Doppler Shifts in Light Scattering from Pure Liquids and Polymer Solutions

A framework for interpreting experiments on Doppler shifts in light scattering from pure liquids and polymer solutions is presented. The spectral density of the light scattered into a given frequency range and angle by a liquid composed of identical, isotropic particles much smaller than the wavelength of the incident light is expressed in terms of the Fourier transform of the two-body space—time correlation function. The formalism is then applied to the problem of light scattering from dilute polymer solutions. The Doppler shifts are evaluated for a polymer molecule undergoing translational Brownian motion, and for a rigid rod-shaped molecule undergoing Brownian motion about its center of mass. It is concluded that for a pure liquid more accurate experiments are needed to determine the form of the long-range part of the space—time correlation function. For polymer solutions, for which Doppler shift measurements have never been done, Doppler shift measurements should yield information about the shape of the polymer molecule, and its translational and rotational diffusion coefficients. In addition, for flexible molecules, information would be obtained about the long wavelength internal modes of motion of the molecule. The recently developed optical maser has made such experiments practical.