Out-of-Phase Stimulated Electron Spin−Echo Appearing in the Evolution of Spin-Correlated Photosynthetic Triplet-Radical Pairs

Abstract

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 tobe 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.