| 1. |
- Kuernsteiner, Philipp, et al.
(författare)
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Matrix effects in the neutralization of He ions at a metal surface containing oxygen
- 2013
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 609, s. 167-171
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Tidskriftsartikel (refereegranskat)abstract
- Charge exchange between He ions and a Ni(111) surface containing oxygen was studied by Low-Energy Ion Scattering, using 1.25 key 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.
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| 2. |
- Bruckner, Barbara, et al.
(författare)
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Electronic excitation of transition metal nitrides by light ions with keV energies
- 2020
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Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 32:40
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the specific electronic energy deposition by protons and He ions with keV energies in different transition metal nitrides of technological interest. Data were obtained from two different time-of-flight ion scattering setups and show excellent agreement. For protons interacting with light nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom were found, which coincide with literature data of N(2)gas for primary energies <= 25 keV. In case of the chemically rather similar nitrides with metal constituents from the 5(th)and 6(th)period, i.e. ZrN and HfN, the electronic stopping cross sections were measured to exceed what has been observed for molecular N(2)gas. For He ions, electronic energy loss in all nitrides was found to be significantly higher compared to the equivalent data of N(2)gas. Additionally, deviations from velocity proportionality of the observed specific electronic energy loss are observed. A comparison with predictions from density functional theory for protons and He ions yields a high apparent efficiency of electronic excitations of the target for the latter projectile. These findings are considered to indicate the contributions of additional mechanisms besides electron hole pair excitations, such as electron capture and loss processes of the projectile or promotion of target electrons in atomic collisions.
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| 3. |
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| 4. |
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| 5. |
- Bruckner, Barbara, et al.
(författare)
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Impact of the experimental approach on the observed electronic energy loss for light keV ions in thin self-supporting films
- 2021
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584. ; 489, s. 82-87
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Tidskriftsartikel (refereegranskat)abstract
- Energy spectra of keV ions interacting with self-supporting foils of Au and W were recorded with a Time-of-Flight Medium-Energy Ion Scattering system in different backscattering and transmission geometries in an otherwise identical experimental setup. For He ions transmitted through Au spectra of detected particles depending on geometry show two distinct components, i.e. different energy loss distributions in the film, whereas for protons no such phenomenon was observed. The two components in the spectrum of transmitted He could be attributed to impact parameter dependent energy loss, being more prominent for He than for protons. The main origin of the necessary impact parameter selection along different trajectories is expected to be texture in the Au-foils. Electronic stopping powers obtained from both, most probable energy loss in transmission and best fits to spectrum width in backscattering are in agreement within experimental uncertainties. This observation applied for all investigated combinations of projectile and target material.
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| 6. |
- Bruckner, Barbara, et al.
(författare)
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Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides
- 2020
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 691
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the impact of surface oxygen on the ion yield for He+ ions scattered from different single crystalline surfaces in low-energy ion scattering. Initially clean Al(111) and Ta(111) were exposed to molecular oxygen and ion spectra for different oxidation stages and different primary energies were recorded. A comparison of ion yields normalized to the differential scattering cross section as well as experimental factors allows obtaining information about the influence of oxygen on charge exchange processes. The decrease in the ion yield of both metals with exposure cannot be explained by different surface coverages exclusively, but requires the neutralization efficiency to be dependent on the chemical structure of the surface. For Ta, additionally, a different energy dependency of the ion yield obtained in the metal and oxide occurs. The ion yield for O shows in both surfaces a significantly weaker energy dependency than the investigated metals.
