ELASTIC ELECTRON BACKSCATTERING FROM SURFACES WITH OVERLAYERS

The most pronounced effects of elastic scattering in solids are observed in electron backscattering experiments, which, as a result, offer the possibility to test the completness of the theoretical models of electron transport. For this purpose, elastic backscattering from the Au/Ni(111) system was studied in the present work. Since the total-elastic-scattering cross section for Au is considerably larger than that for Ni, one would expect from simple arguments that the observed elastic peak intensities increase as the Au overlayer thickness is increased. However, a considerable intensity decrease (by a factor of up to 5) with increasing overlayer thickness is observed at energies up to at least 500 eV. This unexpected behavior can be explained within the proposed theory as due to a particular arrangement of differential-scattering cross sections within the experimental geometry used. At higher energies the backscattered intensities increase, in agreement with expectations. In all cases, the predicted intensities compare very well with experimental observations. The low-energy-electron-diffraction pattern for polycrystalline gold shows a characteristic dark ring whose origin can be ascribed to the complex structure of the differential-elastic-scattering cross section of gold. The decreased elastic-backscattering probability within the dark ring is responsible for the unusual behaviour of intensities observed in the overlayer experiments. The position of the dark ring is well explained by the proposed Monte Carlo algorithm.