Abstrakt
Energy relaxation in light-harvesting complexes has been extensively studied by various ultrafast spectroscopic techniques, the fastest processes being in the sub-100-fs range. At the same time, much slower dynamics have been observed in individual complexes by single-molecule fluorescence spectroscopy (SMS).
In this work, we use a pump-probe-type SMS technique to observe the ultrafast en- ergy relaxation in single light-harvesting complexes LH2 of purple bacteria. After excitation at 800 nm, the measured relaxation time distribution of multiple complexes has a peak at 95 fs and is asym- metric, with a tail at slower relaxation times.
When tuning the exci- tation wavelength, the distribution changes in both its shape and position. The observed behavior agrees with what is to be expected from the LH2 excited states structure.
As we show by a Redfield theory calculation of the relaxation times, the distribution shape cor- responds to the expected effect of Gaussian disorder of the pigment transition energies. By repeatedly measuring few individual com- plexes for minutes, we find that complexes sample the relaxation time distribution on a timescale of seconds.
Furthermore, by compar- ing the distribution from a single long-lived complex with the whole ensemble, we demonstrate that, regarding the relaxation times, the ensemble can be considered ergodic. Our findings thus agree with the commonly used notion of an ensemble of identical LH2 complexes experiencing slow random fluctuations.