HREELS and ELS-LEED Studies of Surface Plasmons on Ag and Pd Single Crystals

Recent progress in experimental investigation of the surface plasmon for d-electron metals with angle resolved electron energy loss spectroscopy and energy loss spectroscopy - low energy electron diffraction is reviewed. The latter technique allows to greatly improve resolution in k-space which constitutes the major limiting factor for measuring surface plasmon dispersion. Applying both techniques we show that for Ag the initial surface plasmon dispersion is linear and positive instead of negative as it is the case for simple metals, For single crystal surfaces we find that the initial slope of the dispersion, the dependence of surface plasmon damping on transferred momentum and the dependence of both quantities on crystal temperature are anisotropic with respect to crystal face. For Ag(111) also the multipole surface plasmon could be identified in the spectra, in accord with the theoretical prediction for a smooth shape of the electronic density profile at the surface. Surface plasmon damping is discussed and shown for Ag to follow the anisotropy of surface plasmon dispersion. For Pd on the contrary a strong negative dispersion of the main loss feature at 7 eV was found