Backscattering Cross Sections for Ionization of Surface-AtomKShells by Electron Impact

Backscattering cross sections for the electron-impact $K$-shell ionization of C, N, O, Na, and Mg adsorbed on a tungsten (100) surface are reported. The ionization event studied is the scattering of $K$-shell electrons to unfilled states within a few eV of the Fermi level of the W substrate. These data are obtained through a new technique which analyzes the currents in ionization-loss features by comparing them with the currents in the elastic peaks. This technique involves numerical simulation of the ionization features using distribution functions. Measured cross sections in the backscattered region are compared with the Burhop theory over an energy range extending from one to four times the ionization energy. The measured and calculated cross sections decrease with increased atomic number and with primary energy. Although some improvement in shape agreement is noted with increased atomic number, the experimentally derived backscattering cross sections exceed the calculated values for the entire energy range investigated. It is shown that the disagreement noted, even for the higherenergy range studied, cannot be attributed to double-scattering processes. The simple theory based on the first Born approximation, although reasonable in total-cross-section determination, is not accurate enough to account for behavior in the backscattered region.