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Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity
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.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0002-5815-3742
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. Experiments were performed using pulsed beams of singly charged ions incident on self-supporting Si(100) nanomembranes, and energy was measured together with the angular distribution after transmission through the sample. The energy loss of H+, H2+, He+, N+, Ne+, 28/29Si+ and Ar+ was analyzed along channeled and random trajectories. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For protons, the difference between channeled and random trajectories increases with ion energy, which is explained by increasing contributions of core-electron excitations more likely to happen at small impact parameters accessible only in random geometry. For heavier ions, the energy loss difference between channeling and random geometry is generally found more pronounced, and, different from protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channeled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit an oscillation with Z1 with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

Keywords [en]
Ion transmission, Energy loss, Charge exchange
National Category
Physical Sciences Condensed Matter Physics
Research subject
Physics with specialization in Ion Physics
Identifiers
URN: urn:nbn:se:uu:diva-409891OAI: oai:DiVA.org:uu-409891DiVA, id: diva2:1427904
Funder
Swedish Research Council, 821-2012-5144Swedish Research Council, 2017-00646_9Swedish Foundation for Strategic Research , RIF14-0053Available from: 2020-05-04 Created: 2020-05-04 Last updated: 2020-05-04
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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Primetzhofer, Daniel

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