Scaling Properties of Ion Neutralization in LEIS

Low Energy Ion Scattering (LEIS) experiment is a widely used analytical tool to characterize the structure and the composition of the surface of a sample. The information on the elemental concentration of the surface is obtained from the energy spectrum of scattered ions. As projectiles, noble gas ions (e.g., He+ ) are used, which are sent to the sample at a large angle with respect to the surface and an ion energy of several keV. If only the scattered ions are analyzed, LEIS is sensitive to the outermost atomic layer. To analyze the composition of a sample surface, the scattering cross section and the ion fraction have to be known for the atomic species of interest. However, the ion fractions are not yet well known. Therefore, the main aim of this proposal is to obtain a reasonably good qualitative understanding of the neutralization process, which is the basis for a safe application of LEIS as a surface analytical tool. To reach this goal, LEIS measurements will be performed using 3 He and 4 He isotopes as projectiles, and selected materials with wide spread neutralization properties (Cr, Fe, Ni, Zn, AL, and a NiAl alloy) as target. Where ever possible, single crystals will be used in order to minimize possible uncertainties due to the badly known structure of polycrystalline samples.

Low Energy Ion Scattering (LEIS) is widely used to analyse quantitatively the composition and the structure of surfaces of polycrystalline samples and of single crystals. For composition analysis, the intensity of scattered noble gas ions with kinetic energy in the range 1 to 10 keV is analysed. For this one has to know the probability of electron exchange between the projectile and the surface, since these processes have a strong influence on the intensity of scattered ions. It is not yet understood, but well known that in general the ion fraction amongst the scattered projectiles is - for a given type of projectile - just an atomic property of the scattering centre and does not depend on presence of other types of atoms in the surface. In other words, in general there are no matrix effects which would make a quantitative analysis more difficult to obtain. The SPIN-LEIS project was dedicated to deepen the understanding of the scattered intensity in LEIS, focussing on the ion fraction and the scattering potential, which defines the scattering probability. In a first step, we studied the fraction of positive He ions backscattered from the Cu(100) surface, in comparison with a polycrystalline copper surface. Strong crystal effects have been found, at low energies where only non-local electron transfer via tunnelling processes is possible, and at high energies, where also electron transfer may occur during the close encounter. The crystal effect at low energies can be understood in terms of different extension of the conduction electrons outside the surface for different crystal orientations, the effects at high energies are related to the fact that in this regime also subsurface atomic layers contribute to the scattered intensity, due to reionisation of neutralised projectiles in a final collision with a surface atom when leaving the surface. We also measured how for a single crystal the scattered intensity depends on the scattering geometry: in polar scans, the angle of incidence is varied, in azimuth scans, the sample is rotated around the surface normal. In these angular scans, the scattered intensity of ions and neutrals was measured. From these measurements we could - by comparison with computer simulations - deduce how many atomic layers contribute to the scattered intensities of ions and neutrals, and found that depending on how the intensity is defined (e.g., in a wide energy window or a narrow one) leads to very different results: for a wide energy window, more than 20 atomic layers contribute to the intensity, and only for a narrow window and after subtraction of the background intensity due to multiple scattering an information depth of one to two atomic layers is obtained. Finally, we could show by computer simulations that for polycrystalline copper the shape of the energy spectrum of projectiles scattered off the surface depends crucially on the scattering potential and on the electronic stopping force.