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  in the range of the fluorescence lifetimes of the fluorophores, τ
; rotational diffusion with a correlation time τ
, and interdye distance dynamics due to FRET with a reconfiguration time τ
(note that τ
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
; and translational diffusion (Figure 4) with a diffusion time τ
. Timescales much greater than τ
are not accessible with freely diffusing molecules, but can be extended by taking advantage of recurrence effects  (i.e. molecules returning to the confocal volume) or with experiments on immobilized molecules (Figure 7).