Clusters of C60 and C70 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 C60Mx and C70Mx (x=0...500; M in (Ca,Sr,Ba)) seem to be caused by the completion of distinct metal layers around a central fullerene molecule. The first layer around C60 or C70 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 (C60)nMx and (C70)nMx, M in (Li,Na,K,Rb,Cs). The units C60M6 and C70M6 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 C60Li12, 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.
 

Carbon, 33 995-1006, 1995.