Showing papers by "Igor Bray published in 2010"

Tracing multiple scattering patterns in absolute (e,2e) cross sections for H2 and He over a 4π solid angle

Abstract: Absolutely normalized ($e,2e$) measurements for ${\mathrm{H}}_{2}$ and He covering the full solid angle of one ejected electron are presented for 16 eV sum energy of both final state continuum electrons. For both targets rich cross-section structures in addition to the binary and recoil lobes are identified and studied as a function of the fixed electron's emission angle and the energy sharing among both electrons. For ${\mathrm{H}}_{2}$ their behavior is consistent with multiple scattering of the projectile as discussed before [Al-Hagan et al., Nature Phys. 5, 59 (2009)]. For He the binary and recoil lobes are significantly larger than for ${\mathrm{H}}_{2}$ and partly cover the multiple scattering structures. To highlight these patterns we propose a alternative representation of the triply differential cross section. Nonperturbative calculations are in good agreement with the He results and show discrepancies for ${\mathrm{H}}_{2}$ in the recoil peak region. For ${\mathrm{H}}_{2}$ a perturbative approach reasonably reproduces the cross-section shape but deviates in absolute magnitude.

Convergent-close-coupling calculations for excitation and ionization processes of electron-hydrogen collisions in Debye plasmas

Abstract: Electron-hydrogen scattering in weakly coupled hot-dense plasmas has been investigated using the convergent-close-coupling method The Yukawa-type Debye-Hueckel potential has been used to describe the plasma screening effects The target structure, excitation dynamics, and ionization process change dramatically as the screening is increased Excitation cross sections for the 1s{yields}2s,2p,3s,3p,3d and 2s{yields}2p,3s,3p,3d transitions and total and total ionization cross sections for the scattering from the 1s and 2s states are presented Calculations cover the energy range from thresholds to high energies (250 eV) for various Debye lengths We find that as the screening increases, the excitation and total cross sections decrease, while the total ionization cross sections increase

Multiconfigurational two-centre convergent close-coupling approach to positron scattering on helium

Abstract: The convergent close-coupling method with two-centre expansions has been developed to calculate positron scattering on helium. The method utilizes a multiconfigurational description of helium wavefunctions. Positronium formation is taken into account explicitly as electron capture into the positronium states. Direct-scattering, positronium-formation and breakup cross sections are calculated at all energies of practical relevance. Convergence in the calculated cross sections is demonstrated by increasing the basis size and orbital quantum number of the included states for each of the centres. Better agreement with experimental data is found when a multiconfigurational description is used for the helium wavefunctions in comparison with the recent frozen-core results.

Relativistic convergent close-coupling method applied to electron scattering from mercury

Abstract: We report on the extension of the recently formulated relativistic convergent close-coupling (RCCC) method to accommodate two-electron and quasi-two-electron targets. We apply the theory to electron scattering from mercury and obtain differential and integrated cross sections for elastic and inelastic scattering. We compared with previous nonrelativistic convergent close-coupling (CCC) calculations and for a number of transitions obtained significantly better agreement with the experiment. The RCCC method is able to resolve structure in the integrated cross sections for the energy regime in the vicinity of the excitation thresholds for the (6s6p) {sup 3}P{sub 0,1,2} states. These cross sections are associated with the formation of negative ion (Hg{sup -}) resonances that could not be resolved with the nonrelativistic CCC method. The RCCC results are compared with the experiment and other relativistic theories.

Two-center convergent close-coupling calculations for positron-lithium collisions

Abstract: We report on two-center convergent close-coupling calculations of positron-lithium collisions. The target is treated as one active electron interacting with an inert ion core. The positronium formation channels are taken into account explicitly utilizing both negative- and positive-energy Laguerre-based states. A large number of channels and high partial waves are used to ensure the convergence of the cross sections. We find the Ramsauer-Townsend minimum in total and elastic cross sections at an impact energy $E$ of about 0.0016 eV. As found previously for H and He, the contributions to the breakup cross section from both the Li and the Ps centers become the same as the threshold is approached.

Spin effects in double photoionization of lithium

Abstract: We apply the nonperturbative convergent close-coupling (CCC) and time-dependent close coupling (TDCC) formalisms to calculate fully differential energy and angular resolved cross sections of double photoionization (DPI) of lithium. The equal energy sharing case is considered in which dynamics of the DPI process can be adequately described by two symmetrized singlet and triplet amplitudes. The angular width of these amplitudes serves as a measure of the strength of the angular correlation between the two ejected electrons. This width is interpreted in terms of the spin of the photoelectron pair.

