The study of Brownian motion by intensity fluctuation spectroscopy

Abstract

The principles and methods of quasielastic light scattering and intensity fluctuation spectroscopy (i.f.s.) are reviewed briefly. Their application to the study of Brownian motion is discussed with emphasis on the behaviour of the velocity autocorrelation function ϕ ( τ ) of a single particle. We consider: (i) The situation usually assumed where ϕ ( τ ) decays rapidly compared with other timescales of interest; (ii) the modification of (i) due to the τ -3/2 ‘long-time tail’ in ϕ ( τ ) including the recent i.f.s. experiment of Boon & Bouiller (1976) which confirms its existence, and finally (iii) systems of charged colloidal particles exhibiting 'liquid-like’ spatial ordering due to long-range Coulombic interactions. Here ϕ ( τ ) has a negative long-time tail. We also consider the many-body dynamics in these systems by exploiting the similarity with neutron scattering by simple atomic liquids. ‘Solid-like’ systems are discussed briefly.