Multiple-quantum EPR coherence in a spin-correlated radical pair system

Multiple-quantum EPR coherence phenomena in a photo-induced spin-correlated radical pair system are examined. Because of the special singlet-triplet mixing mechanism, we have proposed a phase-cycled two-pulse sequence to excite and detect the single-quantum as well as the normally forbidden zero- and double-quantum transitions. Complementary to the method of electron spin echo modulation, this technique can also provide useful structural information about electron-electron exchange and dipolar interactions, in addition to the Zeeman and nuclear hyperfine interactions. Analytical expressions are derived for the multiple-quantum signals.