A two-centre convergent close-coupling approach to positron-helium collisions

Abstract: A two-centre convergent close-coupling method, which includes the positronium-formation channel, has been developed for positron scattering on helium. Convergent, pseudoresonance-free cross sections have been obtained. This is only possible if complete expansions are used on both the positronium and helium centres. The method is valid for all projectile energies and all transitions, including breakup, which is associated with excitation of positive-energy helium and positronium pseudostates. Generally, good agreement between calculated cross sections and available experimental data has been found across all incident energies.

Single ionization of helium by electron impact

Abstract: We suggest that the problem of single ionization of helium by electron impact, leaving the ion in the ground state, has been solved theoretically for the full range of kinematics and collision geometries of practical interest. Following the emphasis on the study of out-of-plane geometries where the cross sections are very small [Schulz et al., Nature 422, 48 (2003)], we find that the convergent close-coupling calculations, in either a frozen- or a multicore treatment of the target, are in excellent agreement with the available measurements. Curiously, some systematic discrepancies are identified for some in-plane cases where the cross sections are an order of magnitude larger. Further measurements are required to resolve these discrepancies. If subsequent measurements confirm the present calculations, then we would have a strong case that the problem has been solved.

Convergent close-coupling calculations of positron scattering on metastable helium

Abstract: The convergent close-coupling method has been applied to positron scattering on a helium atom in the 2 {sup 3}S metastable state. For this system the positronium (Ps) formation channel is open even at zero scattering energy making the inclusion of the Ps channels especially important. Spin algebra is presented for the general case of arbitrary spins. A proof is given of the often-used assumption about the relationship between the amplitudes for ortho-positronium and para-positronium formation. The cross sections for scattering from 2 {sup 3}S are shown to be significantly larger than those obtained for the ground state.

Differential cross sections of double photoionization of lithium

Abstract: We extend our previous application of the convergent close-coupling (CCC) and time-dependent close-coupling (TDCC) methods [Phys. Rev. A 81, 023418 (2010)] to describe energy and angular resolved double photoionization (DPI) of lithium at arbitrary energy sharing. By doing so, we are able to evaluate the recoil ion momentum distribution of DPI of Li and make a comparison with recent measurements of Zhu et al. [Phys. Rev. Lett. 103, 103008 (2009)].

Calculation of electron-impact ionization using the J-matrix method

Abstract: The $J$-matrix approach to electron-atom scattering is applied to ionization processes. We consider the Temkin-Poet model of $e$-H ionization. Convergence issues are studied with greater detail than previously possible using other close-coupling methods. The numerical strengths of the technique are emphasized with the long-term goal of application to ionization-plus-excitation processes.

Quantum-statistical T-matrix approach to line broadening of hydrogen in dense plasmas

Abstract: The electronic self‐energy ∑e is an important input in a quantum‐statistical theory for spectral line profile calculations. It describes the influence of plasma electrons on bound state properties. In dense plasmas, the effect of strong, i.e. close, electron‐emitter collisions can be considered by three‐particle T‐matrix diagrams. These digrams are approximated with the help of an effective two‐particle T‐matrix, which is obtained from convergent close‐coupling calculations with Debye screening. A comparison with other theories is carried out for the 2p level of hydrogen at kBT = 1 eV and ne = 2⋅1023 m−3, and results are given for ne = 1⋅1025 m−3.

A detailed study of electron impact ionization of Ne(2s) and Ar(3s)

Abstract: We compare the results obtained in a variety of numerical calculations for electron impact ionization of the 2s subshell in Ne and the 3s subshell in Ar. Comparison with the recent joint experimental and theoretical study by Kheifets et al (2009 J. Phys. B: At. Mol. Opt. Phys. 42 165204) shows good agreement with the calculations reported there, provided a similar theoretical model is used. The numerical results, however, become severely model dependent in certain cases, thus providing a major challenge to the theoretical treatment of such ionization processes.

