Analytical Theory of Electron-Spin-Echo Modulations in Correlated Radical Pairs

Abstract

A theoretical treatment of the effects of electron-electron coupling on electron-spin-echo envelope modulation (ESEEM) spectra of spin-correlated radical pairs (species which occur as intermediates in a variety of chemical reactions and in the initial steps of photosynthetic solar energy conversion) is presented. The exchange and dipolar interactions between the two unpaired electrons in the radical pair are found to have no effect on the ESEEM spectra if their strength is small compared to the difference in EPR frequencies of the two electrons. However, using perturbation theory, it is shown that strong electron-electron coupling causes shifts in the ESEEM frequencies and changes in modulation depths that are characteristic of the strength of the electronic interactions. The paper concludes with a discussion of ESEEM effects originating from the electron-electron zero-quantum coherence with which spin-correlated radical pairs are formed.