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

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