Colm T. Whelan

Coincidence Studies of Electron and Photon Impact Ionization

Recent Absolute (e,2e) Measurements on Atomic Hydrogen and Helium at Low and Intermediate Energies H. Ehrhardt, J. Roder. Second Born Calculations of (e,2e) Cross Sections at Low Energy Using a Pseudo-State Set F. Rouet, et al. Excitation-Ionization and Excitation-Autoionization of Helium P.J. Marchalant, et al. An Analytical Approach to Resonant and Direct Fragmentation of Many-Body Coulomb Systems J. Berakdar. Ionization Dynamics and Exchange Effects in Pure Three-Body Coulomb Scattering J. Berakdar, J.S. Briggs. Analysis of Integrated Cross Sections and Spin Asymmetries for the Electron-Impact Ionization of One and Two-Electron Atomic Systems J. Berakdar, et al. Double Ionizaton Mechanisms from Triple Coincidence Experiments A. Lahmam-Bennani, et al. Double Ionization of Helium by Electron Impact: A Study of the Two Step Mechanism C. Dal Cappello, et al. Partitioning of Momentum in Electron-Impact Double Ionization J.H. Moore, et al. 5DCS and 4DCS Calculations for Symmetric (e,3e) Impact Ionization Yu.V. Popov, et al. Exact and Approximate Methods of the Rigorous Coulomb Scattering Theory I.V. Farnakeev, et al. Strong and Weak Statements in the Theory of Dipolar (e,2e) Impact Ionization Yu. Popov, et al. Exact and Approximate Methods of the Rigorous Coulomb Scattering Theory I.V. Farnakeev, et al. Strong and Weak Statements in the Theory of Dipolar (e,2e) Impact Ionization Yu. Popov, et al. 25 Additional Articles. Index.

Second-order effects in (e, 2e) excitation-ionization of helium to ( n = 2)

First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to . Results for ionization to the ground-state ion (1s) are also given. The physical content of the approximations is discussed, in particular, the two-step mechanism which appears in the second-order term for excitation-ionization. The second Born term is calculated in the closure approximation using a new numerical method based on prolate spheroidal coordinates. Comparison is made with absolute experimental data from Paris and Rome in very asymmetric coplanar geometry - scattered electron energies of 5500, 1500 and 570 eV and ejected electron energies of 5, 10, 20, 40 and 75 eV. For excitation-ionization the second Born approximation generally gives improved agreement with the experimental data in the recoil region and second-order effects are found to be still significant at 5500 eV. The importance of the second-order term decreases with increasing ejected energy for the cases studied here.

Triply differential single ionization cross sections in coplanar and non-coplanar geometry for fast heavy ion-atom collisions

We have performed a kinematically complete experiment and calculations on single ionization in 100 MeV/amu C6+ + He collisions. For electrons ejected into the scattering plane (defined by the initial and final projectile momentum vectors) our first- and higher-order calculations are in good agreement with the data. In the plane perpendicular to the scattering plane and containing the initial projectile axis a strong forward-backward asymmetry is observed. In this plane both the first-order and the higher-order calculations do not provide good agreement neither with the data nor amongst each other.

Gauge discrepancies in calculations of on helium

We have studied the dependency and sensitivity of the triple differential cross section (TDCS) for on He with respect to the gauge formulation that one employs to represent the TDCS. This analysis has been performed using a variety of analytic initial- and final-state wavefunctions. It was found that the TDCS is very sensitive to the representation of the final-state wavefunction in all geometries and all formulations of the TDCS. When the detected electrons are of unequal energy the TDCS is also sensitive to the initial-state wavefunction. The predictions using all combinations of initial- and final-channel wavefunctions are gauge dependent and vary enormously in absolute size.

Exchange effects in low energy electron impact ionization of the inner and outer shells of argon

First order distorted wave Born approximation (DWBA) triple differential cross sections are reported for low-energy electron-impact ionization of the inner 3s and outer 3p shells of argon. Previous DWBA works have demonstrated that experiment and theory are not in accord for low energy ionization of inert gases and here we investigate the importance of exchange scattering. Different approximations for treating exchange scattering are investigated. It is shown that exchange scattering is particularly important for 3s ionization. Even with a proper treatment of exchange, the first order calculations are still not in satisfactory agreement with experiment. Consequently higher order effects will have to be included to achieve a satisfactory description of the low-energy ionization process. We also investigated both the Hartree-Fock and optimized potential methods for calculating atomic wavefunctions and static potentials and found that both methods produced almost the same cross sections.

Triple differential cross sections for the electron-impact ionization of argon and neon

Cross sections have been measured for the electron-impact ionization of argon and neon in a coplanar symmetric geometry. Results are presented for impact energies of 115.8, 85.8 and 50 eV for argon and 50 eV for neon. A comparison is made with distorted-wave Born approximation calculations which include initial channel polarization and final channel post-collisional interactions. The lowest-energy argon data appear to indicate that polarization effects are very significant, and, in order to explore this, additional measurements have been performed in the coplanar constant geometry.

