ENERGY MIGRATION IN THE LIGHT-HARVESTING ANTENNA OF THE PHOTOSYNTHETIC BACTERIUM RHODOSPIRILLUM-RUBRUM STUDIED BY TIME-RESOLVED EXCITATION ANNIHILATION AT 77 K

The intensity dependence of picosecond kinetics in the light-harvestin g antenna of the photosynthetic bacterium Rhodospirillum rubrum is stu died at 77 K. By changing either the average excitation intensity or t he pulse intensity we have been able to discriminate singlet-singlet a nd singlet-triplet annihilation. It is shown that the kinetics of both annihilation types are well characterized by the concept of percolati ve excitation dynamics leading to the time-dependent annihilation rate s, The time dependence of these two types of annihilation rates is qua litatively different, whereas the dependencies can be related through the same adjustable parameter-a spectral dimension of fractal-like str uctures. The theoretical dependencies give a good fit to the experimen tal kinetics if the spectral dimension is equal to 1.5 and the overall singlet-singlet annihilation rate is close to the value obtained at r oom temperature. The percolative transfer is a consequence of spectral inhomogeneous broadening. The effect is more pronounced at lower temp eratures because of the narrowing of homogeneous spectra.