Umladungsprozesse langsamer Ionen an Festkörperoberflächen

Summary: Research was done on the neutralization behavior of He+ ions backscattered from a Cu surface in the Low Energy Ion Scattering (LEIS) regime, i.e. incident energies of 1 - 10 keV. The essential goal of the project was to perform key experiments to find out, which are the relevant neutralization mechanisms that are contributing in this energy range. The quantity of interest is the ion fraction P+ , i.e. the fraction of projectiles, which are backscattered from surface atoms and leave the surface as singly charged positive ions, and its functional dependence on certain parameters. Within this project, we have shown, that there are two clearly distinguishable energy regimes: At energies below a certain threshold energy, only one neutralization mechanism is active and P+ depends just on the projectiles velocity component v perpendicular to the target surface. At energies above the threshold energy, the situation is more complex. More than one mechanism is contributing, resulting in additional parameter. Results: We systematically studied a peculiarity of charge integrated LEIS spectra, i.e. the so-called surface peak. This feature has been reported many times but an explanation has never been given. We can give now the following explanation: backscattering of projectiles by atoms in the outermost atomic layer is always due to binary collisions, resulting in a very narrow energy distribution. In the LEIS regime, backscattering from the second or even deeper layers is more (at low energies) or less (at high energies) dominated by multiple scattering, resulting in a broad continuum superimposed to the narrow energy distribution of the binary collision. We clearly showed within this project that the ion fraction P+ is rather a function of v than the overall velocity component of the projectile resolving a discrepancy known from literature. We have also shown that there are two regimes for the ion fraction P+ characterized by the functional dependence of P+ on velocity and geometry. Below a certain threshold energy, only one neutralization mechanism (so-called Auger neutralization) is active, which can be described by one parameter, only. We determined this parameter properly within an accuracy of <1%. At energies above the threshold energy additional mechanisms come into play (so-called collision induced neutralization and re-ionization), resulting in a more complex functional dependence of P+ . It is due to the present choice of energy range, scattering angle and scattering geometry, that it was possible to resolve the dependence of P+ on velocity and geometry, while former studies either stayed in the energy regime below the threshold energy or just measured at fixed geometry. Our results provide a more profound understanding of neutralization in LEIS. Additionally our set-up may help to apply LEIS as a quantitative surface analysis tool for various industry applications, e.g. thin films, coatings, catalysts...