Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session K04: Focus Session: The Legacy of Don Madison - Electron-Atom CollisionsFocus Live Streamed
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Chair: Robert Lucchese, Lawrence Berkeley National Laboratory Room: Conference Theater |
Wednesday, June 7, 2023 10:30AM - 11:00AM |
K04.00001: The legacy of Don Madison – electron-atom collisions. Invited Speaker: Klaus Bartschat Don Madison was invited to Münster (Germany) in 1984 to carry out calculations for spin-dependent inelastic electron collisions with mercury, a heavy atom with nuclear charge Z = 80. At the time, he appeared to be the one-and-only hope to do this successfully and thereby assist in the interpretation of experimental data taken in the group of J. Kessler and G.F. Hanne. I got involved when Don faced serious problems with the university’s mainframe computer, then the only machine available there for such work. Meeting Don changed my life -- and not just mine when one looks at the many successful students that he taught and the many colleagues around the world that he collaborated with. |
Wednesday, June 7, 2023 11:00AM - 11:12AM |
K04.00002: Projectile Localization Effects in Electron Vortex Beam Ionization Collisions Allison L Harris In traditional charged particle scattering theory, the incident projectile is considered to be delocalized in the transverse plane with an infinitely large coherence. However, in the last decade, it has been shown that a finite projectile coherence length can significantly alter the collision cross sections and must be considered when comparing theoretical results with experimental data. To date, the effect of coherence on collision cross sections has been studied for heavy ion projectiles, such as protons and carbon nuclei. In these cases, the coherence length was controlled by changing the projectile deBroglie wavelength through its energy, which is directly related to transverse coherence length. For heavy ions, the deBroglie wavelength is quite small and small coherence lengths can be achieved. For non-relativistic plane wave electron projectiles, the deBroglie wavelength is much larger and achieving a small coherence length is not feasible. However, electron vortex projectiles, such as Laguerre-Gauss electrons, offer an opportunity to study projectile coherence in electron-impact collisions. We report here theoretical triple differential cross sections for electron-impact ionization of helium using Laguerre-Gauss projectiles. We show that a localized projectile causes the binary peak to shift to larger ejected electron angles and enhances the recoil peak. As the projectile becomes less localized the cross sections more closely resemble their delocalized counterparts. We also show that the atomic target’s transverse position within the projectile beam can significantly alter the magnitude of the cross section. |
Wednesday, June 7, 2023 11:12AM - 11:24AM |
K04.00003: Deep minima in the Coulomb-Born triply differential cross sections for electron and positron ionization of hydrogen and helium Sandra J Ward Quintanilla, Cody M DeMars Experimental measurements of a deep minimum in the triply differential cross section (TDCS) for electron-impact ionization of helium [1,2] for an energy of 64.6 eV have previously been interpreted in terms of a vortex in the velocity field [3]. The measurements are for out-of-plane symmetric geometry and for a specific gun angle of 67.5°. We have applied the Coulomb-Born (CB1) approximation for the gun angle of 67.5° and for the energies of 64.6eV and 74.6 eV and obtained a minimum in the TDCS for each energy [4]. However, to obtain deep minima in the CB1 TDCSs that corresponds to zeros in the CB1 transition matrix element we varied the gun angle. Corresponding to each zero there is a vortex in the velocity field that is associated with the transition matrix element. Using the CB1 approximation and considering symmetric geometries we also obtained a deep minimum in the TDCS for electron-hydrogen ionization of hydrogen [5], positron-hydrogen ionization [5] and positron-helium ionization [4]. For electron-impact ionization of both helium and hydrogen the CB1 TDCS results have been compared with both the TDCC and 3DW results [4-6]. [1] A. J. Murray and F. H. Read, J. Phys. B 26, L359 (1993). [2] A. J. Murray and F. H. Read, Phys. Rev. A 47, 3724 (1993). [3] J. H. Macek, J. B. Sternberg and S. Yu. Ovchinnikov, Phys. Rev. Lett. 104, 033201 (2010). [4] C. M. DeMars and S. J. Ward, Eur. Phys. J. D. 74, 48 (2020). [5] C. M. DeMars, S. J. Ward, J. Colgan, S. Amami and D. H. Madison, Atoms 8, 26 (2020). [6] J. Colgan, O. Al-Hagan, D.H. Madison, A.J. Murray and M.S. Pindzola, J. Phys. B 42, 171001 (2009). |
Wednesday, June 7, 2023 11:24AM - 11:54AM |
K04.00004: The legacy of Don Madison – electron ionizing collisions with molecules Invited Speaker: James Colgan Don Madison was a pioneer in applying distorted-wave methods to a wide range of atomic and molecular collisions. He made significant contributions to theoretical methods of computing atomic excitation, electron-impact ionization of atoms and molecules, and ion-impact ionization. His work was notable through his long-standing close collaborations with experimental groups, and the flexibility that he demonstrated in applying his methods to a diverse range of targets. In this talk I focus on Don’s work that significantly advanced the field of electron-impact ionization of molecules, and in particular through the prediction of the angular distributions of the outgoing electrons from ionizing collisions. I will review the comparison of Don’s distorted-wave methods with experiment and with other theoretical methods for small and intermediate-size molecules. I’ll also show how such angular distributions are proving important in magnetic fusion plasma simulations. If time allows, I will also discuss Don’s work on heavy-particle collisions with atomic and molecular targets. |
Wednesday, June 7, 2023 11:54AM - 12:06PM |
K04.00005: A New Concept in Positron Polarimetry Joshua Machacek, Stephen Buckman, Sean Hodgman, Timothy J Gay We outline a novel experimental technique for measuring the degree of polarization of a positron beam using an optically pumped, spin-polarized Rb target. The technique is based on the production and measurement of the ortho- and para-positronium fractions through positron collisions with the Rb as a function of their polarization. Using realistic estimates for the cross sections and experimental parameters involved we estimate that a polarization measurement with 1% uncertainty of the measured value can be achieved in a few minutes. |
Wednesday, June 7, 2023 12:06PM - 12:18PM |
K04.00006: A search for chiral asymmetry in secondary electron emission from cysteine induced by longitudinally-polarized electrons Timothy J Gay, Karl J Ahrendsen, Ken W Trantham, Dale Tupa Using a Rb spin filter as a polarized electron source [1], we performed experiments searching for a chirality-dependent secondary electron yield when a 141eV longitudinally spin-polarized electron beam was incident on a solid cysteine surface. We determined the secondary electron yield by measuring the positive current produced when the cysteine target was negatively biased. We found no spin dependent effects to a level of one part in one thousand from this interaction, a collision channel that has not been studied to date. |
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