Electron energy loss spectroscopy of free fullerenes — a dielectric model of the collective excitations

The electronic excitations of the fullerene molecules C60 and C70 have been analysed by electron energy loss spectroscopy (EELS) in the gas phase. The spectra, particularly the collective excitations (plasmons), 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 order to resolve finer structure within the EELS signal we have set up a position-sensitive detector with a CCD array for the spectrometer. The parallel detection of different electron energies within a certain range allows a significant reduction of the recording time of a spectrum. On-chip accumulation of the column signals in the array is performed to record single-electron events.