The electronic stopping cross section (SCS) of Ni for slow H+, H2+, D+ and D2+ ions has been investigated for different types of targets in two complementary experimental geometries, i.e., in transmission and backscattering. To warrant sample purity, both a high purity nickel sheet and nanometer Ni layers were prepared in-situ under ultra-high-vacuum conditions. In an alternative approach, ultra-thin samples were prepared ex-situ as self-supporting foils and as nanometer films on a polished substrate (silicon). Identical SCS results are obtained in backscattering using the in-situ prepared film and the high purity sheet. The ex-situ prepared targets contained considerable concentrations of impurities of low atomic numbers, whose contribution to the SCS can be rectified by applying Bragg's rule using TRIM stopping for the impurities. In this way for the ex-situ targets the accuracy of the resulting SCS data is improved considerably. Concordant stopping cross section data are obtained in both geometries. The achieved accuracy does, however, not permit to spot a possible influence of different impact parameter regimes explored in transmission and in backscattering geometries.

Electronic stopping measurements in chemically reactive targets, e.g., transition and rare earth metals are challenging. These metals often contain low Z impurities, which contribute to electronic stopping. In this article, we present two ways how one can correct for the presence of impurities in the evaluation of proton and He stopping in Ni for primary energies between 1 and 100 keV, either considering or ignoring the contribution of the low Z impurities to multiple scattering. We find, that for protons either method leads to concordant results, but for heavier projectiles, e.g. He ions, the influence on multiple scattering must not be neglected.

The electronic stopping cross sections (SCS) of Ta and Gd for slow protons have been investigated experimentally. The data are compared to the results for Pt and Au to learn how electronic stopping in transition and rare earth metals correlates with features of the electronic band structures. The extraordinarily high SCS observed for protons in Ta and Gd cannot be understood in terms of a free electron gas model, but are related to the high densities of both occupied and unoccupied electronic states in these metals.

In this paper we present results on charge exchange of He+ ions at a polycrystalline Cu surface. Monte Carlo simulations were used to calculate the trajectories of projectiles scattered by an angle Θ=136°. By including Auger neutralization and charge exchange in close collisions, energy spectra of the scattered ions as well as ion fraction values were calculated for primary energies in the range 0.5–5keV and compared to experimental results. In the simulations, the Auger neutralization rate Γ and the probabilities of resonant neutralization (PRN) and reionization (PRI) are treated as free parameters. Using well accepted values from literature for these quantities very good agreement between simulations and experimental data was achieved.

We explore under which conditions low-energy ion scattering experiments are capable of obtaining reliable quantitative results in surface composition analysis. Additionally, we elaborate on the effective information depth of this technique considering the effect of different dominant charge exchange mechanisms. Based on concrete examples, we also point out possible problems and pitfalls in the evaluation of experimental data.

We present electronic stopping power data for H and He ions in Zn and In obtained from backscattering experiments in the range of low velocities; for In data are obtained up to the stopping maximum. These results in combination with previous data are analyzed to unravel the influence of d electrons in electronic stopping of slow light ions. We report on a systematic difference between the stopping behavior of H and He ions at velocities below 0.2 a.u. Furthermore, we find that the d-band offset is not correlated to the velocity at which deviations from velocity proportionality occur. This observation is in contrast to the state-of-the-art models of electronic stopping based on electron-hole pair excitation in a binary Coulomb collision. In this regard we discuss the possible relevance of electron promotion processes in atomic collisions.

Charge exchange by Auger neutralization (AN) plays an important role in surface analysis techniques such as low energy ion scattering (LEIS). Recent advances in the theoretical description of AN have included a model based on a linear combination of atomic orbitals (LCAO) approach, which is able to calculate accurate neutralization probabilities of He+ due to AN in LEIS. Previous investigations have shown that the neutralization probability is strongly influenced by the distance dependent shift of the He 1s level. In this study simulations of He+ scattered from Cu(100) and Cu(110) surfaces at fixed azimuth angles are presented. Additionally, the azimuth dependence of ion- and neutral-yield for He+ scattered from Cu(100) is simulated and compared to experimental data. Calculations were performed using the LCAO model in combination with molecular dynamics simulations. The excellent agreement between simulation and experiment provides evidence that the obtained values for the level shift are a characteristic property of the surface.

