Oscillating anisotropies in a bacteriochlorophyll protein: Evidence for quantum beating between exciton levels Original Research Article

Optical anisotropies of bacteriochlorophyll a Qy electronic transitions in Fenna-Matthews-Olson trimers from the green photosynthetic bacterium Chlorobium tepidum eshibit strong oscillations with ∼ 220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations in the polarized pump-probe signals Δ A∥(t), Δ A⊥] (t) have opposite phase, and they nearly cancel in the isotropic signals. This combined with the fact that they only appear when the laserpulse spectrum overlaps both the lowest-energy groups of excitation levels clustered near 815 and 825 nm, suggests that the oscillations stem from quantum beating between exciton levels, rather than coherent from nuclear motion. In some cases, the anisotropy r(t) exhibits values > 0.4 at times well beyond the laser autocorrelation function.