Time-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale. Yet the fundamental cause of electronic and vibrational dynamics-the coupling between the different energy levels involved-is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range and directly measure electronic couplings in a molecular complex, the Fenna-Matthews-Olson photosynthetic light-harvesting protein. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment-protein complex.
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Acknowledgements We thank Y.-Z. Ma, L. Valkunas and M. Yang for discussions, and C. Goodhope for protein purification. The apparatus for 2D spectroscopy was constructed by I. V. Stiopkin and T.B. This work was supported by the DOE (at LBNL, UC Berkeley and Arizona State University), and by a CRIP grant to M.C. by KOSEF (Korea). T.B. thanks the German Science Foundation (DFG) for an Emmy Noether fellowship, and J.S. thanks the German Academic Exchange Service (DAAD) for a postdoctoral fellowship.
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