Antibunching and Rotational Diffusion in FCS

Abstract

The topics of this chapter, photon antibunching and rotational diffusion, have found far fewer practical applications than translational diffusion and chemical or photophysical effects in FCS. In spite of the interesting features and unique information that these phenomena provide, they have attracted merely academic interest. One reason for this could be the added complexity of the experimental technique. Also, the physical interpretation of the results of real experiments is more involved than the basic theory suggests. The physical basis of both phenomena is different. Rotational diffusion is well known as the reorient at ional component of the Brownian motion. Anti­bunching in fluorescence can be viewed as a property of individual fluorescent molecules to emit photons which are separated in time by a characteristic minimum interval. The reason for treating these phenomena together is that they both appear usually in the nanosecond time range of the autocorrelation function. Therefore, when attempting to measure rotational diffusion, antibunching must also be taken into account . Although photon antibunching can be observed in the emission of various sources (e.g., specific laser configurations, nonlinearly downconverted light), this chapter will be restricted to the fluorescence of organic dye molecules. We shall describe the basic theory, illustrated with some experimental examples and potential applications of FCS concerning antibunching and rotational diffusion.