Non-invertibility of Multiple-Scattered QELSS Spectra
Abstract
Description
We consider the spectrum S(q,t) and field correlation function f(q,t) of light quasielastically scattered from diffusing optical probes in complex viscoelastic fluids. Relationships between the single-scattering f_1(q,t) and the multiple-scattering f_m(t) are examined. We show that it is fundamentally impossibly to invert f_m(t) to obtain f_1(q,t) or particle displacement moments <X^{2n}>, except with assumptions that are certainly not correct in complex, viscoelastic fluids. For diffusing dilute probes in viscoelastic fluids, f_1(q,t) is determined by all even moments <X^{2n}>, n > 0$, of the particle displacement X; this information is lost in f_m(t). In the special case of monodisperse probes in a true simple fluid, f_1(q,t) can be obtained from f_m(t), but only because the functional form of f_1(q,t) is already known.
7 pages and 1 figure
7 pages and 1 figure