The new technique of optical 2D spectroscopy for electronic transitions remedies the deficiency of conventional types of spectroscopy in which only the evolution of populations over time can be determined directly, but not the mechanisms that underlie these changes. Specifically, one can determine couplings, relaxation pathways and rates, and the spatial properties and overlap relationships between excitonic states. This makes it possible to follow energy flow on a molecular length scale with femtosecond time resolution, yielding microscopic insights into reaction mechanisms. In the case of Fenna-Matthews-Olson light-harvesting complex, energy is not simply transferred stepwise down the energy ladder; rather, the delocalization of wavefunctions over several bacteriochlorophyll molecules is exploited to move energy in a smaller number of steps, but larger energy portions than previously supposed.
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