Electron spin echo envelope modulation by electronic spin–spin interactions in radical pairs undergoing electron transfer

Electron spin echo envelope modulation caused by the electronic spin–spin interaction is investigated for the case of a light-induced radical pair undergoing secondary electron transfer. Analytical expressions reveal contributions of both the primary and secondary radical pair to the out-of-phase component of the signal, while only the secondary radical pair contributes to the in-phase component. Spectra exhibit lifetime broadening for the primary pair and dispersive contributions for the secondary pair, but no frequency shifts. Numerical calculations illustrate the dependence of the spectra on the electron transfer rate and on the delay between laser and microwave pulses.