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Fairchild AJ, Chirayath VA, Gladen RW, Koymen AR, Weiss AH, Barbiellini B. Photoemission Spectroscopy Using Virtual Photons Emitted by Positron Sticking: A Complementary Probe for Top-Layer Surface Electronic Structures. PHYSICAL REVIEW LETTERS 2022; 129:106801. [PMID: 36112464 DOI: 10.1103/physrevlett.129.106801] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2022] [Revised: 04/13/2022] [Accepted: 06/08/2022] [Indexed: 06/15/2023]
Abstract
We present a spectroscopic method which utilizes virtual photons to selectively measure the electronic structure of the topmost atomic layer. These virtual photons are created when incident positrons transition from vacuum states to bound surface states on the sample surface and can transfer sufficient energy to excite electrons into the vacuum. The short interaction range of the virtual photons restricts the penetration depth to approximately the Thomas-Fermi screening length. Measurements and analysis of the kinetic energies of the emitted electrons made on a single layer of graphene deposited on Cu and on the clean Cu substrate show that the ejected electrons originate exclusively from the topmost atomic layer. Moreover, we find that the kinetic energies of the emitted electrons reflect the density of states at the surface. These results demonstrate that this technique will be a complementary tool to existing spectroscopic techniques in determining the electronic structure of 2D materials and fragile systems due to the absence of subsurface contributions and probe-induced surface damage.
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Affiliation(s)
- Alexander J Fairchild
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Varghese A Chirayath
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Randall W Gladen
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Ali R Koymen
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Alex H Weiss
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Bernardo Barbiellini
- Department of Physics, School of Engineering Science, LUT University, 53851 Lappeenranta, Finland and Physics Department, Northeastern University, Boston, Massachusetts 02115, USA
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Howell RW. Advancements in the use of Auger electrons in science and medicine during the period 2015-2019. Int J Radiat Biol 2020; 99:2-27. [PMID: 33021416 PMCID: PMC8062591 DOI: 10.1080/09553002.2020.1831706] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2020] [Revised: 09/01/2020] [Accepted: 09/28/2020] [Indexed: 02/06/2023]
Abstract
Auger electrons can be highly radiotoxic when they are used to irradiate specific molecular sites. This has spurred basic science investigations of their radiobiological effects and clinical investigations of their potential for therapy. Focused symposia on the biophysical aspects of Auger processes have been held quadrennially. This 9th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes at Oxford University brought together scientists from many different fields to review past findings, discuss the latest studies, and plot the future work to be done. This review article examines the research in this field that was published during the years 2015-2019 which corresponds to the period since the last meeting in Japan. In addition, this article points to future work yet to be done. There have been a plethora of advancements in our understanding of Auger processes. These advancements range from basic atomic and molecular physics to new ways to implement Auger electron emitters in radiopharmaceutical therapy. The highly localized doses of radiation that are deposited within a 10 nm of the decay site make them precision tools for discovery across the physical, chemical, biological, and medical sciences.
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Affiliation(s)
- Roger W Howell
- Division of Radiation Research, Department of Radiology, New Jersey Medical School, Rutgers University, Newark, NJ, USA
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Direct evidence for low-energy electron emission following O LVV Auger transitions at oxide surfaces. Sci Rep 2020; 10:17993. [PMID: 33093505 PMCID: PMC7582947 DOI: 10.1038/s41598-020-74953-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2020] [Accepted: 10/05/2020] [Indexed: 11/08/2022] Open
Abstract
Oxygen, the third most abundant element in the universe, plays a key role in the chemistry of condensed matter and biological systems. Here, we report evidence for a hitherto unexplored Auger transition in oxides, where a valence band electron fills a vacancy in the 2s state of oxygen, transferring sufficient energy to allow electron emission. We used a beam of positrons with kinetic energies of [Formula: see text] eV to create O 2s holes via matter-antimatter annihilation. This made possible the elimination of the large secondary electron background that has precluded definitive measurements of the low-energy electrons emitted through this process. Our experiments indicate that low-energy electron emission following the Auger decay of O 2s holes from adsorbed oxygen and oxide surfaces are very efficient. Specifically, our results indicate that the low energy electron emission following the Auger decay of O 2s hole is nearly as efficient as electron emission following the relaxation of O 1s holes in [Formula: see text]. This has important implications for the understanding of Auger-stimulated ion desorption, Coulombic decay, photodynamic cancer therapies, and may yield important insights into the radiation-induced reactive sites for corrosion and catalysis.
