ELECTRON-ENERGY-LOSS SPECTROSCOPY OF FREE FULLERENES - A DIELECTRIC MODEL OF THE COLLECTIVE EXCITATIONS

The electronic excitations of the fullerene molecules C-60 and C-70 ha ve been analysed by electron energy loss spectroscopy (EELS) in the ga s phase. The spectra, particularly the collective excitations (plasmon s), have been compared to a dielectric model for the molecules, which is based on the simulation of the molecular structure of fullerenes by a hollow spherical or spheroidal shell with the dielectric properties of graphite. The anisotropic dielectric function of graphite has been modelled by a superposition of appropriate Lorentz oscillators. In or der to resolve finer structure within the EELS signal we have set up a position-sensitive detector with a CCD array for the spectrometer. Th e parallel detection of different electron energies within a certain r ange allows a significant reduction of the recording time of a spectru m. On-chip accumulation of the column signals in the array is performe d to record single-electron events.