Low and intermediate energy coplanar symmetric (e, 2e) cross sections for the ionization of neon

Triple-differential cross sections (TDCS) are measured in a coplanar symmetric energy sharing geometry for a neon target. The experiments were carried out for ionization of the 2p orbital between 500 and 65 eV incident energy and of the 2s orbital at 226.9 and 126.9 eV incident energy. The main difference compared to helium target is the apparition of a structure at 85 degrees for ionization from both orbitals. This structure, clearly seen at 65 eV for the 2p shell, is enhanced as the incident energy decreases. Distorted-wave Born approximation (DWBA) calculations including post-collisional interactions (PCI) and polarization are compared with experimental data. This theoretical model is found to give quite a good fit above 200 eV incident energy for ionization of the 2p shell and from 126.9 to 226.9 eV for ionization of the 2s. Below 100 eV, some disagreement between theory and experiment is observed. Calculations in this energy region, nevertheless, enable the effects of target orbital polarization to be identified.

(e,2e) & related processes

Preface. (e,2e), effective charges, distorted waves and all that C.T. Whelan, R.J. Allan, H.R.J. Walters, X. Zhang. Directions in (e,2e) and related processes H.R.J. Walters, X. Zhang, C.T. Whelan. Near threshold (e,2e) ionisation of helium and atomic hydrogen H. Ehrhardt, T. Rosel. Does asymmetric (e,2e) approach the dipolar limit at intermediate energies? G. Stefani, M.V. Marabello, L. Avaldi, R. Camilloni. Knockout reactions to study atomic and molecular electronic structure: the future J.H. Moore, J.A. Tossell, M.A. Coplan, J.W. Cooper, J.P. Doering. Absolute experimental cross sections for symmetric coplanar (e,2e) collisions of 45 to 500eV electrons with Helium A. Pochat, R.J. Tweed, C.T. Whelan, X. Zhang, H.R.J. Walters, R.J. Allan, F. Gelebart, M. Cherid. Recoil ion momentum spectroscopy: a complementary technique to (e,2e) and (e,3e) L. Spielberger, O. Jagutzki, R. Dorner, K. Froschauer, A. Gensmantel, H. Schmidt-Bocking, J. Ullrich, U. Buck. Arbitrary charged particle-impact ionisation of H and He: scaling properties of the cross sections J. Berakdar. Electron impact ionisation-excitation of helium: calculations using coupled channel parametric potential wave functions R.J. Tweed, S. Mazevet, O. Robaux, J. Langlois. Democratic parameters for (e,2e) and (e,3e) kinematics A.R.P. Rau. Asymptotically exact distorted wave calculations D.H. Madison, S. Jones, A. Franz, P.L. Altick. New experiments needed in (e,2e) L. Frost. New trends: electron impact ionisation of laser excited atoms and double ionisation by electron and photon impact H. Klar. Laser-assisted (e,2e) collisions C.J. Joachain. (e,2e) caclulations using a correlated final state: A. Franz, P.L. Altick. Solid state perspective in thetheory of the Auger decay C. Verdozzi. High-performance computers in (e,2e) and (e,3e) interactions with atoms R.J. Allan. Energy loss and (e,2e) studies of moles on surfaces R.E. Palmer. Influence of Coulomb forces on the angular momenta of products in three or more fragmentary reactions M. Gailitis. Multiconfiguration Hartree-Fock calculation of the autoionization resonance parameters in the (e,2e) reaction on helium atom A.S. Kheifets. Near threshold (g,2e) experiments in the perpendicular plane A. Huetz, P. Selles, D. Waymel, L. Andric, J. Mazeau. Multicoincidence investigation of ionization dynamics L. Avaldi, R. Camilloni, E. Fainelli, R. Multari, G. Stefani. Coupling the coplanar (e,2e) geometry to the perpendicular plane geometry A.J. Murray. Double ionization of noble gases by electron impact C. Dal Cappello, B. Joulakian. Coulomb-Born approximation for the calculation of (e,2e) cross sections. In Memoriam Wolfgang Hink H. Ehrhardt. Processes other than simple outer shell ionization A. Crowe, D.G. McDonald. Investigation of the angular dependence of post collision interaction effects using coincidence techniques B. Lohmann. Relativistic (e,2e) processes under Bethe ridge conditions C.D. Schroter, H.-Th. Prinz, N. Keuler, W. Nakel. Distorted wave calculations of triple differential cross section for inner shell ionization X. Zhang, C.T. Whelan, H.R.J. Walters. Index.

(e,2e), effective charges, distorted waves and all that!

An overview of the theory of (e,2e) processes is presented. Effective charges are introduced and the Peterkop relation discussed. The distored wave Born approximation is considered and applied to the calculation of triple differential cross-sections. A derivation of the distorted wave impulse approximation is given and the difficulties encountered in the choice of off-shell Coulomb T-matrix highlighted. It is shown that 3 body effects are significant in both the initial and final channels for the ionisation of He at energies of 50eV and below.

First and second Born calculations of (e, 2e) excitation-ionization of helium

We report detailed calculations of the first Born triple-differential cross section for (e, 2e) excitation-ionization of ground state He to He+ (n = 2). These are in accord with the very recent work of Kheifets et al (1999 J. Phys. B: At. Mol. Opt. Phys. 32 L433) and confirm that the first Born amplitude is now known very accurately. We illustrate the sensitivity of the first Born cross section to the choice of initial and final state wavefunctions. We combine our accurate first Born amplitude with an estimate of the second Born term evaluated in the closure approximation. As in previous work (Marchalant et al 1998 J. Phys. B: At. Mol. Opt. Phys. 31 1141) we find that second-order effects are significant even up to energies as high as 5.5 keV. Agreement with experiment generally remains not very satisfactory.