M P Scott

Electron - atom scattering at low and intermediate energies using a pseudo-state/ R-matrix basis

The use of a pseudo-state expansion within the standard low-energy R-matrix framework to facilitate the study of electron scattering by complex atoms and ions at both low and intermediate energies is discussed. Electron scattering from atomic hydrogen is considered as an example, and results for elastic scattering phase shifts and excitation cross sections are found to be in excellent agreement with recent IERM results in these energy regions. The advantage of this procedure is that existing computer codes, which have been developed over many years, can be directly extended to study electron scattering from a general N-electron target atom or ion.

Atomic data for opacity calculations. XXI. The neon sequence

Photoabsorption processes are studied for the ground and excited terms of the members of the neon isoelectronic sequence considered to be of significant astrophysical abundance. A two-state, LS close-coupling calculation is carried out. Oscillator strengths and total photoionization cross section data are calculated for even and odd LS terms with S=0,1;L<or=4, and an effective principal quantum number nu <or=10.

Electron scattering by atomic hydrogen at intermediate energies. I. Integrated elastic, 1s-2s and 1s-2p cross sections

Integrated cross sections for 1s-1s, 1s-2s and 1s-2p electron-hydrogen-atom scattering processes are reported for scattering energies ranging from the ionisation threshold to 4 Ryd. For the total angular momentum L in the range 0<or=L<or=4 the intermediate energy R-matrix theory of Burke et al. (1981) has been used. From L=5 to L=Lmax, where Lmax varies between 10 and 16, calculations have been performed using the standard R-matrix close-coupling method and employing the nine-state basis (three eigenstates plus six pseudostates) of Fon et al. (1981). To compensate for incompleteness in the R-matrix bases, corrections have been applied to all partial waves 0<or=L<or=Lmax using the plane-wave second Born approximation. In addition, the second Born approximation has also been used to extrapolate the partial-wave series from L=Lmax+1 to infinity. The agreement with theoretical work of Callaway et al. (1987) is good for both elastic and inelastic processes. The calculated 1s-2s and 1s-2p cross sections are in reasonable accord with experimental results originating from the 1s-2p relative data of Long et al. (1968) independently normalised to theory at high energy and with the 2s/2p ratio measurements of Kauppila et al. (1970). However at 4 Ryd the 1s-2p result lies below an absolute measurement of Williams (1981) and confirms an earlier theoretical result of van Wyngaarden and Walters (1986). Finally the total inelastic cross section is reported.

Differential cross sections and electron-impact coherence parameters for electron scattering from helium atoms

We have applied the R-matrix with pseudo-states method (RMPS) to study electron - helium scattering at intermediate energies. Differential cross sections for elastic and inelastic collisions and electron-impact coherence parameters for excitation of the transition are presented for incident energies of 30, 50 and 80 eV. Excellent agreement with experiment and with the predictions of the convergent close-coupling calculation by Fursa and Bray is obtained. Since correlation effects and target continuum states are both accurately represented in a completely general way by this method, the associated program package can now be used to obtain reliable results at intermediate energies for arbitrary complex atomic and ionic targets of importance in applications.

2DRMP: A suite of two-dimensional R-matrix propagation codes

The R-matrix method has proved to be a remarkably stable, robust and efficient technique for solving the close-coupling equations that arise in electron and photon collisions with atoms, ions and molecules. During the last thirty-four years a series of related R-matrix program packages have been published periodically in CPC. These packages are primarily concerned with low-energy scattering where the incident energy is insufficient to ionise the target. In this paper we describe 2DRMP, a suite of two-dimensional R-matrix propagation programs aimed at creating virtual experiments on high performance and grid architectures to enable the study of electron scattering from H-like atoms and ions at intermediate energies.

Autoionizing states in electron-impact ionization of C3+

Results are reported for electron-impact ionization of C3+ in the energy range 290-350 eV using the R -matrix with pseudo-states method (RMPS). Pseudo-states and autoionizing states are explicitly included in the R -matrix expansion to take fully into account for the first time the interaction between the direct and indirect ionization processes. The calculated resonance structure in the ionization cross section is in good agreement with the experiment of Muller et al but there is a small discrepancy in the normalization of the cross section between theory and experiment.

Benchmark calculations for e - H scattering between the n = 2 and n = 3 thresholds

We present eigenphase sums and partial cross sections for elastic scattering and electron-impact excitation of the n = 2 states of atomic hydrogen for incident electron energies between the n = 2 and n = 3 thresholds. These results were obtained by using the convergent close-coupling (CCC), R-matrix with pseudo-states (RMPS), and intermediate-energy R-matrix (IERM) methods. The agreement between the predictions from all three approaches is excellent, indicating that the effects of both the discrete and the continuum states in the close-coupling expansion have been treated to convergence. Very good agreement is also obtained with the experimental data of Williams.

Cross Sections for the K-Shell Photoabsorption of Neutral Iron

The Fe I Kα emission lines are observed during solar flares and in the emission spectra of other astrophysical sources, such as active galactic nuclei. This paper presents cross sections for the K-shell photoionization of neutral iron, which are urgently required for the modeling of these astrophysical phenomena. A 16 target-state representation is utilized, where each target state is represented by an extensive configuration-interaction-type wave function. The consequent introduction of electron correlation effects is seen to significantly alter the magnitude of the background cross section in comparison with earlier work, while extensive resonance structure is also resolved near threshold for the first time.

Computational aspects of the two-dimensional propagation of R-matrices on MPPs

Abstract The simulation of electron collisions, at intermediate energies, with atoms, ions and molecules in highly excited Rydberg states is a physically important and computationally challenging problem. A suitable computational technique, to meet this challenge for the general two-electron problem, involves an adaptation of the intermediate energy R -matrix approach, in which the ( r 1 , r 2 ) plane of the inner region is partitioned into a set of connected subregions and a global -matrix is propagated, in two dimensions, across this plane. In this paper we examine the computational aspects of this technique and, in particular, describe a parallel strategy for exploiting the architecture of distributed memory parallel computers.

Fast Computation of the Slater Integrals

Slater integrals are two-dimensional radial integrals whose integrand is constructed from normalized eigenfunctions of the Schrodinger equation. These integrals occur in many atomic structure and scattering computations. However, in two-dimensional R-matrix propagation they represent a significant computational bottleneck. The problem involves two steps, that is, the numerical solution of the Schrodinger equation and the computation of the integrals, respectively. By exploiting the characteristic features of the problem we seek to devise a two-stage computational strategy, where the second stage is influenced and informed by the first. In particular, we focus on the development of extended frequency-dependent quadrature rules (EFDQRs) that improve the accuracy and significantly reduce the computation time of the integrals. The performance of these ad hoc rules is examined and compared to the currently used Newton--Cotes method in the construction of Hamiltonian matrices involving up to 300 x 106 integrals. A gain of two orders of magnitude in the CPU time is obtained.