Plasmon excitations in graphitic carbon spheres measured by EELS

The determination of the physical properties of individual nanometer-size particles has made rapid progress with the availability of local probe techniques during the past years. Electron energy-loss spectroscopy in a high-resolution transmission electron microscope is one experimental tool that can give insight into the intriguing properties of such small particles. The interpretation of the experimental data of the plasmon excitations is well established in the case of isotropic particles of different geometries. For the case of anisotropic particles such as multiwall fullerenes (carbon onions), the interpretation schemes had to be reviewed. In a recent publication, we have proposed a formalism based on nonrelativistic local dielectric response theory for high-energy transmission electron microscopy electrons penetrating or passing close by an anisotropic particle [St\"ockli et al., Phys. Rev. B 57, 15599 (1998)]. Here we report a detailed comparison of experimental data with the excitation probabilities obtained within this formalism. We show that there is an excellent agreement between theory and experiment. In consequence, we are able to interpret the plasmon loss data of multiwall fullerenes and draw conclusions on their physical properties.