Si 4 clusters are soft-landed on an inert Van-der-Waals surface and the island formation is studied using x-ray and ultraviolet photoelectron spectroscopy. The experimental data indicate that the clusters do not fuse to Si islands but survive as individuals. The potential-energy surface of two Si clusters is calculated and a barrier against fusion is found, in agreement with the experiments. Both finding support the existence of a form of silicon consisting of Si 4 clusters.

Abstract.A PACIS (pulsed arc cluster-ion source) developed for high average cluster-ion currents is presented. The performance of the PACIS at different operational modes is described, and the suitability for cluster-deposition experiments is discussed in comparison with other cluster-ion sources. Maximum currents of mass-selected cluster ions of 3-6 nA of small Si The preparation of nanostructures on surfaces is one of the major tasks in technology and basic research. Applications of nanostructured surfaces are abundant and cover a wide range from heterogeneous catalysis to high-density computer memories. From the point of view of basic research, many of the properties of small three-dimensional nanostructures and two-dimensional islands on surfaces are not well understood yet. For example, there is no systematic study of the size-dependence of the electronic structure of clusters of simple metals on surfaces, although free clusters of such metals show strong variations depending on the number of delocalized electrons [1]. Concerning the chemical properties, there are first results on the size-dependence of the catalytic properties of small clusters on inert substrates [2], but there are only very few systematic studies yet (see below). The kind of research focussing on the electronic and chemical properties of monodispersed small clusters on surfaces is only at its beginning. The reason is that it is difficult to deposit clusters with a perfectly monodispersed size distribution on a surface.

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