Intra- and extramolecular vibrations of fulleride surfaces

The surfaces of K-doped C60 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 T1u symmetry, as well as their usual behavior upon doping, the K4C60 and K6C60 phases display two K-related, dipole-active, extramolecular phonons. In K3C60, 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 Ag vibrations dominate the intramolecular energy region. The properties and the modified K/C60 stoichiometry of the outmost surfaces of fullerides are discussed accordingly.