Abstract: A new improved fluorescence correlation spectroscopy apparatus for determining motional states of particles, in which the scheme of fluorescence correlation spectroscopy with traveling interference fringe excitation (FCSTFE) is applied, is presented. In this method, the modulated fluorescence signal from particles excited by a moving interference fringe is detected, and cosine and sine Fourier coefficients at the frequency of the traveling fringe are recorded. The autocorrelation function of the fluorescence intensity consists of terms which are characterized by the size of the excitation region and the spacing of the fringe. A lock-in detection method is adopted for extracting cosine and sine Fourier coefficients at the frequency of the traveling fringe $LB@F$-cos$/(t), F$-sin$/(t)$RB@. Autocorrelation functions of F$-cos$/(t) and F$-sin$/(t) express the motion of the fluorescent particles. We constructed the apparatus using acousto-optic modulator (AOM) devices to generate the traveling fringes. The potential of this method for the extraction of information about the dynamics of particles in biological systems is discussed. !24
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