Photon Counting, Correlation Functions, and the Critical Region

Abstract

The statistical distribution of the number of photoelectrons emitted in a short time interval from a phototube illuminated by laser light scattered from a fluid is examined. Moments of this distribution measure correlated scattering of photons and are related to density correlations of various orders in the fluid. It is pointed out that a measurement of the second moment will indicate whether corrections are needed to the usual interpretation of the scattered-light spectrum. Deviations of the higher moments from the value for uncorrelated scattering would allow measurement of the spatial dependence of multiparticle correlations. The strength of the correlated scattering contribution to the various moments is estimated near a critical point, where it is expected to be largest, using a scaling-law equation of state and available data for simple fluids. According to this estimate, measurement of these multi-particle correlations will be very difficult in a simple fluid. It is pointed out that it may be easier near the critical mixing point of a binary fluid.