Characterization of the local structure of fluids by apertured cross-correlation functions

Apertured cross-correlation functions (ACCF's) are proposed and demonstrated to be useful constructs in the study of local structure in disordered systems. A formal expression for the apertured intensity cross-correlation function is obtained in terms of multiparticle ACCF's. This expression is examined in several limits and is seen to be related to a variety of correlation functions which can be studied via present or proposed intensity cross-correlation experiments. A two-dimensional hard-disk system is investigated by computer simulation to demonstrate the use of ACCF's. It is found that three- and four-body correlations must, in general, be included to give accurate representation of the apertured intensity cross-correlation function. A harmonic-model calculation is reviewed, and possible uses of the cross-correlation technique are discussed.