Variations of LEED intensities with angle of incidence and the influence on spot profiles.

Abstract

We have performed a systematic theoretical study of the effects of variations in the direction of the incoming electron beam on low-energy electron-diffraction (LEED) intensities for the surfaces Ni{111} and p(2\ifmmode\times\else\texttimes\fi{}2)C-Ni{111}. Large intensity modulations are observed for variations of the order of 5\ifmmode^\circ\else\textdegree\fi{} in the polar angle of incidence, and the R factor comparison between theoretical I-V curves rise to 1.0 (i.e., no correlation) after varying the polar angle by about 10\ifmmode^\circ\else\textdegree\fi{}--15\ifmmode^\circ\else\textdegree\fi{}. Such data can be used as an independent set of I-V curves to increase the energy range of LEED analyses. Another benefit of collecting data at off-normal incidence directions is that multiple R-factor minima can be shifted or eliminated by changes of 20\ifmmode^\circ\else\textdegree\fi{}--30\ifmmode^\circ\else\textdegree\fi{} in the polar angle. However, we could not see a clear improvement in the sensitivity towards the lateral positions of surface atoms for polar angles below 40\ifmmode^\circ\else\textdegree\fi{}. We have also studied possible effects on measured spot profiles for data taken by scanning through the incidence direction. Serious differences in the relative satellite intensities of split spots which are equal at constant incident angle and additional broadening and distortions of spot shapes could be found for certain angular scan ranges and energies.