Selective excitation in pulsed EPR of spin-correlated radical pairs: electron–electron interactions, zero-, single-, and double-quantum relaxation and spectral diffusion

Experiments are described in which a low-amplitude microwave pulse excites only one out of four allowed transitions of a spin-correlated radical pair (SCRP). A second high-amplitude pulse produces an FID whose temporal shape is strongly modulated with frequencies determined by electron–electron spin-exchange and dipolar interactions. The dependence of the FID intensity on the delay between the two pulses is determined by the relaxation of populations of energy levels connected by allowed single-quantum transitions and by forbidden zero- and double-quantum transitions. The first direct measurement of the zero-quantum population relaxation rate for P+QA − radical pairs in bacterial photosynthetic reaction centers shows it is temperature independent.