Relativistic effects in EELS of nanoporous alumina membranes

Electrochemically prepared porous alumina membranes with a pore diameter of 58 nm have been investigated by electron energy-loss spectroscopy in a transmission electron microscope operated at 120 keV and 200 keV. Energy-loss spectra are recorded for electrons traveling along the pores at different impact parameters. The dominant spectral features near the pore wall are shown to originate in the excitation of surface plasmons. Additional loss features at 7 eV (8 eV) for 200 keV (120 keV) are observed, whose origin is associated to Cherenkov radiation modified by the sample nanostructure. Maxwell equations are solved for different geometrical models of the target, providing a good qualitative description of the experimental spectra. Quantitative agreement is obtained by considering multiple plasmon excitation, suggesting a promising tool to investigate pore surfaces.