INHOMOGENEITY OF RADICAL PAIR ENERGIES IN PHOTOSYNTHETIC REACTION CENTERS REVEALED BY DIFFERENCES IN RECOMBINATION DYNAMICS OF P-) WHEN DETECTED IN DELAYED EMISSION AND IN ABSORPTION(H(A)

Nonuniform radical pair recombination reactions originating from energ etic inhomogeneity can be revealed by comparing the kinetics monitored in transient absorption and in delayed emission. The dynamics of the high-energy tail of a distribution (weighted by the Boltzmann factor f or repopulation of the emitting state) determines the kinetics observe d in emission, while in absorption the bulk average is reflected. In r eaction centers of Rb. sphaeroides we found the recombination dynamics of the radical pair P+H(A) to be faster and their magnetic field depe ndence to exhibit a significantly broader resonance line width when de tecting in emission than when monitoring in absorption. This points to larger values of both the singlet and triplet recombination rates for the high-lying radical pair states. The observed increase of both rat es with increasing energy can be understood only if superexchange coup ling via P+B(A)- is prevailing for these recombination reactions, sinc e its energy denominator decreases for the high-lying states. The obse rved weak temperature dependence of the average amplitude of the delay ed fluorescence as compared to the amplitude of the prompt emission ca n be modelled by averaging over a Gaussian distribution of the free-en ergy differences with a maximum at DELTAG0 = 0.25 eV and a width of 2s igma congruent-to 0.1 eV. The deviations from monoexponential kinetics of primary charge separation and the weak electric field effects on t he fluorescence are attributed to this energetic inhomogeneity.