Picosecond absorbance difference spectra of the antenna of photosynthetic purple bacteria. The influence of exciton interactions and librations.

The influence of exciton interactions and pseudolocal librational modes on the relaxation of photo-induced absorption changes of the light-harvesting antenna of the photosynthetic purple bacterium, Rhodospirillum rubrum, was studied in the ps time domain. A hypothesis is put forward that the long-lived librational modes can occur in the circular aggregate of light-harvesting bacteriochlorophyll molecules of the core antenna. The model spectra obtained quantitatively fit the shape of the photo-induced picosecond absorbance difference spectra. The model predicts the decay of photo-induced spectra with the time constant within tens of picoseconds.