Fluorescence correlation spectroscopy (FCS) is capable of probing dynamic p
rocesses in living biological systems. From photon fluctuation of fluoresci
ng particles which diffuse through a small detection volume, FCS reveals in
formation on the concentration and the structure of the particles, as well
as information on microscopic environment. In this note, we study the radia
tion forces experienced by Rayleigh particles in a laser field in details,
and analyze the effects of gradient field on FCS measurements.