Out-of-phase stimulated electron spin-echo appearing in the evolution of spin-correlated photosynthetic triplet-radical pairs

A strong out-of-phase stimulated electron spin-echo is observed for the Q(A)(-) radical in the spin-correlated triplet-radical pair (3)PQ(A)(-) in photosynthetic bacterial reaction centers. The formation of this echo is shown to be induced by spin polarization of Q(A)(-) and by decay of the triplet state. The out-of-phase and in-phase echoes show deep envelope modulation induced by electron-electron dipole interaction between the partners in the pair, The analysis of this modulation provides dipolar frequencies. The interspin distance in (3)PQ(A)(-) is shown to be the same as in the radical pair P(+)Q(A)(-). This new type of experiment appears to be widely applicable for the study of chemical reactions and intermolecular distances in solids.