ANALYTICAL THEORY OF ELECTRON-SPIN-ECHO MODULATIONS IN CORRELATED RADICAL PAIRS

A theoretical treatment of the effects of electron-electron coupling o n electron-spin-echo envelope modulation(ESEEM) spectra of spin-correl ated 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 interaction s between the two unpaired electrons in the radical pair are found to have no effect on the ESEEM spectra if their strength is small compare d to the difference in EPR frequencies of the two electrons. However, using perturbation theory, it is shown that strong electron-electron c oupling causes shifts in the ESEEM frequencies and changes in modulati on depths that are characteristic of the strength of the electronic in teractions. The paper concludes with a discussion of ESEEM effects ori ginating from the electron-electron zero-quantum coherence with which spin-correlated radical pairs are formed. (C) 1995 Academic Press, Inc .