Bulletin of the American Physical Society
70th Annual Gaseous Electronics Conference
Volume 62, Number 10
Monday–Friday, November 6–10, 2017; Pittsburgh, Pennsylvania
Session QR3: Collisions with Atoms |
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Chair: Sandra Quintanilla, University of North Texas Room: Oakmont Junior Ballroom |
Thursday, November 9, 2017 8:00AM - 8:30AM |
QR3.00001: Free-free experiments: the search for dressed-atom effects Invited Speaker: N.L.S. Martin The absorption or emission of radiation during the collision of charged particles with atoms and molecules is investigated in free-free experiments. Up to now almost all such experiments have been in agreement with a simple theory which assumes that the interaction of the radiation with the atom itself has no effect on the scattering process. Very recently the first experiments to observe the unambiguous breakdown of this assumption have been carried out in xenon by Morimoto, Kanya, and Yamanouchi.\footnote{Y. Morimoto, R. Kanya, and K. Yamanouchi, Phys.\ Rev.\ Lett.\ {\bf 115}, 123201 (2015)} An estimate of the dressing of the target by the radiation's electric field may be made in terms of the electric dipole polarizability $\alpha$ of the target. The effects in Xe ($\alpha=28$~au) were extremely difficult to measure because they occur at very small scattering angles. We have begun to carry out electron-collision experiments for atomic processes which involve polarizabilities an order of magnitude larger than elastic scattering from Xe. Two such processes are elastic scattering from Potassium, and inelastic scattering into the first excited states of argon; both involve polarizabilities $\alpha \sim 300$~au. I will give a progress report on our experiments. [Preview Abstract] |
Thursday, November 9, 2017 8:30AM - 8:45AM |
QR3.00002: Electron Impact Excitation of Xenon Dirk Luggenhoelscher, Uwe Czarnetzki, Oleg Zatsarinny, Klaus Bartschat We have applied a novel experimental technique to measure cross sections for electron-impact excitation of the 5p$^5$6s and 5p$^5$6p states of xenon from its 5p$^6$ ground state. This is a complex collision system, for which benchmarking of theory against experiment is needed. The experiment is performed using ultrashort current pulses released from an electrode by femtosecond laser pulses with 80 MHz repetition rate. In order to minimize space charge effects, only about 10$^4$ electrons are generated in each pulse. Electrons are accelerated by a homogeneous electric field to energies of typically 250 eV. The fluorescence light generated in collisions with Xe atoms at low pressure (Pa range) is imaged perpendicularly and provides a direct image of the energy-dependent excitation cross section. The calculations were carried out with a fully relativistic and parallelized version of the B-spline R-matrix code [1], using a 75-state close-coupling model [2] with the target structure obtained earlier [3]. [1] O.~Zatsarinny, Comp.\ Phys.\ Commun.~{\bf 174} (2006) 273. [2] O.~Zatsarinny and K.~Bartschat, J.~Phys.~B: At.\ Mol.\ Opt. Phys.~{\bf 43} (2010) 074031. [3] O.~Zatsarinny and K.~Bartschat, Phys.\ Scr.\ T{\bf 134} (2009) 014020. [Preview Abstract] |
Thursday, November 9, 2017 8:45AM - 9:00AM |
QR3.00003: B-spine R-matrix with pseudostates calculations for electron-impact excitation and ionization of magnesium. Oleg Zatsarinny, Klaus Bartschat The B-spline R-matrix with Pseudo-States method [1,2] was employed to treat electron collisions with magnesium atoms. Predictions for elastic scattering, excitation, ionization, and ionization-excitation were obtained for all transitions between the lowest 25 states of Mg in the energy range from threshold to 100~eV. The accuracy of the results was checked by comparing with available experimental data and with results obtained in different approximations with increasing number of coupled states. The largest scattering model included 716 coupled states, most of which were pseudo-states that simulate the effect of the high-lying Rydberg continuum and, most importantly, the ionization continuum on the results for transitions between the discrete states of interest. Similar to our work on e-Be collisions~[3], this effect is particularly strong at ``intermediate'' incident energies of a few times the ionization threshold. The dataset generated from the largest model is estimated to be accurate to within a few percent for the cross sections of relevance for plasma modelling. [1] O.~Zatsarinny, Comp.\ Phys.\ Commun.~{\bf 174} (2006) 273. [2] O.~Zatsarinny and K.~Bartschat, J.~Phys.\ B~{\bf 46} (2013) 112001. [3] D.~V.~Fursa and I.~Bray, J.~Phys.\ B~{\bf 49} (2016) 235701. [Preview Abstract] |
Thursday, November 9, 2017 9:00AM - 9:30AM |
QR3.00004: Double ionization of helium by electron and proton impact. A Generalized Sturmian Functions Approach. Invited Speaker: Marcelo Ambrosio In this contribution we explore the double ionization of helium by fast electrons and protons. A first-Born framework is considered for the projectile-target interaction, while all the intra-target ones are considered to all orders. This reduces the number of active components from four particles to three, and the resulting three-body problem can be tackled with today’s computational resources [1-3]. We apply the Generalized Sturmian Functions method to obtain the scattering wavefunction, and from its asymptotic part we extract the transition amplitudes. We present theoretical fully differential cross sections calculated for a wide variety of energy and momentum transfer regimes. A general good agreement is observed when they are compared with experimental data [4-7]. For electronic projectiles, for given momentum transfer values, higher order effects are appreciable in the measured cross sections, and some differences with first Born calculations are noticeable [5]. However, in the case of protonic projectiles we observe that a first Born treatment is enough to reproduce the features of the available experiments [6,7]. Analyzing the different energy and momentum transfer regimes, we are able to distinguish which collisional mechanisms are more preeminent in each regime [7]. This work was done in collaboration with Dr. Darío Mitnik, Prof. Lorenzo Ugo Ancarani, Dr. Gustavo Gasaneo, Antonio Gómez and Enzo Gaggioli. [1] J. Berakdar et al., Phys. Rep. 374, 91 (2003). [2] I. Bray et al., Phys. Rep. 520, 135 (2012). [3] G. Gasaneo et al., Adv. Quantum Chem. 67, 153 (2013). [4] M. J. Ambrosio et al.,J. Phys. B 48, 055204 (2015). [5] M. J. Ambrosio et al., Phys. Rev. A 93, 032705 (2016). [6] M. J. Ambrosio et al., Phys. Rev. A 92, 042704 (2015). [7] M. J. Ambrosio et al., Eur. Phys. J. D 71, 127 (2017). [Preview Abstract] |
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