Comparison of the intensity correlation function and the intermediate scattering function of fluids: a molecular dynamics study of the Siegert relation

The Siegert relation is a link between the intensity correlation function I2(k, t) and the intermediate scattering function F(k, t). This relation is commonly used in the analysis of dynamic light scattering data. Molecular dynamics computer simulation renders possible a test of this relation since F(k, t) are directly accessible. In addition we study the influence of the size of the “scattering volume” for which F(k,t) are extracted. The validity of the Siegert relation is confirmed for “large” scattering volumes while for small systems deviations are found. These deviations are caused on hand by number fluctuations and on the other hand by the dynamics of the local order in the fluid.