Newtonian solvent is $g^{(1s)}(q,t)$ entirely determined by $\langle (\Delta x(t))^{2} \rangle$. Furthermore, the Langevin equation approach that ties $g^{(1s)}(q,t)$ to $\langle (\Delta x(t))^{2} \rangle$ *also requires with equal certainty* that $g^{(1s)}(q,t)$ relaxes as a simple exponential $\exp(- \Gamma t)$, $\Gamma$ being a time-independent constant. Contrariwise, *if the spectrum is not a simple exponential in time, $g^{(1s)}(q,t)$ is not determined by $\langle (\Delta x(t))^{2} \rangle$.* Several related subsidiary errors are discussed.
author:
- 'George D. J. Phillies'
title:  Interpretation of Quasielastic Scattering Spectra of Probe Species in Complex Fluids 
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Introduction
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Quasielastic scattering of light, x-rays, and neutrons has proven to be a powerful experimental tool for the study of complex fluids. For dilute macromolecule solutions, quasielastic scattering has extensive analytic applications for particle sizing[@phillies1990z]. With non-dilute solutions and more complex systems, quasielastic scattering reveals consequences of intermacromolecular and other interactions.

In a substantial enhancement of the classical method, the fluid of interest is doped with trace concentrations of monodisperse probe particles. Probes have ranged from small molecules to micron-size colloidal particles. The diffusion of probe particles through the complex fluid is then observed. Early successful studies of probe diffusion in complex fluids using quasielastic light scattering were by Hallett 