PERSISTENT HOLE-BURNING AND FEMTOSECOND PUMP-PROBE ABSORPTION-SPECTROSCOPY OF GREEN SULFUR PHOTOSYNTHETIC BACTERIA ANTENNAE

The excited state relaxation in antenna of green sulphur photosyntheti c bacteria was studied at low temperatures both directly, by femtoseco nd one-colour pump-probe absorption spectroscopy, and indirectly, by p ersistent spectral hole burning. The transient absorption profiles yie lded several exponential components. The fastest decay component (life time 250-350 fs) probably reflects an excitonic relaxation within the aggregate. At natural anaerobic conditions, the main component at arou nd 775 nm is characterized by a lifetime of 5.8 ps that agrees with th e lifetime of the lowest excited state of bacteriochlorophyll c determ ined from zero-phonon hole widths (delta(ZPH) = 1.85 cm(-1)). We inter pret it as the mean time of energy transfer between bacteriochlorophyl l c clusters. At aerobic conditions, the excitation decays mainly with a 2.6 and 16 ps lifetime. The observed hole widths are correspondingl y broader (delta(ZPH) = 4.0 cm(-1)). This is due to quenching by a hit herto unidentified mechanism protecting photosynthetic apparatus again st a singlet-oxygen-induced damage. (C) 1998 Elsevier Science B.V. All rights reserved.