Intra- and extramolecular vibrations of fulleride surfaces

The surfaces of K-doped (formula presented) 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 (formula presented) symmetry, as well as their usual behavior upon doping, the (formula presented) and (formula presented) phases display two K-related, dipole-active, extramolecular phonons. In (formula presented) 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 (formula presented) vibrations dominate the intramolecular energy region. The properties and the modified (formula presented) stoichiometry of the outmost surfaces of fullerides are discussed accordingly.