A fluorescence correlation spectroscopy experiment that combines two-photon excitation and a standingwaveinterference pattern is presented. The experimental correlation function can be analyzed using a simple expression involving (1) an exponential decay with time constant (tau)_(f), which reflects diffusion across the interference fringes, and (2) a longer-lived decay with time constant (tau)_(omega), which reflects diffusion in and out of the focal spot. The diffusion of Rhodamine 110 in water and ethylene glycol is measured using this method. The ability to simultaneously measure diffusion on two different time and lengthscales makes this experiment especially useful in environments where anomalous diffusion is suspected.
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