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Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics. Johannes Kepler Univ Linz, Atom Phys & Surface, A-4040 Linz, Austria.ORCID iD: 0000-0003-2208-6581
Johannes Kepler Univ Linz, Atom Phys & Surface, A-4040 Linz, Austria.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0002-5815-3742
2020 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 691, article id 121491Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2020. Vol. 691, article id 121491
Keywords [en]
Low energy ion scattering, Ion yields, Charge exchange, Oxygen exposure, Al(111), Ta(111)
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-394735DOI: 10.1016/j.susc.2019.121491OAI: oai:DiVA.org:uu-394735DiVA, id: diva2:1359576
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2019-10-28Bibliographically approved
In thesis
1. New aspects of electronic interactions of keV ions with matter
Open this publication in new window or tab >>New aspects of electronic interactions of keV ions with matter
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1876
Keywords
ToF-MEIS, LEIS, electronic stopping, charge exchange, scattering potential
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-395952 (URN)978-91-513-0803-6 (ISBN)
Public defence
2019-12-06, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2019-11-08 Created: 2019-10-28 Last updated: 2019-11-08

Open Access in DiVA

The full text will be freely available from 2021-09-06 00:00
Available from 2021-09-06 00:00

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Publisher's full texthttp://www.sciencedirect.com/science/article/pii/S0039602819306065

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Bruckner, BarbaraPrimetzhofer, Daniel

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