Surface-plasmon dispersion and multipole surface plasmons in Al(111)

We have measured the collective electronic excitations of the Al(111) surface by means of angle-resolved high-resolution electron-energy-loss spectroscopy. Loss spectra reveal both the monopole and the multipole surface plasmons. The measured dispersion of the monopole surface plasmon is negative, as predicted by calculations of the dynamic response of the electron at the surface of a free-electron-like sample. The aluminum multipole surface plasmon, which was not detected in previous electron-energy-loss investigations, is clearly observed in present spectra.