Using absorption detected magnetic resonance (ADMR) the triplet states of the photosynthetic reaction centers of Rhodobacter (Rb.) sphaeroides R26 and Rhodopseudomonas (Rps.) viridis have been observed in an exceptionally wide temperature range from 4.2 up to 150 and 240 K, respectively. The observed changes in the zero field splitting (zfs) parameters show the presence of a higher lying triplet state separated by an energy gap of about 16 meV. The temperature dependence of the ADMR signal intensities are explained by the combined effect of spin lattice relaxation (SLR) in the triplet state and in the precursor radical pair state.
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