  Size and composition of clusters produced by adiabatic expansion of binary
gas mixtures (Ar-Kr, Kr-Xe, and N2-Ar) with various component concentrations
are studied by using electron-diffraction technique. The resulting homogeneous
and heterogeneous clusters are shown to have compositions substantially
different from those of the primary gas mixtures and dependent on cluster size.
We have found that the key parameters needed for an analysis of cluster
composition are the critical cluster radius and the heavier component
concentration in the gas mixture that can be used to establish the regions of
existence of homogeneous and heterogeneous clusters. These critical values
determine the coefficient of the enrichment of clusters with the heavier
component with respect to its concentration in the primary gas mixture.
Theoretical relations are obtained which provide a good quantitative
description of the experimental results and can be expected to be also valid
for finding the composition of binary clusters of other van der Waals systems.
