Diffraction effects in the incident and exit paths of electrons backscattered from Cu(001) at medium energies

We analyze experimentally and theoretically the angular distribution of ele ctrons backscattered from Cu(001) at medium energies. We present polar inte nsity plots (PIP's) of backscattered electrons with important qualitative d ifferences with PIP's of x-ray photoemitted electrons acquired in similar c onditions. In particular, our PIP's of backscattered electrons present peak s, ascribed to diffraction effects in the incident part of the electron tra jectory, and a symmetry that are not observed in the PIP's of photoelectron s. The theoretical analysis has been made using a model presented recently to explain the change of the angular pattern in passing from low to medium energies. We present calculations performed including curved-wave-front and multiple-scattering effects that are in very good agreement with the exper iments. The results show that the main structures in the PIP's are produced by forward-focusing interferences (similar to those of photoelectron diffr action) in either the incoming or the outgoing parts of the electron trajec tory, and by interferences between waves with important forward scattering in both the incoming and outgoing parts of the electron trajectory. [S0163- 1829(99)03431-1].