Intensity dependence of high-order autocorrelation functions in fluorescence correlation spectroscopy

This paper describes several instrumental contributions to experimentally obtained high-order photoelectron fluctuation autocorrelation functions in single-photon counting fluorescence correlation spectroscopy. As shown, the high-order functions are intensity-dependent for at least three reasons: the presence of background light; the stochastic nature of photon collection and detection; and the temporal response of the photoelectron pulse counter. Theoretical expressions that describe the effects are derived and experimentally confirmed, and methods of determining the high-order fluorescence fluctuation autocorrelation functions of interest from experimentally obtained photoelectron fluctuation autocorrelation functions are outlined.