Position-displacement correlations in QELSS spectra of non-dilute colloids

This paper examines relationships between the quasielastic light scattering spectrum S(q, t) and the distribution functions for particle displacements over various times. For dilute probes in a complex, non-scattering fluid, S(q, t) is determined by the even moments of the one-particle displacement distribution function P(X, t). For concentrated scattering particles, S(q, t) is not determined by P(X, t). Instead, S(q, t) is determined in part by P(X, t) and in part by a spatial Fourier transform of the two particle displacement distribution function P(2)(X, t, R(12)). Here, X is the displacement of particle 1 during t, and R(12) is the component (at t = 0), parallel to the scattering vector q, of the vector from particle 1 to a second particle 2.