| 1. |
- Bruckner, Barbara, et al.
(författare)
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A note on extracting electronic stopping from energy spectra of backscattered slow ions applying Bragg's rule
- 2018
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : ELSEVIER SCIENCE BV. - 0168-583X .- 1872-9584. ; 423, s. 82-86
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Goebl, D., et al.
(författare)
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Quasi-resonant neutralization of He+ ions at a germanium surface
- 2013
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:48, s. 485006-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Roth, D., et al.
(författare)
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A procedure to determine electronic energy loss from relative measurements with TOF-LEIS
- 2013
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 317:Part A, s. 61-65
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Tidskriftsartikel (refereegranskat)abstract
- 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 0.07–0.56 a.u., corresponding to proton energies in the range 120 eV–8 keV, the electronic stopping cross section is linear in the ion velocity. Extrapolation of the data yields a velocity threshold of vth = 0.0274 a.u. ± 10%. At velocities below 0.18 a.u., i.e. at proton energies below 810 eV, electronic stopping in Ge is – compared to Cu – found to be more efficient by up to a factor of 2.6.
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| 4. |
- Roth, D., et al.
(författare)
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Electronic Stopping of Slow Protons in Transition and Rare Earth Metals : Breakdown of the Free Electron Gas Concept
- 2017
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 118:10
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Goebl, D., et al.
(författare)
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Auger neutralization of He+ on Cu surfaces : Simulation of azimuthal scans
- 2013
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 317:Part A, s. 23-27
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Monreal, R. C., et al.
(författare)
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Effects of the atomic level shift in the Auger neutralization rates of noble metal surfaces
- 2013
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 315, s. 206-212
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Tidskriftsartikel (refereegranskat)abstract
- In this work we compare characteristics of Auger neutralization of He 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.
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| 7. |
- Primetzhofer, Daniel, et al.
(författare)
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Local vs. non-local energy loss of low energy ions : Influence of charge exchange processes in close collisions
- 2013
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 317:Part A, s. 8-12
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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