EPR of spin-correlated radical pairs. Analytical treatment of selective excitation including zero-quantum coherence

We present a simple analytical treatment of the electron paramagnetic resonance (EPR) spectra of spin-correlated radical pairs, including the zero-quantum coherence with which they are created. We concentrate on the time dependence of the spectrum of a prototype radical pair, with exchange and dipolar couplings but no nuclei, in transient nutation experiments with a without a delay between the formation of the pair and the application of the microwave field. We show how the zero-quantum coherence manifests itself and suggest how it may be turned to advantage. An appendix describes the singlet-triplet interconversion of radical pairs in the language of product operators.