Femtosecond pump-probe spectroscopy of the B850 antenna complex of Rhodobacter sphaeroides at room temperature

The photosynthetic bacterium Rhodobacter sphaeroides contains a light-harvesting antenna complex (LH2) with a ring of interacting bacteriochlorophyll molecules (B850). Excitation of membrane-bound LH2 complexes with low-intensity, femtosecond pulses causes changes in absorption and stimulated emission that initially depend on the excitation wavelength but relax to a quasiequilibrium with a time constant of 100 +/- 20 fs. Excitation on the blue side of the B850 absorption band is followed by a shift of the signals to longer wavelengths and a decrease in amplitude, whereas the relaxations following excitation on the red side consist mainly of a decrease in amplitude. The signals have an apparent initial anisotropy of approximately 0.5 when the complex is excited with broadband pulses, and 0.35-0.4 with narrower pulses. The anisotropy decays to 0.1 with a time constant of about 30 fs. The anisotropies are similar at wavelengths on either side of the absorption band and are relatively insensitive to the excitation wavelength. Contributions of coherent pump-probe coupling and perturbed free induction decay to the measured anisotropies are considered. Pump-probe coupling could increase the initial anisotropy but cannot account for the decay kinetics. Using a density-matrix formalism, we show that the initial light-induced signals are consistent with coherent excitation of multiple exciton levels in an inhomogeneous ensemble of LH2 complexes and that the main features of the spectral relaxations and the anisotropy can be explained by electronic dephasing and thermal equilibration within the manifold of exciton levels.