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Intra- and extramolecular vibrations of fulleride surfaces

The surfaces of K-doped C 6 0 thin films have been studied by high-resolution electron energy loss spectroscopy in reflection geometry. In addition to showing the expected intramolecular infrared-active modes of T 1 u symmetry, as well as their usual behavior upon doping, the K 4 C 6 0 and K 6 C 6 0 phases display two K-related, dipole-active, extramolecular phonons. In K 3 C 6 0 , the electronic screening limits the dipolar probing depth to the topmost layer and only one K-related phonon is observed. Our data also reveal that the Raman-active A g vibrations dominate the intramolecular energy region. The properties and the modified K/C 6 0 stoichiometry of the outmost surfaces of fullerides are discussed accordingly.