Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session NR2: Electron-Impact Ionization of Atoms and Molecules |
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Chair: Murthada Khakoo, California State University - Fullerton Room: 255E |
Thursday, November 17, 2011 9:30AM - 10:00AM |
NR2.00001: Theoretical and experimental (e,2e) studies for isoelectronic atoms and molecules -- Ne and CH4 Invited Speaker: Very recently, experimental and theoretical studies of electron impact ionization of He and H$_{2}$ revealed new physical effects for ionization of both atoms and molecules. In this talk, a similar study will be reported for atoms and molecules with 10 electrons. The theoretical calculations for this study were performed at Rolla and the experimental measurements were made at Manchester, England by Andrew Murray's group and at Heidelberg Germany by Alexander Dorn's group. All the experimental measurements were performed for relatively low electron energies. In Manchester, measurements were made for the highest occupied molecular orbital (HOMO) and the next highest occupied molecular orbital (NHOMO) of CH$_{4}$ for coplanar and perpendicular plane symmetric geometry. In Heidelberg, measurements were made for the 1t$_{2}$ state in coplanar and half perpendicular plane asymmetric geometry. The experimental measurements will be compared to M3DW (molecular 3-body distorted wave) results for CH$_{4}$ and atomic 3DW results for Ne. [Preview Abstract] |
Thursday, November 17, 2011 10:00AM - 10:15AM |
NR2.00002: A relativistic treatment of electron ionization of xenon Allan Stauffer, Alexey Illarionov Xenon is a heavy atom with a 5p$^{6}$ valence shell. This outer shell is split into two subshells with an energy difference of 1.3 eV. Experimental measurements exist for the ionization of this atom which resolve these two subshells. Moreover, ionization experiments have been carried out with spin-polarized electrons. A relativistic treatment of these processes based on the Dirac equations has two particular advantages. By providing distinct orbitals for the two valence subshells with different ionization thresholds, we have a more realistic description of the target atom than is available in a standard Schroedinger approximation. Since the Dirac equations explicitly contain the spin of the electrons, we have a natural way of treating spin-polarized processes. We will present results based on a simple plane and coulomb representation of the scattered and ejected electron as well as a more elaborate distorted-wave approach. [Preview Abstract] |
Thursday, November 17, 2011 10:15AM - 10:30AM |
NR2.00003: Indistinguishability in Excitation-Ionization B. Milum, A.L. Harris, D.H. Madison The study of electron impact excitation-ionization of helium is of continuing interest in order to better understand the dynamics of four-body processes. In the process of excitation-ionization, a projectile electron collides with a helium atom causing one atomic electron to be ionized and the other atomic electron to be left in an excited state of the He$^{+}$ ion. The participation of both atomic electrons in this process makes excitation-ionization a four-body problem. Current models for this particular four-body process assume that the projectile electron can be distinguished from the atomic electrons. However, nature and experiment cannot distinguish one electron from another. We will present theoretical fully differential cross section results using the 4-body Distorted Wave (4DW) model where the possibility of electron exchange is included. The results will be compared with experiment and the 4DW model where exchange is neglected. [Preview Abstract] |
Thursday, November 17, 2011 10:30AM - 10:45AM |
NR2.00004: Using generalized Sturmian basis for two- and three-body scattering problems Ancarani Lorenzo Ugo, Gasaneo Gustavo A methodology based on generalized Sturmian functions is put forward to solve two- and three-body scattering problems. It uses a spectral method which allows for the inclusion of the correct asymptotic behavior when solving the associated driven Schr\"{o}dinger equation. For the two--body case we demonstrate the equivalence between the ECS and the Sturmian approaches, and illustrate the latter by using Hulth\'{e}n Sturmian functions. Contrary to the Exterior Complex Scaling approach, no artificial cut-off of the potential is required in the Surmian approach. For the three--body scattering problem, the theoretical framework is presented in hyperspherical coordinates and a set of hyperspherical Generalized Sturmian functions possessing outgoing asymptotic behavior is introduced. The Sturmian procedure is a direct generalization of the method discussed for the two-body problem, thus the comparison with the ECS method is similar. For both the two- and three-body cases, Sturmian basis are efficient as they possess the correct outgoing behavior, diagonalize part of the potentials involved and are essentially localized in the region where the unsolved interaction is not negligible. Moreover, with the Sturmian basis the operator (H-E) is represented by a diagonal matrix whose elements are simply the Sturmian eigenvalues. [Preview Abstract] |
Thursday, November 17, 2011 10:45AM - 11:00AM |
