Bulletin of the American Physical Society
2005 58th Gaseous Electronics Conference
Sunday–Thursday, October 16–20, 2005; San Jose, California
Session QW1: Electron and Positron Collisions |
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Chair: Murtadha A. Khakoo, California State University-Fullerton Room: Doubletree Hotel Pine |
Wednesday, October 19, 2005 10:00AM - 10:30AM |
QW1.00001: High-Precision Cross Sections for Electron-Atom Collisions in Laser and Lighting Applications Invited Speaker: In recent years, much progress has been achieved in calculating reliable cross-section data for electron scattering from atoms and ions. In particular, the ``convergent close-coupling'' (CCC) [1] and \hbox{``$R$-matrix} with pseudo-states'' (RMPS) [2] methods have been extremely successful in describing elastic scattering as well as electron-impact excitation and ionization of light quasi-one and quasi-two electron targets, such as atomic hydrogen, helium, the alkalis, and the alkali-earth elements. However, accurate calculations of electron collisions with more complex targets, notably the heavy noble gases Ne$-$Xe, heavy quasi-one electron targets such as Zn, Ba, or Hg, and transition metals such as Fe or Mo~[3], continue to be a major challenge. We have recently further developed a new version of the $R$-matrix (close-coupling) method, using a $B$-spline basis with non-orthogonal sets of term-dependent orbitals~[4]. This method allows us to generate target descriptions of unprecedented accuracy in collision calculations. Example results~[5-7] for some of the systems mentioned above illustrate that the flexibility of the $B$-spline $R$-matrix (BSR) method to describe both the $N$-electron target and the ($N\!+\!1$)-electron collision problems is of crucial importance for obtaining highly accurate cross sections, particularly in the low-energy near-threshold regime, which is often dominated by resonance structure. \newline \newline [1] I. Bray, D.V. Fursa, A.S. Kheifets, and A.T. Stelbovics, J. Phys. B {\bf 35} (2002) R117.\newline [2] K. Bartschat, Comp. Phys. Commun. {\bf 114} (1998) 168.\newline [3] K. Bartschat, in {\it Atomic and Molecular Data and Their Applications}, D.R. Schultz, P.R. Krstic, and F. Owbny (eds.), AIP Conf. Proc. \#636 (2002) 192.\newline [4] O. Zatsarinny and C. Froese Fischer, J.~Phys. B~{\bf 33} (2000) 313.\newline [5] O. Zatsarinny and K. Bartschat, J.~Phys. B~{\bf 37} (2004), 2173 and 4693.\newline [6] O. Zatsarinny and K. Bartschat, Phys. Rev. A {\bf 71} (2005), 022716.\newline [7] O. Zatsarinny, K. Bartschat, L. Bandurina, and V. Gedeon, Phys. Rev. A {\bf 71} (2005) 042702. [Preview Abstract] |
Wednesday, October 19, 2005 10:30AM - 11:00AM |
QW1.00002: Ionization of Simple Molecules by Ion or Electron Impact in a Reaction Microscope Invited Speaker: We have studied single ionization of simple molecules by fast charged particle impact in a reaction microscope. By measuring the momenta of the emitted electron and the recoil ionic fragment in coincidence, channel-selective low-energy electron spectra have been recorded. For non-dissociative ionization of H$_{2}$ by 6 MeV protons, the electron energy distribution agrees well with a CDW-EIS prediction [1] except for E$_{e}$ $<$ 1 eV where an significant enhancement is observed. It is due to the autoionization of rovibrational levels of Rydberg states of H$_{2}$, which occurs by converting vibrational energy into kinetic energy of the emitted electron. First fully differential cross sections have been obtained bearing the ``signature'' of this molecular mechanism, which lies beyond the Born-Oppenheimer approximation [2]. \newline Recently, considerable interest has been raised by the observation of two-center interference effects in the electron emission from H$_{2}$, in analogy to Young's double-slit experiment [3]. They are predicted to be more pronounced if one could fix the orientation of the molecular axis at the instant of the collision [4]. For dissociative ionization of H$_{2}$ by 6 MeV protons we had access to this information. Molecular-frame angular distributions of the emitted electrons have been compared to the CDW-EIS calculation [5]. \newline Argon dimers as well as atomic Ar, both present in the same gas-jet, are ionized by 1keV electron impact in a kinematically complete experiment carried out in an upgraded reaction microscope. The obtained electron spectra for Ar$_{2}$ and Ar are compared directly in order to identify interference structures, which are expected to be much more visible than for H$_{2}$ since the interatomic distance of Ar$_{2}$ is comparable to the de Broglie wavelength of the emitted electron. \newline \newline [1] M.E. Galassi \textit{et al.,} Phys. Rev. A \textbf{66}, 052705 (2002)\newline [2] C. Dimopoulou \textit{et al.,} Phys. Rev. Lett. \textbf{93}, 123203 (2004)\newline [3] N. Stoltherfoht \textit{et al}., Phys. Rev. Lett. \textbf{87}, 023201 (2001)\newline [4] G. Laurent \textit{et al.}, J. Phys. B \textbf{35}, L495 (2002)\newline [5] C. Dimopoulou \textit{et al.,} J. Phys. B \textbf{38}, 593 (2005) [Preview Abstract] |
