Kinematic Low-Energy-Electron-Diffraction Intensities from Averaged Data: Theoretical Test for Ag(111) and Al(100)

The accuracy of a recently proposed method of extracting kinematic low-energy-electron-diffraction (LEED) intensities from averages of experimental data taken at constant momentum transfer is examined by using calculated intensities in lieu of the data. These "dynamical" intensities are evaluated using the isotropic-scatterer inelastic-collision model. As the model parameters are known a priori, both the kinematical and dynamical intensities are specified exactly without appeal to any empirical procedures. Comparison of the kinematical and averaged-dynamical intensities reveals systematic discrepancies between the two on the order of 10-100% for parameters characteristic of electrons with energy $50\le E\le 400$ eV incident on clean surfaces of monatomic metals. These discrepancies arise because the averaging does not remove completely the effects of multiple scattering from strongly scattering ion cores. A general (geometrical) formulation of the relation between the kinematical and dynamical intensities is proposed. Within the framework of this formulation appropriate measures of the averaged intensities, kinematical intensities, and their differences are constructed. For physically reasonable model parameters, the averaging method is shown to be capable of determining the values of expansions (contractions) of the uppermost-layer spacings at clean single-crystal surfaces to within a few percent.