In recent energy loss measurements, band structure effects in electronic stopping have been observed for materials with finite excitation thresholds, for example, noble metals such as Cu and Au. To further investigate the influence of the position of the d band relative to the Fermi edge, electronic stopping of hydrogen and helium ions in Ag and Pt was determined. For Ag, the electronic stopping power exhibits a velocity dependence similar to Cu and Au. No particular effect due to the comparatively large d-band offset in Ag is found. In the case of Pt, the electronic stopping power is virtually velocity proportional for H+ ions and exhibits a distinct velocity dependence for He+ ions. For hydrogen the results are compatible with modeling the conduction band as a free electron gas with an energy-dependent effective number of electrons. For He+, however, the observed effects point towards an additional energy loss mechanism, e.g., by charge-exchange processes.

Electronic stopping of H and He ions in metals and insulators is analyzed at velocities below 0.2 atomic units, i.e. below 1 keV for H and below 4 keV for He. In metals, stopping of H ions is affected by d-electrons only when the d-band extends up to the Fermi energy; for He ions, also d-bands well below the Fermi energy contribute significantly to electronic stopping. In insulators, the low threshold velocity for electronic stopping cannot be explained by electron-hole pair excitation; charge exchange cycles, however, may govern the threshold behavior of electronic stopping in ionic crystals.

In this work we compare characteristics of Auger neutralization of [Formula: see text] ions at noble metal and free-electron metal surfaces. For noble metals, we find that the position of the energy level of He with respect to the Fermi level has a non-negligible influence on the values of the calculated Auger rates through the evaluation of the surface dielectric susceptibility. We conclude that even though our calculated rates are accurate, further theoretical effort is needed to obtain realistic values of the energy level of He in front of these surfaces.

The determination of the electronic stopping power for low-energy ions is an experimentally demanding task. In this paper we elaborate on the different effects of nuclear stopping and multiple scattering on the energy spectra for different experimental geometries, i.e., transmission through thin foils and backscattering from thin films. By calculating distributions of path lengths and scattering angles we demonstrate how electronic stopping, nuclear stopping, and multiple scattering add up to the total energy loss. We show that at low energies it is important to properly disentangle these effects to extract electronic stopping from the measured energy loss spectra.

The electronic stopping cross section of protons in polycrystalline Ge is deduced from TOF-LEIS measurements using an amorphized Ge wafer and a thick Cu reference sample. The evaluation is based on the comparison of height ratios of backscattering spectra from both, experiment and Monte Carlo simulations, which allow for multiple scattering of the protons in the sample. This procedure yields the electronic stopping cross section of Ge for protons with a relative uncertainty of less than 10% per data point. In the velocity regime 0.07–0.56a.u., corresponding to proton energies in the range 120eV–8keV, the electronic stopping cross section is linear in the ion velocity. Extrapolation of the data yields a velocity threshold of vth=0.0274a.u.±10%. At velocities below 0.18a.u., i.e. at proton energies below 810eV, electronic stopping in Ge is – compared to Cu – found to be more efficient by up to a factor of 2.6.

When low-energy He ions are scattered from a Ge surface, the fraction of positive ions exhibits characteristic oscillations as a function of ion energy. These oscillations are caused by quasi-resonant neutralization (qRN), a process which is active for materials with a narrow band nearly resonant with the unperturbed He 1s-level. In this paper we measure the fraction of He+ backscattered from Ge(100). In conjunction with recently developed theoretical methods, we extract quantitative information on the efficiency of qRN. Our evaluation reveals that qRN is a highly efficient process leading to ion fractions two orders of magnitude lower than in systems for which neutralization is only due to Auger processes.

