Page 188 - Carbon Nanotubes

et
180 U. ZIMMERMAN ai.
preted to signal the onset of metal-metal bonding. An
exception to the electronically determined cluster sta-
bility is C60Li12, which was observed to be particularly
stable independent of the cluster charge. Supported by
MNDO calculations, we found that the geometrical ar-
rangement of atoms in this cluster, one above each
pentagon of the fullerene, was most important for the
stability. At higher alkali metal coverage of the ful-
lerene, an electronic shell structure similar to pure
metal clusters is observed in the ionization threshold
of the clusters.

Acknowledgements-We would like to thank H. Schaber for
his outstanding technical assistance, U. Naher for many stim-
ulating discussions, and A. Mittelbach for providing the C,,
used in the experiments.

,
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