SPIN-CORRELATED RADICAL PAIRS - MICROWAVE PULSE EFFECTS ON LIFETIMES,ELECTRON-SPIN ECHO ENVELOPE MODULATIONS, AND OPTIMUM CONDITIONS FOR DETECTION BY ELECTRON-SPIN ECHO SPECTROSCOPY

A theoretical description is presented of the use of electron spin ech o spectroscopy to study the spin-correlated radical pairs encountered in photosynthetic systems. The analysis includes an extra microwave pu lse, prior to the spin echo, whose purpose is to alter the radical pai r lifetime by populating spin energy levels from which the pair is una ble to recombine directly. The optimal flip-angles of the three non-se lective pulses are derived, the time dependence produced by spin-latti ce 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 th e exchange and dipolar interactions, and hence the separation, of the two electron spins are discussed. (C) 1998 Elsevier Science B.V.