Threshold phenomena in electron impact ionisation of atoms explored by multi-particle imaging techniques
Final Report Abstract
The envisaged experiments to explore the threshold phenomena for fundamental four-body Coulomb fragmentation in electron impact double ionization of helium and lithium in the threshold regime at 2 eV excess energy were not feasible. This was due to the technical difficulties to obtain an appropriate pulsed electron beam (narrow energy spread and short temporal width) with the photocathode electron source and to combine a magneto-optical trap of lithium atom with the reaction microscope for simultaneous detection of both final states electrons and the recoil ion. Therefore, the major objectives of this project were focused on the studies of electron impact ionization of a number of heavier and technologically relevant atoms and molecules covering a broad range of experimental kinematics and a large part of the 4π solid angle of electron emission. Highly interesting results have been obtained from these studies. The measurements of helium ionization confirmed that the theoretical description of single ionization of helium by electron impact leaving the ion in the ground state can be sufficiently accurate for many practical purposes. For heavier or complex many-electron targets, such as Ar and Ne, theory is being greatly challenged when it comes to treating electron impact ionization. Comprehensive experiments of (e, 2e) on Ar and Ne were obtained to benchmark a series of theoretical approaches of DWBA, DWBA-PCI, DWB1-RM, DWB2-RM, and BSR. It was found that the recently developed non-perturbative BSR method is generally in best agreement with experiment for the scattering and perpendicular planes, as well as for the 3D emission patterns. Particularly, for Ne target, the experimental data and the BSR results reveal an unprecedented degree of agreement not only in shape but also in the relative magnitude of the triple-differential cross section over a significant range of scattering angles and ejection energies. This study represents an important step towards the understanding of the ionization of complex atomic targets at low impact energies. Finally, minor discrepancies between experiment and theory exist for the complex many-electron targets. A range of new and interesting phenomena was raised with the investigations of multi-center targets like molecules or cluster targets. The experiments on small argon clusters found that the increased emission of ejected electrons in the backward direction as well as an overall enhancement in the cross section for out-of-plane emission. These observations emphasize the particular importance of out-of-plane investigations. Comparison of the cross sections for clusters with those of atomic argon reveal clear signature of multiple-scattering reactions. The experiments on randomly aligned H2 molecules found perpendicular plane structures which can be assigned to multiple scattering processes of the projectile. The size and shape of these lobes sensitively depends on the details of the collision kinematics. The challenging experiments for electron impact ionization of aligned H2 revealed the first observation of distinct alignment dependence of molecular-frame (e, 2e) cross sections. It was found that the re-scattering of the ejected electron in the molecular potential results in an effective focusing along the molecular axis.
Publications
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“Signatures of projectile–nucleus scattering in three-dimensional (e, 2e) cross sections for argon”, J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 035202
Xueguang Ren, Arne Senftleben, Thomas Pflüger, Alexander Dorn, Klaus Bartschat, Joachim Ullrich
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“Tracing multiple scattering patterns in absolute (e,2e) cross sections for H2 and He over a 4π solid angle”, Physical Review A, 82 (2010) 032712
X. Ren, A. Senftleben, T. Pflüger, A. Dorn, J. Colgan, M.S. Pindzola, O. Al-Hagan, D.H. Madison, I. Bray, D.V. Fursa, J. Ullrich
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“Benchmark experiment for electron impact ionization of argon: absolute triple-differential cross sections via three-dimensional electron emission images”, Physical Review A, 83 (2011) 052714
Xueguang Ren, Arne Senftleben, Thomas Pflüger, Alexander Dorn, Klaus Bartschat, Joachim Ullrich
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“Observation of multiple-scattering in (e, 2e) experiments on small argon clusters”, Physical Review Letters, 107 (2011) 223201
Thomas Pflüger, Arne Senftleben, Xueguang Ren, Alexander Dorn, Joachim Ullrich
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“Low-energy electron-impact ionization of argon: Three-dimensional cross section”, Physical Review A, 85 (2012) 032702
X. Ren, T. Pflüger, J. Ullrich, O. Zatsarinny, K. Bartschat, D.H. Madison and A. Dorn
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“Strong molecular alignment dependence of H2 electron impact ionization dynamics”, Physical Review Letters, 109 (2012) 123202
X. Ren, T. Pflüger, S. Xu, J. Colgan, M.S. Pindzola, A. Senftleben, J. Ullrich, A. Dorn
