Spin-correlated radical pairs: microwave pulse effects on lifetimes, electron spin echo envelope modulations, and optimum conditions for detection by electron spin echo spectroscopy

Abstract

A theoretical description is presented of the use of electron spin echo spectroscopy to study the spin-correlated radical pairs encountered in photosynthetic systems. The analysis includes an extra microwave pulse, prior to the spin echo, whose purpose is to alter the radical pair lifetime by populating spin energy levels from which the pair is unable to recombine directly. The optimal flip-angles of the three non-selective pulses are derived, the time dependence produced by spin-lattice relaxation and recombination is predicted, and the echo modulations arising from electron–electron and electron–nuclear interactions are calculated. The implications of the latter for the determination of the exchange and dipolar interactions, and hence the separation, of the two electron spins are discussed.