NR2.00005: Calculating electron-impact ionization cross sections for molecules without making orientation averaging approximation Adam Upshaw, Don Madison We have been using the M3DW (molecular 3-body distorted wave) approximation plus the OAMO (orientation averaged molecular orbital) to calculate cross sections for electron-impact ionization of molecules. The approximation yielded good agreement with experiment for H$_{2}$ and reasonable agreement for N$_{2}$. However, the agreement was not that good for H$_{2}$O, CH$_{4}$, and larger molecules and at least part of the problem must be attributed to the OAMO approximation. Consequently, we have dropped the OAMO approximation and have performed a proper numerical average over al orientations. M3DW results both with and without making the OAMO approximation will be compared with experimental measurements. [Preview Abstract] |
Thursday, November 17, 2011 11:00AM - 11:30AM |
NR2.00006: (e,2e) Experiments on Small Noble Gas Clusters: Search for Multicenter and Interference Effects Invited Speaker: Single ionization of small argon and neon clusters at intermediate energies (i.e. 100 eV and 61 eV, respectively) have been performed. Triple differential cross-sections have been acquired over the complete solid angle of electron emission. 3D emission patterns for dimers and small clusters show significant differences compared to the ionization of the respective atomic target which can be attributed to multiple scattering collisions and molecular wave function symmetry effects. Coincidence measurements of two charged cluster fragments allow to obtain more detailed insight in multiple ionization reactions as well as energy and charge transfer processes between different atoms within the cluster. [Preview Abstract] |
Thursday, November 17, 2011 11:30AM - 11:45AM |
NR2.00007: Experimental and theoretical triple differential cross sections for electron impact ionization of aligned H$_{2}$ Esam Ali, Allison Harris, Julian Lower, Erich Weigold, Chuangang Ning, Don Madison Most experiments measuring electron-impact ionization of molecules do not determine the orientation of the molecule at the time of ionization. One way to determine the orientation is to simultaneously ionize the molecule and excite the residual ion to a state that will dissociate. The orientation of the molecule can then be determined by detecting one of the dissociation fragments since the fragments will leave in the direction of orientation. Experimental and theoretical TDCS (triple differential cross sections) results will be presented for excitation-ionization of the $2s\sigma _g ,\;2p\sigma _u ,\;\mbox{and}\;\mbox{2p}\pi _u $ states of H$_{2}$ for three different orientations of the molecule. [Preview Abstract] |
Thursday, November 17, 2011 11:45AM - 12:00PM |
NR2.00008: Theoretical and Experimental Triple Differential Cross Sections for Electron Impact Ionization of Neon S.M. Amami, Don Madison, Thomas Pflueger, Xueguang Ren, Arne Senftleben, Alexander Dorn, Joachim Ullrich Three-dimensional triple differential cross sections have been calculated and measured for 61 eV electron-impact ionization of the 2p state of neon. Three-dimensional distributions for the ejected electron will be presented for fixed incident projectile energy and scattering angle. Comparison between theoretical DWBA (distorted wave Born approximation), M3DW (molecular 3-body distorted wave), and experiment will be given for ejected electron energies ranging between 2 eV to 20 eV and scattered projectile angles ranging between 20 degrees and 70 degrees. Although the M3DW is in much better agreement with experiment than the DWBA, the experiment exhibits some structure not predicted by theory [Preview Abstract] |
Thursday, November 17, 2011 12:00PM - 12:15PM |
NR2.00009: Fully Nonperturbative Treatment of Ionization and Ionization with Excitation of Noble Gases Oleg Zatsarinny, Klaus Bartschat We have extended the B-spline R-matrix (BSR) method [1] to include a large number of pseudo-states. These pseudo-states are not only important for the accurate treatment of electron-impact excitation processes at intermediate energies, but they also allow for the calculation of ionization processes, using the same basic ideas as in the convergent close-coupling and standard R-matrix with pseudo-states (RMPS) approaches [2,3]. In addition to total cross sections, our general and flexible BSRMPS method yields excellent agreement for the highly correlated ionization with excitation process in helium [4]. Calculations for the more complex Ne and Ar targets will be shown at the conference and compared with recent experimental data [5].\\[4pt] [1] O. Zatsarinny, Comp. Phys. Commun. {\bf 174} (2006) 273.\\[0pt] [2] D. V. Fursa and I. Bray, Phys. Rev. A {\bf 52} (1995) 1279.\\[0pt] [3] K. Bartschat and I. Bray, J. Phys. B {\bf 29} (1996) L577.\\[0pt] [4] O. Zatsarinny and K. Bartschat, Phys. Rev. Lett. {\bf 107} (2011) 023203.\\[0pt] [5] X. Ren {\it et al.}, Phys. Rev. A {\bf 83} (2011) 052714. [Preview Abstract] |
Thursday, November 17, 2011 12:15PM - 12:30PM |
NR2.00010: On the applicability of the Exterior Complex Scaling method for scattering problems including Coulombic potentials Ancarani Lorenzo Ugo, Gasaneo Gustavo, Mitnik Dario We study some formal aspects of the exterior complex scaling approach when implemented for both short and long-range potentials. The method requires an artificial cut-off of the potential in order to avoid exponential divergencies. Preserving the cut-off strategy, we propose alternative proposals to the ECS recipe which lead to a well-defined scattering problem. For a Coulombic long-range two-body potential we carefully analyze the use of a free wave function as asymptotic term, and then propose two variants of distorted--wave reformulation. A numerical illustration is provided. We then study in detail the cut-off procedure and identify some inconsistencies. To avoid them a proposal is put forward and its efficiency is demonstrated with analytical solutions for the pure Coulomb potential. [Preview Abstract] |
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