Abstract
Clusters of C 60 and C 70 coated with alkali or alkaline earth metals are investigated using photoionization time-of-flight mass spectrometry. Intensity anomalies in the mass spectra of clusters with composition C 60M x and C 70M x ( x = 0… 500; M ϵ| Ca, Sr, Ba) seem to be caused by the completion of distinct metal layers around a central fullerene molecule. The first layer around C 60 or C 70 contains 32 or 37 atoms, respectively, equal to the number of carbon rings constituting the fullerene cage. Unlike the alkaline earth metal-coated fullerenes, the electronic rather than the geometric configuration seems to be the factor determining the stability of clusters with composition (C 60) nM x and (C 70) nM x , M ϵ| Li, Na, K, Rb, Cs. The units C 60M 6 and C 70M 6 are found to be particularly stable building blocks of the clusters. At higher alkali metal coverage, metal-metal bonding and an electronic shell structure appear. An exception was found for C 60Li 12, which is very stable independently of charge. Semiempirical quantum chemical calculations support that the geometric arrangement of atoms is responsible for the stability in this case.
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