Low-Energy Electron Diffraction from Ag(111): Intensity-versus-Energy Curves and Absolute Intensity of the (00) Beam

The intensity of the (00) beam of a (111) surface of Ag has been measured with a Faraday cage as a function of the energy of the incident beam (10 < E < 280 eV), the grazing angle of incidence (46.5° < φ < 83.5°), two azimuths differing by 180°, and the temperature. The I vs E curves, when compared with data for Ag ( 111 ) of other workers who have used different methods of surface preparation, show good agreement in the structure over the whole range of incident angles, indicating that LEED is not strongly sensitive to surface condition. The I vs E curves for the two azimuths are identical, a necessary result of the reciprocity theorem. For comparison with the I vs E structure, a complete 3-beam geometric model is used. This differs from a simple Ewald construction in that it considers also the Bragg conditions between intermediate beams and the final beam. It also requires that there be no difference in the effect of intermediate forward and backward scattered beams. It is shown that the number of possible beams is much too large even at low energies to make positive identification of any structure in the I vs E curves. A comparison with a rigorous multiple-scattering theory yields agreement in the number and position of peaks, but not in heights and widths of peaks. The possibility of comparison of absolute intensities in theory and experiment is investigated and an attempt is made to remove the major differences. Intensity vs temperature measurements are made at closely spaced energies in order to extract the rigid-lattice scattering. Correction of this intensity for surface plasma losses leads finally to maximum scattered intensities of 2% at 100 eV, 10% at 60 eV, and up to 40% at energies below 20 eV.