Scaling properties of light scattering spectra for particles moving with helical trajectories.

This investigation indicates that model quasi-elastic light scattering spectra from particles moving on a helical trajectory exhibit nonscaling behavior. At low q light scattering samples the average progressive speed of the center of mass, while at large q it is the trajectory speed which is important. The extent of this non-scaling depends critically on the pitch and radius of the helix. For spherical scatterers the largest effect is seen when the ratio of the pitch to the radius is ~0.1. Interpreting data from such a system by using a point particle approximation would lead to a sizable underestimate of the speed of the scatterer. In the high q limit the scaling level of the model correlation functions is independent of the product of the curvature of the helix and q. The results for rotating disk-shaped ellipsoidal scatterers parallel those for spheres. The critical parameter for cases where the radius of the disk is large compared with the radius of the helix is the radius of the disk itself. When the radius of the disk is larger than the wavelength of the light the scaling behavior is determined primarily by rotational motion. Deviation from this behavior due to helical motion occurs only at extremely low values of q.