Stochastic Thermal-Diffusion Forced Rayleigh Scattering

The holographic grating technique of thermal-diffusion forced Rayleigh scattering (TDFRS) utilizes the Ludwig–Soret effect to induce a concentration modulation within a binary liquid. The signal generation is described in terms of a linear response formalism, and the memory function for the concentration mode g(t) and its Fourier transform, the diffusion susceptibility, are measured by means of pseudostochastic random binary sequences with flat power spectra in combination with fast Fourier transform and correlation techniques. For polydisperse polymer solutions the individual modes contribute proportional to their concentration to g(t), contrary to photon-correlation spectroscopy, where the correlation function is dominated by the high molar mass components. Other advantages of stochastic TDFRS are time-scale delocalization of dust spikes and frequency multiplexing. Measurements are reported on monodisperse and bimodal polystyrene in toluene.