Mechanism of the low-ejection-energy (e,2e) reaction on a graphite surface

We develop a theoretical model to describe a slow electron ejection from a crystal by electron impact at a moderate incident energy. The electron impact ionization is considered within the first Born approximation. The projectile is treated as a plane wave whereas the target electron initial and final states are described by the bulk one-electron wave functions in the momentum space representation. To allow the ionized electron to escape from the crystal the final state in the bulk of the crystal is matched in energy and a parallel component of momentum by a plane wave in the vacuum. This theoretical model is used to simulate the binding-energy spectra obtained by the grazing-angle reflection mode (e,2e) reaction on the surface of highly oriented pyrolytic graphite. [S0163-1829~98!03011-2]