Excited-state energy equilibration via subpicosecond energy transfer within the inhomogeneously broadened light-harvesting antenna of LH-1-only Rhodobacter sphaeroides mutants M2192 at room temperature and 4.2 K

Using pump-probe spectroscopy, the dynamics of energy transfer within the inhomogeneously broadened light-harvesting antenna of LH-1-only mutants of the photosynthetic bacterium Rhodobacter sphaeroides was studied at room temperature and 4.2 K. In both cases, the transient difference spectra shift approximately 140 cm(-1) to lower energy, with most of the shift occurring within the first picosecond after excitation. Employing an inhomogeneous distribution for the excited-state energies of the subunits in the LH-1 antenna and using a weak coupling (Forster-type) energy-transfer mechanism, the observations can well be simulated with a transfer time between optimally overlapping antenna subunits at a single lattice distance of approximately 0.28 ps at room temperature (RT) and 0.40 ps at 4.2 K. We find that the fwhm width of the inhomogeneous distribution function decreases from 400 cm(-1) at RT to 200 cm(-1) at 4.2 K.