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

摘要

The electronic excitations of the fullerene molecules C_60 and C_70 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 hell 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 differnet electron energies within a certain range allows a significant reduction of the recording time of a spectrum. On-chip accumulation of thecolumn signals in the array is performed to record single-electron events.