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| 7. |
- Bruckner, Barbara
(författare)
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New aspects of electronic interactions of keV ions with matter
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low- and medium-energy ion scattering are powerful techniques to perform high-resolution depth profiling with sub-nanometer resolution. Typically, ions with primary energies between a few keV and a few hundred keV are used to probe the sample and backscattered projectiles are detected. To obtain highly accurate composition profiles, knowledge on physical processes governing ion-matter interaction is crucial. Apart from the main (back-)scattering process, which yields a detectable signal, the projectile loses energy in interactions with both electrons and nuclei (stopping) along its path in matter. In all these interactions, also the charge state of the probing particle can be altered. Information on this multitude of interaction mechanisms can be deduced from two different experimental approaches: either in backscattering or transmission geometry. Especially towards lower primary energies, available experimental data are found more scarce. This situation is particularly true for more complex targets, i.e. reactive transition metals and their compounds. This absence of quantitative information on energy loss or charge exchange processes hampers in many cases the quality of characterization despite the high technological relevance of these materials.To contribute to an improvement of this status quo, this thesis focuses on (i) an analysis of sources of uncertainties in the evaluation of electronic energy loss, (ii) experiments to obtain stopping data for protons and He ions in different reactive samples and (iii) studies of charge exchange between projectile and target.The first part presents a discussion of two possible sources of systematic errors, i.e. the composition of the investigated sample (thin films of the reactive transition metals often have low Z impurities like H, C, N and O), and deficiencies in the available models for the scattering potential. Concerning impurities in the films, it is shown that a correction according to Bragg's rule yields good agreement with data obtained from clean samples, even for energies down to a few keV, as long as the concentration levels of the impurities are low. In the second part experimentally deduced electronic energy loss data for transition metal nitrides as well as self-supporting Au and W-foils are presented. In the latter study a comparative approach using backscattering and transmission experiments is performed with measurements in both geometries conducted on the same sample, and in the same scattering chamber with only the position of the detector varied. In the final section the influence of surface oxygen on the energy spectra of backscattered ions at primary energies ≤ 5 keV is investigated. Depending on the host material O is found to enhance or suppress sub-surface signals. Additionally, also the change in neutralization efficiency for surface oxides in comparison to clean metal surfaces is studied for single crystalline Al(111) and Ta(111).
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| 8. |
- Bruckner, Barbara, et al.
(författare)
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On the influence of uncertainties in scattering potentials on quantitative analysis using keV ions
- 2020
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584. ; 470, s. 21-27
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Tidskriftsartikel (refereegranskat)abstract
- Experimental spectra from Medium-Energy Ion Scattering were compared to Monte-Carlo simulations (employing the TRBS code) to obtain information on the scattering potential. The impact of uncertainties in the interatomic potential on quantification of sample properties such as thickness, composition or electronic stopping was investigated for different scattering geometries: backscattering and transmission. For backscattered He ions with tens of keV primary energy the scattering potential was found to overestimate the multiple scattering background in the energy spectra resulting in an uncertainty of < 3 % in quantitative analysis. Light ions transmitted through a sample for equivalent path length in the medium are only affected minorly by changes in the scattering potential. This effect becomes more distinct for heavier primary ions.
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| 9. |
- Bruckner, Barbara, et al.
(författare)
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The impact of surface oxidation on energy spectra of keV ions scattered from transition metals
- 2019
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Ingår i: Applied Surface Science. - : ELSEVIER SCIENCE BV. - 0169-4332 .- 1873-5584. ; 479, s. 1287-1292
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Tidskriftsartikel (refereegranskat)abstract
- Studying the initial stages of surface oxidation is of great relevance to understand how oxygen alters the physical and chemical properties at the interface of the host material to the environment and is therefore, crucial for improvement in manifold technological applications. We investigated the influence of surface oxygen on ion spectra recorded for keV noble gas ions backscattered from metal surfaces in low energy ion scattering (LEIS). Initially pure Zn and Ta surfaces, chosen for their well-characterized properties in ion-neutralization in LEIS, have been oxidized and ion spectra for pure and oxidized surfaces have been compared. Oxygen on the surface significantly influences shape and intensity of the backscattered ion spectrum at all energies: for both metal systems, the surface scattered ion yield of the metal is drastically decreasing under oxygen presence. The observed decrease, however, cannot be explained by the reduction in the surface areal density of the metal constituents exclusively. At least for Zn an additional significant change in charge exchange behavior is necessary to explain the observations. In contrast to the generally observed decrease in the yield of ions scattered from the outermost surface, the change in shape and intensity of the reionization background are found to show opposing trends and different energy dependencies for Zn and Ta.
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| 10. |
- Sortica, Mauricio A., et al.
(författare)
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On the Z(1)-dependence of electronic stopping in TiN
- 2019
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- We present a thorough experimental study of electronic stopping of H, He, B, N, Ne and Al ions in TiN with the aim to learn about the energy loss mechanisms of slow ions. The energy loss was measured by means of time-of-flight medium-energy ion scattering. Thin films of TiN on silicon with a delta-layer of W at the TiN/Si interface were used as targets. We compare our results to non-linear density functional theory calculations, examining electron-hole pair excitations by screened ions in a free electron gas in the static limit, with a density equivalent to the expected value for TiN. These calculations predict oscillations in the electronic stopping power for increasing atomic number Z(1) of the projectile. An increasing discrepancy between our experimental results and predictions by theory for increasing Z(1) was observed. This observation can be attributed to contributions from energy loss channels different from electron-hole pair excitation in binary Coulomb collisions.
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