This award supports Professor Jan A. Northby of the University of Rhode Island for collaboration in research with Professor J. Peter Toennies of the Max Planck Institute for Flow Research in Gottingen, West Germany. They are collaborating in the study of the properties of microscopic helium clusters which are formed in cryogenic free jet expansions. Their approach involves producing and studying helium cluster ions at the U.S. lab and neutral helium clusters at the German lab, then comparing results. The broad objective of their research is to examine the evolution of macroscopic phenomena, in particular of superfluidity, as the number of atoms increases. Their current efforts are concentrated in developing the technology for producing and characterizing helium cluster beams covering the size range where departures from macroscopic behavior can be expected. The German laboratory has two molecular beam machines dedicated to helium cluster experiments and uniquely suited to the proposed collaborative research; however, they have only recently begun working with condensed helium. Professor Northby has had considerable experience with rare gas clusters initially formed as ions and has benefited from Dr. Toennies expertise in molecular beam work. The complementary expertise and facilities of the investigators create a very strong collaborative arrangement. Clusters of atoms form the bridge between atomic properties and the properties of the bulk. Interest in the study of size-dependent properties in clusters of all kinds of materials has grown dramatically in the past decade. From the experimental side, this has been driven by the development of free jet expansion techniques for the production of cluster beams. From the theoretical side, growth has been driven by the tremendous expansion in computer power which now permits detailed ab initio calculations of the properties of quite large finite systems. The proposed research should lead to fundamental new understanding of superfluidity and the transition of matter from the atomic to the bulk state. This could have a practical spin-off for film growth and other areas.