COMPARISON OF THE INTENSITY CORRELATION-FUNCTION AND THE INTERMEDIATESCATTERING FUNCTION OF FLUIDS - A MOLECULAR-DYNAMICS STUDY OF THE SIEGERT RELATION

The Siegert relation is a link between the intensity correlation funct ion I-2(k, t) and the intermediate scattering function F(k, t). This r elation is commonly used in the analysis of dynamic light scattering d ata. Molecular dynamics computer simulation renders possible a test of this relation since F(k, t) and I-2(k, t) are directly accessible. In addition we study the influence of the size of the ''scattering volum e'' for which F(k, t) and I-2(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 one hand by number fluctuations and on the other hand by the dynamics of the local order in the fluid.