  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 employ a pump-probe type SMS technique to observe the ultrafast
energy 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 asymmetric, with a tail at slower
relaxation times. When tuning the excitation wavelength, the distribution
changes in both its shape and position. The observed behaviour 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
corresponds to the expected effect of Gaussian disorder of the pigment
transition energies. By repeatedly measuring few individual complexes for
minutes, we find that complexes sample the relaxation time distribution on a
timescale of seconds. Furthermore, by comparing the distribution from three
long-lived complexes with the whole ensemble, we demonstrate that 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.