Wednesday, October 19, 2005 11:00AM - 11:15AM |
QW1.00003: Low Energy Proton Impact Ionization M. Foster, D.H. Madison, J.L. Peacher, M. Schulz Recent experiments have measured fully differential cross sections (FDCS) for single ionization of helium by 75 keV proton impact for fixed ejected electron energies and different momentum transfers. These measurements show major discrepancies in the absolute magnitude between the experiment and the theoretical, 3DW (three-distorted-wave) model. The 3DW model is a fully quantum mechanical calculation that has accurately predicted FDCS for higher energy C$^{6+}$ impact ionization of helium. However, the 3DW model treats the collision as a three-body process (projectile, ion, ejected electron). The lack of agreement between the 3DW model and experiment for low energy collisions suggests that a three-body model may not be appropriate for lower collision energies (especially, when considering that a proton with energy of 75 keV is equivalent to electron energy of 40 eV, which is only 15 eV above the ionization threshold for helium). These experiments further demonstrate the fact that the fundamental physics governing a simple collision process is still not well understood. Consequently, we will present a complete four-body model known as the 6DW (six- distorted-wave) model. The 6DW model takes all two particle Coulomb interactions (six in total) into account on equal footing. [Preview Abstract] |
Wednesday, October 19, 2005 11:15AM - 11:30AM |
QW1.00004: Excitation of H$_2$ by electron impact Yong-Ki Kim, M. Asgar Ali Cross sections for the excitation of H$_2$ by electron impact from its ground electronic state to the first two dipole- and spin-allowed electronic states (B and C) have been calculated by modifying plane-wave Born cross sections (BE scaling) as was done successfully for neutral atoms.\footnote{Y.-K. Kim, Phys. Rev. A {\bf 64}, 032713 (2001).} The scaled cross sections are in good agreement with the experimental data by Khakoo and coworkers.\footnote{M.A. Khakoo and S. Trajmar, Phys. Rev. A {\bf 34}, 146 (1986)}$^,$\footnote{J. Wrkich et al. J. Phys. B {\bf 35}, 4695 (2002).} Calcualtion of BE scaled excitation cross sections for other molecules is in progress, and the results will be presented at the conference. [Preview Abstract] |
Wednesday, October 19, 2005 11:30AM - 11:45AM |
QW1.00005: Positron-molecule annihilation, Feshbach resonances and bound states J.A. Young, C.M. Surko Positron-matter interactions are unique as the positron has no exchange interaction with electrons, is repelled by nuclei, can form positronium and can annihilate with electrons. Using monoenergetic positrons from a trap-based beam, we have been able to measure the first energy-resolved, positron-on-molecule annihilation spectra below the positronium formation threshold [1,2]. Strong peaks in annihilation rate are observed at energies just below the vibrational modes of various molecules. These peaks are due to vibrational Feshbach resonances (VFR) and provide evidence of positron-molecule binding. In alkanes, the binding energy grows linearly and the annihilation rate exponentially with molecular size. In this paper, the properties of these VFR are further explored. The dependence on target morphology is studied for the ring hydrocarbons, benzene, cyclohexane and cyclopropane. A comparison is presented of positron-annihilation and infrared-absorption spectra. Finally, evidence is presented for a second, ``positronically excited'' bound state in largest alkane molecules studied.\\ ~[1] S. J. Gilbert, \textit{et al.}, \textit{Phys. Rev. Lett}., \textbf {88}, 043201 (2002).\\ ~[2] L. D. Barnes, \textit{et al.}, \textit{Phys. Rev. A} \textbf{67}, 032706 (2003). [Preview Abstract] |
Wednesday, October 19, 2005 11:45AM - 12:00PM |
QW1.00006: Positron transport in argon M. Suvakov, Z. Lj. Petrovic, S.J. Buckman We have compiled a complete set of cross sections for positrons in argon based on recent measurements and theory. All aspects of the positron transport differ significantly from that of electrons. The positronium channel leads to a loss of positrons and therefore will be analogous to electron attachment in electron transport. At the same time ionization is treated as an inelastic, conservative process. A Monte Carlo program has been used to calculate positron transport coefficients. The most significant feature is the effect of positronium formation at low E/N. The transport coefficients show a huge effect of non- conservative collisions as the bulk drift velocity becomes almost two orders of magnitude smaller than the flux drift velocity. At higher E/N, however, the two drift velocities have the same order of magnitude. [Preview Abstract] |
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