Session QR2: Electron-Atom Collisions

Thursday, October 4, 2007

Reaction microscopes allow the measurement of the momentum vectors of all fragments produced in ionizing collisions. We have developed such a multi-particle spectrometer for the investigation of electron scattering processes. Here novel studies of fundamental few-body reactions will be presented. For single ionization of helium by fast (E = 1000 eV) and by slow (E = 105 eV) electron impact the three-dimensional electron emission patterns show structures which, so far, have not been observed in conventional experiments. Kinematically complete experiments for double ionization of helium by electrons with energies close to the threshold enable for the first time a detailed insight into the behaviour of three strongly interacting continuum electrons in the field of the residual ion. Finally, results for ionization of simple molecules as hydrogen or water are discussed where the dependence of the ionization dynamics from the alignment of the molecular axis with respect to the incoming projectile beam can be studied.   

Thursday, October 4, 2007

The Australian Positron Beamline Facility was built to provide a source of positrons for scattering experiments. Recently, the first high resolution positron beam was produced and has been used to measure positron scattering cross sections from helium. The beamline is based on techniques developed by Cliff Surko at UCSD and uses a muffer gas trap in a strong magnetic field to trap, cool and form the positrons into a pulsed beam. A beam of 150 meV energy resolution has been obtained and was used to measure the absolute differential positron scattering cross section for helium, below the positronium formation threshold. This is the first time that these measurements have been made and the results will be compared to state of the art theoretical calculations, as a strict test of their validity. Plans for further work using this facility will also be outlined.   

Thursday, October 4, 2007

We have used the $B$-spline $R$-matrix (close-coupling) method with non-orthogonal sets of orbitals~[1] to calculate inner-shell photo\-detachment of Na$\rm ^-(3s^2)$ for incident photon energies between 30 and 50$\,$eV. Satisfactory agreement is obtained with the measurements of Covington {\it et al.}~[2]. With the same computational model for the e-Na (half) collision, we can also treat electron impact excitation of the $\rm (2p^5 3s^2)^2P_{3/1,1/2}$ auto\-ionizing states in sodium. Our results for the latter process will be compared with new experimental data, obtained with an energy width (FWHM) of $200\!-\!250\,$meV in the incident beam~[3]. This improvement in the energy resolution made it possible, for the first time, to resolve the near-threshold excitation of the two fine-structure components $\rm (2p^5 3s^2)^2P_{3/2,1/2}$ separately. \par\noindent [1] O.~Zatsarinny, Comp.~Phys.~Commun. {\bf 174} (2006) 273. [2] A.M.~Covington {\it et al.}, J. Phys. B~{\bf 34} (2001) L735. [3] A.A. Borovik, O. Zatsarinny, and K. Bartschat; Book of Abstracts ICPEAC XXV (in press).   

Thursday, October 4, 2007

The importance of projectile interactions in fully differential cross sections (FDCS) will be explored for the problem of simultaneous excitation-ionization by electron impact. We will compare the results of two theories- the first Born approximation (FBA) and the four-body distorted wave model (4DW). In the first Born approximation (FBA), the projectile electron is treated as a plane wave, the ejected electron is treated as a Hartree Fock distorted wave, and the final state Coulomb interaction between the two continuum electrons is ignored. In the 4DW model, all continuum electrons are treated as Hartree Fock distorted waves, and a Coulomb distortion factor is included in the final state to account for the interaction between the two outgoing electrons. Results will be presented for an incident electron energy of 500 eV and will be compared to experimental data.   

Thursday, October 4, 2007

We have applied a hybrid method, combining a second-order distorted-wave method for a fast ionizing projectile with an $R$-matrix (close-coupling) approach for the initial bound state and the ejected electron~[1, 2], to calculate electron and positron impact ionization of helium, magnesium, and calcium. Our results will be compared with recent experimental data and predictions from other theoretical approaches. For various kinematical situations, in-plane and out-of-plane, we analyze the sensitivity of the theoretical results to the details of the computational models, as well as the projectile charge. New benchmark experiments to test the various theoretical approaches will be suggested. \par\vspace{0.1truecm}\noindent [1] Y. Fang and K. Bartschat, J. Phys. B {\bf 34} (2001) L19. \newline [2] K. Bartschat and O. Vorov, Phys. Rev. A {\bf 72} (2005) 022728.   

Thursday, October 4, 2007

In recent years there have been a number of experimental and theoretical investigations of electron-impact excitation (EIE) cross sections with improved measurement techniques and state-of-the-art calculations. These investigations not only reveal many interesting aspects of basic physics of electron collision with neutral atoms, but they are extremely important for many gaseous electronics applications such as lighting, plasma processing, and gas lasers. We will present the current status of many theoretical calculations of EIE cross sections using distorted-wave (DW) and close-coupling R-matrix and B-spline R-matrix methods for several noble gases such as Ar, Kr, and Xe in particular and compare them with recent improved measurements by the University of Wisconsin group using a Fourier-transform spectrometer. Although in some cases there is qualitative agreement between theoretical and experimental results, even with the advancements of both theoretical and experimental techniques and improved data, quantitative discrepancies still exist in most cases. We will present some of the issues regarding the disagreement and also discuss the significant effects of EIE data for various applications. In particular, results from this study have been used to model an electron beam pumped Ar-Xe laser at NRL. We will also comment on the complexity involved in calculating the structure and EIE cross sections of a complex open-shell element such as Mo and its implications for modeling a molybdenum discharge lamp.