Fig. 3. Fluorescence intensity kinetics of single LHCII complexes after the onset of illumination (at t = 0) for different excitation intensities and the same excitation modulation, ton + toff = 50 μs + 50 μs = 100 μs. These kinetics (AOM histograms) were extracted by histogramming the absolute fluorescence photon arrival times into one modulation cycle with a binning time of 100 ns (gray-shaded region in the inset). The measurement at the highest excitation intensity (green curve) had a shorter on-time of 3.3 μs in order to prevent fast photo-bleaching at such a high excitation intensity. Inset: Illustration of the stepwise amplitude modulation of the excitation light via an acousto-optic modulator. By effectively turning the excitation laser on and off on the μs time range we can control the time-dependent changes in the concentration of triplets, as schematically shown with the dashed blue line. The varying concentration of triplets can then be observed via the measured fluorescence intensity kinetics.
