Inelastic Low-Energy-Electron Diffraction for Surface-Plasmon Studies: Extended Measurements on Epitaxial A1(111)

Abstract

Inelastic low-energy-electron diffraction measurements on a clean A1(111) surface have been greatly extended and improved. This provides the experimental basis for a much more accurate determination of the high-momentum surface-plasmon dispersion and lifetime. The surface is prepared in situ in an ultrahigh-vacuum scanning diffractometer by epitaxial deposition on a Si(111) single-crystal wafer. Specular beam inelastic intensity profiles are measured in the $<11\mathrm{}\overline{2}>$ and $<1\mathrm{}\overline{1}0>\mathrm{}$ directions at 15° and 25° incidence. The primary beam energy is chosen to correspond closely to both diffraction-before-loss and loss-before-diffraction processes involving different vestigial Bragg peaks at roughly 50 and 100 eV. Present emphasis is on loss profiles, which have recently been shown to be essentially free of complicating dynamical effects. The use of a 100-meV data grid and a more efficient computer differentiation technique permits determination of loss peak positions to 50 meV under favorable conditions. Internal consistency of the data is demonstrated here and in a more comprehensive analysis by Duke and Landman, published separately. The analysis leads to a substantially different evaluation of the surface-plasmon dispersion than previously obtained. There is evidence that the visibly smoother substrate used in the present work does not affect this result significantly. Experimental procedures and results are discussed in detail.