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Chirayath VA, Gladen RW, McDonald AD, Fairchild AJ, Joglekar PV, Satyal S, Lim ZH, Shead TN, Chrysler MD, Mukherjee S, Barnett BM, Byrnes NK, Koymen AR, Greaves RG, Weiss AH. A multi-stop time-of-flight spectrometer for the measurement of positron annihilation-induced electrons in coincidence with the Doppler-shifted annihilation gamma photon. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2020; 91:033903. [PMID: 32260020 DOI: 10.1063/1.5140789] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2019] [Accepted: 02/07/2020] [Indexed: 06/11/2023]
Abstract
In this study, we describe an advanced multi-functional, variable-energy positron beam system capable of measuring the energies of multiple "positron-induced" electrons in coincidence with the Doppler-shifted gamma photon resulting from the annihilation of the correlated positron. The measurements were carried out using the unique characteristics of the digital time-of-flight spectrometer and the gamma spectrometer available with the advanced positron beam system. These measurements have resulted in (i) the first digital time-of-flight spectrum of positron annihilation-induced Auger electrons generated using coincident signals from a high-purity Ge detector and a micro-channel plate, (ii) a two-dimensional array of the energy of Doppler-broadened annihilation gamma and the time-of-flight of positron-annihilation induced Auger electrons/secondary electrons measured in coincidence with the annihilation gamma photon, and (iii) the time-of-flight spectra of multiple secondary electrons ejected from a bilayer graphene surface as a result of the impact and/or annihilation of positrons. The novelty of the gamma-electron coincidence spectroscopy has been demonstrated by extracting the Doppler-broadened spectrum of gamma photons emitted due to the annihilation of positrons exclusively with 1s electrons of carbon. The width of the extracted Doppler-broadened gamma spectrum has been found to be consistent with the expected broadening of the annihilation gamma spectrum due to the momentum of the 1s electrons in carbon.
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Affiliation(s)
- V A Chirayath
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - R W Gladen
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - A D McDonald
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - A J Fairchild
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - P V Joglekar
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - S Satyal
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - Z H Lim
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - T N Shead
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - M D Chrysler
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - S Mukherjee
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - B M Barnett
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - N K Byrnes
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - A R Koymen
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - R G Greaves
- First Point Scientific Inc., Agoura Hills, California 91301, USA
| | - A H Weiss
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
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Auger electron emission initiated by the creation of valence-band holes in graphene by positron annihilation. Nat Commun 2017; 8:16116. [PMID: 28703225 PMCID: PMC5511367 DOI: 10.1038/ncomms16116] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2017] [Accepted: 05/31/2017] [Indexed: 11/08/2022] Open
Abstract
Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated background resulting from primary beam-induced secondary electrons. Here, we report the direct measurement of the energy spectra of electrons emitted from single layer graphene as a result of the decay of deep holes in the valence band. These measurements were made possible by eliminating competing backgrounds by employing low-energy positrons (<1.25 eV) to create valence-band holes by annihilation. Our experimental results, supported by theoretical calculations, indicate that between 80 and 100% of the deep valence-band holes in graphene are filled via an Auger transition. Auger processes are at the core of electron emission in solid-state physics, however measuring the spectra of electrons emitted solely as a result of Auger transitions remains a challenge. Here, the authors measure the electron energy spectrum in graphene and observe the prominence of Auger-like processes in its valence band.
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Mukherjee S, Shastry K, Anto CV, Joglekar PV, Nadesalingam MP, Xie S, Jiang N, Weiss AH. Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2016; 87:035114. [PMID: 27036826 DOI: 10.1063/1.4943858] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Accepted: 02/25/2016] [Indexed: 06/05/2023]
Abstract
We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flight spectrometer. The spectrometer's new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted from the sample surface at kinetic energies ranging from ∼500 eV to 0 eV. The spectrometer's unique characteristics made it possible to perform (a) first experiments demonstrating the direct transition of a positron from an unbound scattering state to a bound surface state and (b) the first experiments demonstrating that Auger electron spectra can be obtained down to 0 eV without the beam induced secondary electron background obscuring the low energy part of the spectra. Data are presented which show alternative means of estimating positron surface state binding energy and background-free Auger spectra.
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Affiliation(s)
- S Mukherjee
- Radiochemistry Division, Bhabha Atomic Research Center, Trombay, Mumbai, India
| | - K Shastry
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - C V Anto
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - P V Joglekar
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - M P Nadesalingam
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - S Xie
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - N Jiang
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
| | - A H Weiss
- Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA
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