COUPLING EFFECTS IN THE EXCITATIONS BY AN EXTERNAL ELECTRON-BEAM NEARCLOSE PARTICLES

The energy-loss experienced by swift scanning transmisson electron mic roscopy electrons is calculated for a target constituted by two spheri cal particles in the framework of dielectric theory, using bispherical coordinates. Deviations from the behavior of the isolated sphere are investigated. The coupling between close particles and its effects on the energy-loss spectrum is found to depend significantly on the elect ron trajectory and target situation (particle size and interparticle d istance). Excitations above the planar surface plasmon energy are foun d to be the dominant contribution to the loss spectrum for trajectorie s between the two spheres, while low-energy excitations, under those o f the isolated sphere, appear for trajectories out of that region. The different possibilities are analyzed and compared with experimental r esults for aluminum particles and provisions of recent effective mediu m theories. The inelastic part of the induced electric field around th e spheres is also studied showing a very important localization of the excitation in the space between both particles, even for trajectories outside that region. The expressions obtained are available for any l ocal dielectric function.