Clusters of argon, krypton, and xenon are grown in a free jet and ionized by electron impact. The size of these clusters, (Rg)+n, extends up tonbartil;1000. Individual cluster sizes are mass resolved up tonbartil;570 in the case of Ar+n. The well known, but puzzling differences in the size distributions of Kr and Xe clusters disappear beyondnbartil;130, while those between Ar and Xe disappear beyondnbartil;220. The most pronounced lsquo;lsquo;magic numbersrsquo;rsquo; in the distributions of large cluster ions occur atn=147 (148 for Ar), 309, and 561, in striking agreement with the number of atoms required to build icosahedral clusters with 3, 4, and 5 complete coordination shells, respectively. Closure of the 6th icosahedral coordination shell is indicated by another strong intensity drop atnbartil;923 in the unresolved part of the spectra. Several additional intensity extrema are observed between major shell closures. A simple structural model, assuming an icosahedral core decorated by the additional atoms, accounts for these anomalies reasonably well up ton=561.
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