METAL-COATED FULLERENES

Clusters of C-60 and C-70 coated with alkali or alkaline earth metals are investigated using photoionization time-of-flight mass spectrometr y. Intensity anomalies in the mass spectra of clusters with compositio n C(60)M(x) and C(70)M(x) (X = 0...500; M is an element of {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 con tains 32 or 37 atoms, respectively, equal to the number of carbon ring s constituting the fullerene cage. Unlike the alkaline earth metal-coa ted fullerenes, the electronic rather than the geometric configuration seems to be the factor determining the stability of clusters with com position (C-60)(n)M(x) and (C-70)(n)M(x), M is an element of {Li, Na, K, Rb, Cs}. The units C(60)M(6) and C(70)M(6) are found to be particul arly stable building blocks of the clusters. At higher alkali metal co verage, 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 t he geometric arrangement of atoms is responsible for the stability in this case.