Role of multiple dipole scattering in high-resolution electron-energy-loss spectroscopy

A cleaved crystal of GaSe was analyzed by high-resolution electron-energy-loss spectroscopy (HREELS) and its optical parameters were deduced by using the dielectric theory. The angular and intensity distributions of the multiple energy losses and gains of low-energy electrons scattered on GaSe(0001) were examined in detail, in order to assess the relevance of the dielectric theory for describing higher-order excitations than the first Born approximation. We find a high linear correlation between the angular shift of the successive peaks recorded on the spectrum, and we demonstrate that peak intensities are underestimated in specular measurements. Experimental as well as theoretical implications for HREELS are discussed.