Selective excitation in pulsed EPR of a spin-correlated triplet-radical pair

Abstract

Experiments are described in which a low-amplitude microwave pulse excites only one out of three allowed transitions of the quinone radical (Q A −) in a spin-correlated triplet-radical pair 3 PQ A − of the bacterial photosynthetic reaction center. A second high-amplitude pulse produces a FID whose temporal shape is strongly modulated with frequencies determined by electron–electron dipolar interaction in the pair. The FID is detected in both the in-phase and the out-of-phase channels. The out-of-phase FID is a result of switching off the magnetic dipolar interaction between 3 P and Q A − due to decay of 3 P during the time interval between the two pulses. Refocusing of FID by an additional non-selective pulse allows a dead-time free measurement of this modulation. The influence of the dead-time problem on the distance determination is discussed.