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Disparate Energy Scaling of Trajectory-Dependent Electronic Excitations for Slow Protons and He Ions
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0001-9180-6525
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: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 124, no 9, article id 096601Article in journal (Refereed) Published
Abstract [en]

We have simultaneously measured angular distributions and electronic energy loss of helium ions and protons directly transmitted through self-supporting, single-crystalline silicon foils. We have compared the energy loss along channeled and random trajectories for incident ion energies between 50 and 200 keV. For all studied cases the energy loss in channeling geometry is found lower than in random geometry. In the case of protons, this difference increases with initial ion energy. This behavior can be explained by the increasing contribution of excitations of core electrons, which are more likely to happen at small impact parameters accessible only in random geometry. For helium ions we observe a reverse trend—a decrease of the difference between channeled and random energy loss for increasing ion energy. Because of the inefficiency of core-electron excitations even at small impact parameters at such low energies, another mechanism has to be the cause for the observed difference. We provide evidence that the observation originates from reionization events induced by close collisions of helium ions occurring only along random trajectories.

Place, publisher, year, edition, pages
2020. Vol. 124, no 9, article id 096601
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:uu:diva-409698DOI: 10.1103/physrevlett.124.096601ISI: 000517416700008PubMedID: 32202865OAI: oai:DiVA.org:uu-409698DiVA, id: diva2:1426691
Funder
Swedish Research Council, 821-2012-5144; 2017-00646_9Swedish Foundation for Strategic Research , RIF14-0053Available from: 2020-04-27 Created: 2020-04-27 Last updated: 2020-05-28Bibliographically approved
In thesis
1. Beyond scattering – what more can be learned from pulsed keV ion beams?
Open this publication in new window or tab >>Beyond scattering – what more can be learned from pulsed keV ion beams?
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Interactions of energetic ions with matter govern processes as diverse as the influence of solar wind, hadron therapy for cancer treatment and plasma-wall interactions in fusion devices, and are used for controlled manipulation of materials properties as well as analytical methods. The scattering of ions from target nuclei and electrons does not only lead to energy deposition, but can induce the emission of different secondary particles including electrons, photons, sputtered target ions and neutrals as well as nuclear reaction products. In the medium-energy regime (ion energies between several ten to a few hundred keV), ions are expected to primarily interact with valence electrons. Dynamic electronic excitations are, however, not understood in full detail, and remain an active field of experimental and theoretical research. In addition, whereas scattered ions are employed for high-resolution depth profiling in medium energy ion scattering (MEIS), research on secondary particle emission in this regime is scarce.

This thesis explores possibilities to experimentally study ion-solid interactions in the medium-energy regime beyond a backscattering approach. The capability for detection of electrons, photons and sputtered ions was integrated into the time-of-flight (ToF-) MEIS set-up at Uppsala University. Additionally, transmission of ions in combination with crystalline samples was employed to study impact-parameter dependent electronic excitations. In all cases, the use of pulsed ion beams with nanosecond pulse widths proves to be imperative for achieving energy measurements with sufficient resolution as well as low doses for non-destructive interactions even with sensitive samples.

Trajectory-dependent energy loss of various ions in Si(100) was studied. For all ions heavier than protons, experimental evidence shows that, if close collisions are not suppressed by channelling, consequent charge-exchange events increase the mean charge state of the ion and heavily influence the experienced energy loss. Furthermore, measurements of electron emission are presented. For medium-energy ions, electrons emitted in forward direction from carbon foils exhibit energies between 10 and 400 eV. Scaling with ion velocity indicates binary collisions as the primary energy transfer mechanism. Detected photons also have energies of a few eV, i.e. on the order of typical valence transitions in solids. For photon emission, pronounced chemical matrix effects are observed. Finally, the sputtering process at medium energies was studied. Target bulk constituents exhibit similar behaviour as known from established methods at lower energies, i.e. sputtering by nuclear collision cascades. In contrast, the desorption of surface species seems to be governed by electronic energy transfer mechanisms.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 90
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1945
Keywords
Charge exchange, Deep UV photons, Electron emission, Silicon, Sputtering, TOF-MEIS
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-409892 (URN)978-91-513-0964-4 (ISBN)
Public defence
2020-06-12, Polhelmsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-05-20 Created: 2020-05-04 Last updated: 2020-05-20

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Lohmann, SvenjaPrimetzhofer, Daniel

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