Features of the Primary Electron Spin Echo Signal for Spin-Correlated Radical Pairs

The phase of the primary electron spin echo signal was analyzed on model radical pairs (RPs) in a singlet spin state. An interpretation is given of the fact known in the literature that the echo signal of such pairs may not have a component with a “normal” phase, which is expected for the echo signal of non-interacting unpaired electrons in the RP. For the first time, it has been theoretically shown that forbidden EPR transitions can make a significant contribution to the phase shift of the magnetization of RP spins, caused by the dipole–dipole hyperfine interaction of unpaired RP electrons with magnetic nuclei. The “anomalous” phase of the spin echo signal of interacting spins in radical pairs is a demonstration that spin dynamics is accompanied by reversible transitions of dipole polarization (spin magnetization) and quadrupole spin polarization.