Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session FT3: Electron-Impact Ionization |
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Chair: Allison Harris, Henderson State University Room: Classroom 202 |
Tuesday, October 23, 2012 3:30PM - 4:00PM |
FT3.00001: Single and double ionization of helium: a Sturmian approach Invited Speaker: Lorenzo Ugo Ancarani The quantal description of three-body break-up processes is a~notoriously difficult problem, one major obstacle being the imposition of proper asymptotic boundary~conditions. In this contribution, single and double ionization of helium are investigated through a Sturmian approach in hyperspherical coordinates. A similar approach, in spherical coordinates, has been applied successfully to a number of two-electron systems (see, e.g., [1]). It is known, though, that for the three particles break-up the global asymptotic behavior (Peterkop) is best described in hyperspherical coordinates. The use of these more natural coordinates, within a Sturmian approach, provide the basis functions with more adequate outgoing asymptotic conditions [2]. In this way the scattering wave function expansion is~restricted to the region where the interaction between the particles takes place, considerably increasing the convergence rates. The application of the recently proposed hyperspherical Sturmian approach [2] is applied to ionization processes of helium. The scattering wave function and related differential cross sections will be presented.\\[4pt] [1] A. L. Frapiccini et al, J. Phys. B \textbf{43} 101001 (2010); J. M. Randazzo et al, Phys. Rev. A \textbf{84}, 052715 (2011).\\[0pt] [2] G. Gasaneo and L. U. Ancarani, J. Phys. A \textbf{45}, 045304 (2012). [Preview Abstract] |
Tuesday, October 23, 2012 4:00PM - 4:15PM |
FT3.00002: Nonperturbative B-Spline R-Matrix with Pseudostates Calculations for Electron Impact Ionization of Helium Oleg Zatsarinny, Klaus Bartschat The theoretical and numerical approach used in a recent publication [1] describing a non\-perturbative treatment of ionization and simultaneous ionization plus excitation of helium by electron impact will be discussed. We then present a variety of comparisons between the present predictions, experimental data, and results from other non\-perturbative theories, such as convergent close-coupling and time-dependent close-coupling, for ionization without excitation. The overall excellent agreement with the other available data provides confidence in using the \hbox{$B$-spline} \hbox{$R$-matrix} with pseudo\-states approach for this benchmark system, as well as extending it to more complex situations for which no other non\-perturbative methods are currently available.\\[4pt] [1] O. Zatsarinny and K. Bartschat, Phys. Rev. Lett.~{\bf 107} (2011) 023203. [Preview Abstract] |
Tuesday, October 23, 2012 4:15PM - 4:30PM |
FT3.00003: Low energy (e,2e) experimental and theoretical 3-dimensional study of Neon Sadek Amami, Don Madison, Hari Saha, Thomas Pflueger, Xueguang Ren, Arne Senftleben, Alexander Dorn, Joachin Ullrich Three-dimensional triple differential cross sections have been calculated and measured for 61eV electron-impact ionization of the 2p state of neon. Three-dimensional distributions for the ejected electron will be presented for a fixed incident projectile energy and scattered projectile angles ranging between 20 degrees and 70 degrees and ejected electron energies ranging between 2eV to 20eV. The theoretical model used for the calculations is the DWBA (distorted wave Born approximation). The importance of PCI (post collision interaction between the scattered and ejected electron) will be examined by either including or excluding this effect in the final state wavefunction. The importance of the interaction between the ejected electron and the residual atomic electrons will be examined by comparing results using distorted waves calculated in a static potential with Hartree-Fock distorted waves. [Preview Abstract] |
Tuesday, October 23, 2012 4:30PM - 5:00PM |
FT3.00004: Relativistic Calculations of Electron Ionization of Xenon Invited Speaker: Allan Stauffer We are interested in the ionization of heavy atoms by electrons of intermediate energy. Since the incident particles do not have relativistic energies, the question arises as to why a relativistic treatment of this process is preferable. The answer lies both in the treatment of the target as well as the incident particle. In our case, a relativistic treatment of the target system is done within the j-j coupling scheme where the spin and angular momenta of each electron are coupled to a total angular momentum j. Thus the valence p shell of xenon is split into two subshells, one with j = 3/2 and one with j = 1/2. Calculations of the target wave functions can be readily carried out using an available program [1]. There is a fine structure splitting of 1.31 eV between these two subshells. Thus the energy required to ionize these two subshells is sufficiently different that they can be distinguished experimentally. The Dirac equations which describe the free electrons in a distorted-wave approximation with non-local exchange explicitly contain the spin of the electron. Thus the treatment of spin-polarized scattering is straightforward and does not require any recoupling of angular momenta as in a non-relativistic scheme. Recent experiments [2,3] have measured the ionization of the j = 3/2 valence electrons of xenon when the incident electron makes an arbitrary angle with the plane containing the outgoing electron which have identical energies. We will present calculations for this process to compare with the measurements and discuss the results in terms of the models proposed for the scattering mechanisms giving rise to these non-coplanar events.\\[4pt] [1] P. Jonsson, X. He, C. Froese Fischer and I. P. Grant, Comput. Phys. Commun. 177, 597-622 (2007).\\[0pt] [2] K. L. Nixon, A. J. Murray and C. Kaiser, J. Phys. B 43, 085202 (2010).\\[0pt] [3] K. L. Nixon and A. J. Murray, Phys. Rev. A 85, 022716 (2012). [Preview Abstract] |
Tuesday, October 23, 2012 5:00PM - 5:15PM |
FT3.00005: Importance of Molecular alignment in (e,2e) collisions Esam Ali, Don Madison, Allison Harris, Julian Lower, Erich Weigold, Chuangang Ning 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 three excited states of H2 for three different orientations of the molecule at incident electron energy of 176 eV. [Preview Abstract] |
Tuesday, October 23, 2012 5:15PM - 5:30PM |
FT3.00006: Single differential cross sections for electron-hydrogen ionization: the quantum mechanical flux formula revisited Juan Martin Randazzo, Lorenzo Ugo Ancarani, Gustavo Gasaneo, Flavio Colavecchia One way of extracting single differential cross sections (SDCS) for the electron-hydrogen ionization process is based on using the quantum mechanical flux operator evaluated at asymptotic distances. This procedure is formally correct; however, numerical evaluations are necessarily performed at finite distances. As a consequence, unphysical oscillations appear at very unequal energy sharing [1] and, for this reason, the flux formula was somehow discarded by the community. In this contribution we propose two corrections based on alternative ways of defining the energy fraction. The first one uses the components of the probability flux instead of the usual asymptotic kinematical (or geometrical) approximation. The second comes from a finite distance energy reinterpretation related to a simple, classical, energy conservation analysis. Results of calculations for the s-wave approximation of the e-H processes, performed at various impact energies and for both singlet and triplet symmetry, are presented. Once our flux formula corrections are applied, the unphysical behavior previously observed is removed, and our SDCS results compare favorably with benchmark theoretical data.\\[4pt] [1] M. Baertschy et al,~Phys. Rev. A, 60 (1999) R13. [Preview Abstract] |
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