Benchmark calculations for electron impact ionization and ionization-excitation of magnesium

Abstract: We compare results of various numerical calculations for electron impact ionization of the ground state Mg (3s2) leading to the final ionic ground state Mg+(3s) or the excited ionic states Mg+(3p, 4s, 3d) Although our numerical results exhibit some model dependence, especially for small values of the triple-differential cross section, we cannot resolve the significant differences between experiment and theory reported by Bolognesi et al (2008 J Phys B: At Mol Opt Phys 41 065203) Extensive theoretical modelling allows us to identify possible sources of this strong disagreement and to suggest future investigations

Benchmark Integral Cross Sections for Electron Impact Excitation of the n=2 States in Helium

Abstract: In this paper, we present integral cross sections (ICS) for electron impact excitation of the n = 2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 23S, 21S, 23P and 21P, has been determined by integration of the angular differential cross sections (DCS) over all of 0° to 180°, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 21P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 21P ICSs is generally found in the measured impact energy region.

Ionization of helium by 64.6 eV electrons

Abstract: A comprehensive computational study using the convergent close-coupling method of 64.6 eV electron-impact ionization of the ground state of helium is presented. The kinematics considered range from the very asymmetric energy-sharing through to equal energy-sharing. The cross sections given range from the total to fully differential, with the latter being calculated for in- and out-of-plane geometries. Generally excellent agreement with available experiment is found, but some systematic discrepancies are also identified.

Single-photon double K-shell ionization of low-Z atoms

Abstract: The photon energy dependence of the double K-shell ionization of light atoms is reported. Experimental double-to-single photoionization cross section ratios for Mg, Al, Si and Ca were obtained from measurements of high-resolution x-ray emission spectra. The double photoionization (DPI) cross-sections for K-shell hollow atom production are compared to convergent close-coupling calculations (CCC) for neutral atoms and He-like ions. The relative importance of the initial-state and final-state electron-electron interactions to the K-shell DPI in many-electron atoms and two-electron ions is addressed. Physical mechanisms and scaling laws of the K-shell double photoionization are examined. A semiempirical universal scaling of the DPI cross sections with the effective nuclear charge for neutral atoms 2≤Z≤47 is established.

Spin-resolved electron-impact ionisation of atoms

Abstract: We shall consider various aspects of electron-impact ionisation of atoms. Firstly, the recent progress in the formulation of the problem will be outlined. Then application of the convergent close-coupling (CCC) method to the calculation of the total ionisation spin asymmetries will be described for a range of neutral through to highly charged targets. We shall also consider the importance of electron spin in the problem of single and double photoionisation of lithium. Lastly, application of the CCC method to 64.6 eV e-He ionisation fully differential cross sections for in- and out-of-plane geometries will be considered.

Three-dimensional cross sections for electron impact ionization of atoms and molecules

Abstract: Using a multi-particle momentum spectrometer (reaction microscope), three-dimensional and fully differential cross sections (FDCS) for electron impact ionization are obtained, providing benchmark data for comprehensive tests of theoretical calculations. Since all final-state particles, including the scattered projectile were detected, a good momentum transfer resolution was obtained also for heavy targets like Ar. Results for ionization of the Ar 3p- and He 1s-orbitals by 200 eV electron impact are presented. The cross section patterns display rich structure, which is partially reproduced by theory, although differences persist out-of-the scattering plane. Kinematically complete experiments for electron impact ionization of simple diatomic molecules have attracted increased attention concerning molecular structure effects. (e, 2e) on He and H2 is studied at equivalent collision kinematics to explore the differences of atomic and molecular ionization. Here FDCS were obtained covering the whole solid angle.

Convergence study of the close-coupling approach to positron-helium collisions

Abstract: Positron-helium collisions have been studied using a two-center close-coupling method. Convergent, pseudoresonance-free cross sections have been obtained by using complete expansions on both the helium and positronium centers. The low energy elastic cross sections, phase-shifts and total cross sections are compared with experimental data and results of other theories.

Surface-Integral Approach to the Coulomb Few-Body Scattering Problem

Abstract: We present main features of a surface-integral approach to the Coulomb few-body scat- tering problem. This approach is valid for both short-range and Coulombic longe-range interactions. We give new general denitions for the potential scattering amplitude. For the Coulombic potentials the generalized amplitude gives the physical on-shell amplitude without recourse to a renormaliza- tion procedure. New post and prior forms for the amplitudes of breakup, direct and rearrangement scattering in a Coulomb three-body system are also presented. The Green's functions and formal solutions of the Schrodinger equation in integral form are not used. Therefore, for the purpose of dening the scattering amplitudes the knowledge of a complicated analytic structure of the Green's function in the complex-energy plane is not required.