ENERGY-LOSS OF STEM ELECTRONS IN COUPLED SURFACES

In the frame of the self-energy formalism, the energy loss spectrum fo r STEM electrons moving close to a sphere coupled to a planar surface is studied. As an application the problem of the coupling between a sm all metallic particle and a large supporting surface is studied, as we ll as its dependence on both the size of the particle and the dielectr ic nature of the support. The coupling between both surfaces is found to be relevant only for particles of radius less than upsilonomega-1, where omega is the energy transfer and upsilon is the electron velocit y. This result is explained in terms of the range of the Coulomb inter action. Our results are in qualitative agreement with recent experimen tal data.