Figure 5

Correlation spectroscopy can be used to probe a wide range of timescales and processes. Illustration of processes that can contribute to the fluorescence correlation functions of freely diffusing molecules, with their characteristic timescales and corresponding amplitudes. Illustrated are: photon antibunching [112] in the range of the fluorescence lifetimes of the fluorophores, τ _fl ; rotational diffusion with a correlation time τ _rot , and interdye distance dynamics due to FRET with a reconfiguration time τ _r (note that τ _rot and τ _r can be in a similar range, but can be distinguished by the donor-acceptor crosscorrelation (Figure 6), which shows anticorrelated behaviour in the case of distance dynamics); triplet state blinking on a timescale τ _T ; and translational diffusion (Figure 4) with a diffusion time τ _diff . Timescales much greater than τ _diff are not accessible with freely diffusing molecules, but can be extended by taking advantage of recurrence effects [18] (i.e. molecules returning to the confocal volume) or with experiments on immobilized molecules (Figure 7).