPR spectrum in solutions in the region of linear response. The differences in understanding the manifestations of spin exchange in the early and new paradigms are discussed in detail. A common feature of these paradigms is that the components of the spectrum broaden and shift to the center of gravity of the EPR spectrum with an increase in the rate of transfer of spin coherence in the region of slow exchange, while in the region of fast transfer of spin coherence, the entire spectrum turns into one narrow homogeneous line in the center of gravity. Within both paradigms, the spectrum is represented as the sum of the resonance lines. The fundamental differences between the two paradigms relate to the nature and form of each resonance line. The establishment of the fact that each resonance line has a mixed absorption + dispersion form has become the cornerstone of the new paradigm. In the new paradigm, each line of the spectrum is associated with a specific mode of coordinated (collective) motion of the coherence of spins. The exchange narrowing of the spectrum at a high spin exchange rate is explained by the fact that in this situation only one collective mode is excited by the microwave field. The remaining modes are dark states and are not observed in the EPR experiment.