Excitation of dielectric spheres by external electron beams

Classical dielectric theory is developed to provide an analytical expression for the energy losses experienced by a fast charged particle passing close to a dielectric sphere. The results are compared with energy loss spectra obtained from Ag catalyst particles in a scanning transmission electron microscope. In practice a considerable number of angular momentum components are required, particularly if the effects of thin oxide or other surface coatings are to be included. For close surface encounters at large spheres the results confirm the validity of a simple quasi-planar approximation.