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Beyond scattering – what more can be learned from pulsed keV ion beams?
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics. (Ion physics)ORCID iD: 0000-0001-9180-6525
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
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 [en]
Charge exchange, Deep UV photons, Electron emission, Silicon, Sputtering, TOF-MEIS
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:uu:diva-409892ISBN: 978-91-513-0964-4 (print)OAI: oai:DiVA.org:uu-409892DiVA, id: diva2:1428167
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
List of papers
1. Disparate Energy Scaling of Trajectory-Dependent Electronic Excitations for Slow Protons and He Ions
Open this publication in new window or tab >>Disparate Energy Scaling of Trajectory-Dependent Electronic Excitations for Slow Protons and He Ions
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.

National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-409698 (URN)10.1103/physrevlett.124.096601 (DOI)000517416700008 ()32202865 (PubMedID)
Funder
Swedish Research Council, 821-2012-5144; 2017-00646_9Swedish Foundation for Strategic Research , RIF14-0053
Available from: 2020-04-27 Created: 2020-04-27 Last updated: 2020-05-28Bibliographically approved
2. Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity
Open this publication in new window or tab >>Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity
(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
Ion transmission, Energy loss, Charge exchange
National Category
Physical Sciences Condensed Matter Physics
Research subject
Physics with specialization in Ion Physics
Identifiers
urn:nbn:se:uu:diva-409891 (URN)
Funder
Swedish Research Council, 821-2012-5144Swedish Research Council, 2017-00646_9Swedish Foundation for Strategic Research , RIF14-0053
Available from: 2020-05-04 Created: 2020-05-04 Last updated: 2020-05-04
3. Assessing electron emission induced by pulsed ion beams: a time-of-flight approach
Open this publication in new window or tab >>Assessing electron emission induced by pulsed ion beams: a time-of-flight approach
Show others...
(English)In: Article in journal (Other academic) Submitted
Abstract [en]

We present a method to measure the kinetic energy of electrons emitted upon ion impact via their time-of-flight. Pulsed beams of H+, D2+, He+ and Ne+ ions with velocities between 0.4 and 3.5 a.u. are transmitted through thin, self-supporting carbon and gold foils. Transmitted ions and secondary electrons are detected with a position-sensitive detector behind the sample and their respective energies are determined via their flight times. A coincidence criterion can be applied in the acquisition software. Measured electron energies range between 10 and 400 eV. Above ion velocities of 1 a.u. the most probable electron energy scales with ion velocity pointing towards a kinetic emission mechanism. At lower ion velocities, the electron energy stays constant and lies above the maximum energy transfer possible in a classical binary collision between ion and electron. Potential applications and technical challenges of measuring electron energies and yields with a time-of-flight approach are discussed.

Keywords
Electron emission, Time-of-flight (TOF), Ion-solid interaction, Kinetic emission, Self-supporting foils
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-409773 (URN)
Funder
Swedish Research Council, 821-2012-5144Swedish Research Council, 2017-00646_9Swedish Foundation for Strategic Research , RIF14-0053
Available from: 2020-04-28 Created: 2020-04-28 Last updated: 2020-05-18
4. Analysis of photon emission induced by light and heavy ions in time-of-flight medium energy ion scattering
Open this publication in new window or tab >>Analysis of photon emission induced by light and heavy ions in time-of-flight medium energy ion scattering
2018 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 417, p. 75-80Article in journal (Refereed) Published
Abstract [en]

We present a systematic analysis of the photon emission observed due to impact of pulsed keV ion beams in time-of-flight medium energy ion scattering (ToF-MEIS) experiments. Hereby, hydrogen, helium and neon ions served as projectiles and thin gold and titanium nitride films on different substrates were employed as target materials. The present experimental evidence indicates that a significant fraction of the photons has energies of around 10 eV, i.e. on the order of typical valence and conduction band transitions in solids. Furthermore, the scaling properties of the photon emission with respect to several experimental parameters were studied. A dependence of the photon yield on the projectile velocity was observed in all experiments. The photon yield exhibits a dependence on the film thickness and the scattering angle, which can be explained by photon production along the path of the incident ion through the material. Additionally, a strong dependence on the projectile type was found with the photon emission being higher for heavier projectiles. This difference is larger than the respective difference in electronic stopping cross section. The photon yield shows a strong material dependence, and according to a comparison of SiO2 and Si seems to be subject to matrix effects. (C) 2017 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Photons, Deep UV, TOF-MEIS, Au
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-348917 (URN)10.1016/j.nimb.2017.08.005 (DOI)000426030500014 ()
Conference
15th International Conference on Particle Induced X-ray Emission (PIXE), APR 02-07, 2017, Split, CROATIA
Funder
Swedish Foundation for Strategic Research , RIF14-0053Swedish Research Council, 821-2012-5144
Available from: 2018-04-25 Created: 2018-04-25 Last updated: 2020-05-04Bibliographically approved
5. Ion-induced particle desorption in time-of-flight medium energy ion scattering
Open this publication in new window or tab >>Ion-induced particle desorption in time-of-flight medium energy ion scattering
2018 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 423, p. 22-26Article in journal (Refereed) Published
Abstract [en]

Secondary ions emitted from solids upon ion impact are studied in a time-of-flight medium energy ion scattering (ToF-MEIS) set-up. In order to investigate characteristics of the emission processes and to evaluate the potential for surface and thin film analysis, experiments employing TiN and Al samples were conducted. The ejected ions exhibit a low initial kinetic energy of a few eV, thus, requiring a sufficiently high acceleration voltage for detection. Molecular and atomic ions of different charge states originating both from surface contaminations and the sample material are found, and relative yields of several species were determined. Experimental evidence that points towards a predominantly electronic sputtering process is presented. For emitted Ti target atoms an additional nuclear sputtering component is suggested.

Keywords
Desorption, Electronic sputtering, TOF-MEIS, TiN
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-351227 (URN)10.1016/j.nimb.2018.02.016 (DOI)000430901400004 ()
Funder
Swedish Research Council, 821-2012-5144Swedish Foundation for Strategic Research , RIF14-0053
Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2020-05-04Bibliographically approved

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