Charge exchange between He ions and a Ni(111) surface containing oxygen was studied by Low-Energy Ion Scattering, using 1.25keV He+ as primary ions. The energy resolved yield of positive ions was detected after backscattering from Ni or O for different exposures of Ni(111) to molecular oxygen. Pronounced changes in the neutralization efficiency due to the presence of oxygen are observed for both, the adsorbate phase at low oxygen dose, and the NiO phase at high dose. The presence of O in the surface makes resonant charge transfer in a close collision possible. Evidence for a strong matrix effect is found: O in NiO neutralizes much more efficiently than O in the adsorbate phase. Independently, the different interaction stages of Ni–O and the surface structure were monitored by Photoelectron-Emission-Microscopy and Low-Energy Electron Diffraction.

We investigate the contribution of charge exchange processes in close collisions between projectile and target atoms to the electronic energy loss of low energy ions. We measure the energy loss of slow hydrogen and He ions in ultrathin Al films through which the ions are transmitted before and after backscattering by the atoms of a Ta substrate. The individual contributions to the energy loss are analyzed. The roles of thresholds for reionization and of scattering kinematics as key parameters for the coupling between elastic and inelastic losses are discussed. The implications of the obtained results for different experimental approaches to deduce stopping cross sections are outlined.

Electronic energy loss of light ions transmitted through nanometer films of Al has been studied at very low ion velocities. For hydrogen, the electronic stopping power S is found to be perfectly proportional to velocity, as expected for a free electron gas. For He, the same is anticipated, but S shows a transition between two distinct regimes, in both of which S is velocity proportional-however, with remarkably different slopes. This finding can be explained as a consequence of charge exchange in close encounters between He and Al atoms, which represents an additional energy loss channel.

In Low Energy Ion Scattering (LEIS), Auger-neutralization is an omnipresent charge exchange mechanism, especially when noble gas ions are used as projectiles, with a primary energy below the threshold energy, Eth, for collision induced charge exchange processes (neutralization and reionization). Recent experiments revealed a significant dependence of the ion survival probability, P+, on the crystal plane, when He+ ions are scattered from a metal surface. This is in contrast to the fact, that the neutralization probability in LEIS is usually assumed to be independent of the chemical environment of the collision partner (absence of matrix effects). In order to investigate this crystal effect, an existent theory on Auger-neutralization (based on a Linear Combination of Atomic Orbitals) is adapted to the LEIS geometry. With this model, Auger-neutralization rates are calculated for a Ag(1 1 0) surface. Trajectories for He particles scattered from this surface into different azimuth directions are obtained by means of Molecular Dynamics simulations. Subsequently, the ion survival probability is calculated and compared to measurements. Good agreement is obtained which gives confidence in the applicability of this model in the LEIS regime. Moreover, it was possible to obtain detailed information on the properties of the neutralization process.

Charge exchange of 4He+ and 3He+ ions with surfaces of polycrystalline aluminium and Al(111) was investigated in the low-energy ion scattering (LEIS) regime. Ion spectra were recorded for primary energies ranging from 280 to 4000eV by using an electrostatic analyzer (ESA). A very low threshold energy Eth for collision induced charge exchange (CI) was deduced from the shape of experimental spectra. Ion fractions P+ were evaluated. No systematic difference in P+ was observed for both, the two surfaces investigated and the two different projectiles.

A comparative study of Auger neutralization (AN) of He ions at noble metal surfaces is presented in order to reveal how the electronic structure of the sample influences this charge exchange process. Comparison of calculated ion fractions to experimental data obtained in low energy ion scattering (LEIS) shows that good agreement is achieved only if the relevant aspects of the He-metal interaction are properly taken into consideration. For instance AN depends sensitively on the distance-dependent position of the projectile level, which varies significantly when considering different target materials.