Abnormal electron spin echo and multiple-quantum coherence in a spin-correlated radical pair system

The phenomena of the abnormal “out-of-phase” electron spin echo in a photo-induced spin-correlated radical pair system are examined theoretically. It is shown that such abnormal phenomena are a consequence of initial non-Boltzmann distribution and zero-quantum coherence produced by laser excitation. The analysis of echo amplitude versus the pulse-angle of the microwave pulse reveals two sources for the formation of the echo. The method of excitation and detection of multiple-quantum coherence using a phase-cycled 2-pulse sequence is also discussed. Such a technique is complementary to the ESE method.