  High-resolution electron energy loss spectroscopy (HREELS) is one of the most
powerful methods to detect the dispersion of plasmons. However, we find that in
the HREELS measurement, the scattering geometric configuration will seriously
affect the identification of plasmons. Here, taking graphene as an example,
using the HREELS capable of two-dimensional energy-momentum mapping combined
with the intensity distribution calculations, we visually display the intensity
distribution of the scattering geometric factor. We demonstrate that the energy
loss peaks from the scattering geometric effect may be misinterpreted as the
features of an acoustic plasmon. In any HREELS measurement, it is necessary to
evaluate the effect of the scattering geometry quantitatively to identify the
intrinsic surface excitations.
