Time Scales of Coherent Dynamics in the Light-Harvesting Complex 2 (LH2) of Rhodobacter sphaeroides

The initial dynamics of energy transfer in the light-harvesting complex 2 from Rhodobacter sphaeroides were investigated with polarization controlled two-dimensional spectroscopy. This method allows only, the coherent electronic motions to be observed, revealing the time scale of dephasing among the excited states. We observe persistent coherence among all states and assign ensemble dephasing rates for the various coherences. A simple model is utilized to connect the spectroscopic transitions to the molecular structure, allowing us to distinguish coherences between the two rings of chromophores and coherences within the rings. We also compare dephasing rates between excited states to dephasing rates between the ground and excited states, revealing that the coherences between excited states dephase on a slower time scale than coherences between the ground and excited states.