Abstract
The excited state relaxation in antenna of green sulphur photosynthetic bacteria was studied at low temperatures both directly, by femtosecond one-colour pump-probe absorption spectroscopy, and indirectly, by persistent spectral hole burning. The transient absorption profiles yielded several exponential components. The fastest decay component (lifetime 250–350 fs) probably reflects an excitonic relaxation within the aggregate. At natural anaerobic conditions, the main component at around 775 nm is characterized by a lifetime of 5.8 ps that agrees with the lifetime of the lowest excited state of bacteriochlorophyll c determined from zero-phonon hole widths ( δ ZPH = 1.85 cm −1). We interpret it as the mean time of energy transfer between bacteriochlorophyll c clusters. At aerobic conditions, the excitation decays mainly with a 2.6 and 16 ps lifetime. The observed hole widths are correspondingly broader ( δ ZPH = 4.0 cm −). This is due to quenching by a hitherto unidentified mechanism protecting photosynthetic apparatus against a singlet-oxygen-induced damage.
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