Sh. A. Abdel-Naby

Electron-ion recombination of Be-like C, N, and O

The absolute total recombination reaction rate coefficients for Be-like C, N, and O have been measured using the CRYRING storage ring and compared with the results from distorted-wave theory. For the theory results, it is found that shifts to NIST energy values for the core excited energies of the recombining system are not sufficient to accurately match all of the resonance positions and heights at lower energies. These theory results represent the quality of most archived theory DR data. The accurate calculation of these low energy resonances still presents a significant challenge to theory. In addition, trielectronic recombination resonances, associated with the formation of triply excited states during recombination, have been observed in the total recombination reaction rate coefficient spectra of N3+ and O4+. Finally, we construct a dielectronic recombination Maxwellian rate coefficient from the experimental results for low n resonances, and from the theoretical results for high n resonances. In the case of O4+, the trielectronic recombination resonances have a strong influence on the low temperature Maxwellian rate coefficient. Our best hybrid Maxwellian rate coefficient is compared with archived distorted-wave theory data, and is found to be in reasonable agreement, even at the low temperatures.

K-shell Photoabsorption Studies of the Carbon Isonuclear Sequence

K-shell photoabsorption cross sections for the isonuclear C I-C IV ions have been computed using the R-matrix method. Above the K-shell threshold, the present results are in good agreement with the independent-particle results of Reilman & Manson. Below threshold, we also compute the strong 1s → np absorption resonances with the inclusion of important spectator Auger broadening effects. For the lowest 1s → 2p, 3p resonances, comparisons to available C II, C III, and C IV experimental results show good agreement in general for the resonance strengths and positions. Our results also provide detailed information on the C I K-shell photoabsorption cross section including the strong resonance features, since very limited laboratory experimental data exist. The resultant R-matrix cross sections are then used to model the Chandra X-ray absorption spectrum of the blazar Mkn 421.

Single and double photoionization of Be and Mg

A new version of the time-dependent close-coupling method is used to calculate the single and double photoionization of the Be and Mg atoms. Total cross sections are calculated using an implicit time propagator with a core orthogonalization method on a variable radial mesh. The double to single photoionization cross section ratios are found to be in good agreement with experiment for both Be and Mg.

Pentuple energy and angle differential cross sections for the electron-impact double ionization of helium

Energy and angle differential cross sections for the electron-impact double ionization of helium are calculated using a non-perturbative time-dependent close-coupling method. Collision probabilities are found by the projection of a time-evolved nine-dimensional coordinate space wavefunction onto fully antisymmetric products of spatial and spin functions representing three outgoing Coulomb waves. At an incident energy of 106 eV, we present double energy differential cross sections and pentuple energy and angle differential cross sections. The pentuple energy and angle differential cross sections are found to be in reasonable agreement with the scaled shapes observed in recent (e, 3e) reaction microscope experiments. Integration of the differential cross sections over all energies and angles yields a total ionization cross section that is also in reasonable agreement with absolute crossed-beams experiments.

K-shell photoabsorption of magnesium ions

X-ray photoabsorption cross sections have been computed for all magnesium ions with three or more electrons using the R-matrix method. A comparison with other available data for Mg II-Mg X shows good qualitative agreement in the resultant resonance shapes. However, for the lower ionization stages, and for singly ionized Mg II in particular, the previous R-matrix results overestimate the K-edge position due to the neglect of important orbital relaxation effects, and a global shift downward in photon energy of those cross sections is therefore warranted. We have found that the cross sections for Mg I and Mg II are further complicated by the M-shell (n = 3) occupancy. As a result, the treatment of spectator Auger decay of 1s → np resonances using a method based on multichannel quantum defect theory and an optical potential becomes problematic, making it necessary to implement an alternative, approximate treatment of Auger decay for neutral Mg I. The new cross sections are used to fit the Mg K edge in XMM-Newton spectra of the low-mass X-ray binary GS 1826-238, where most of the interstellar Mg is found to be in ionized form.

Atomic Collision Processes for Astrophysical and Laboratory Plasmas

An accurate knowledge of atomic collision processes is important for a better understanding of many astrophysical and laboratory plasmas. Collision databases which contain electron-impact excitation, ionization, and recombination cross sections and temperature dependent rate coefficients have been constructed using perturbative distorted-wave methods and non-perturbative R-matrix pseudo-states and time-dependent close-coupling methods. We present recent atomic collision results.

Neutron-impact ionization of He

The time-dependent close-coupling method is used to calculate neutron-impact single and double ionization cross sections of the He atom. We also present the ratio of double to single ionization for He as a guide to experimental checks of theory at low energies and experimental confirmation of the rapid rise of the ratio at high energies unique to neutron-impact ionization of atoms.

Electron-impact ionization of Al

Perturbative distorted-wave and non-perturbative close-coupling methods are used to calculate electron-impact ionization cross sections for the ground state of the neutral Al atom. Configuration-average distorted-wave calculations are made for both direct ionization and excitation-autoionization contributions. The total perturbative results are found to be almost a factor of 2 higher than experiment over a wide energy range. On the other hand, the R-matrix with pseudo-states results for total ionization are found to be in good agreement with experiment. Comparison of time-dependent close-coupling calculations for the direct ionization with the R-matrix with pseudo-state calculations for total ionization reveals that both the direct ionization and excitation-autoionization contributions are strongly affected by correlation effects.

K-Shell Photoabsorption of Magnesium Ions

We report the results of extensive R-matrix calculations for the X-ray photoabsorption of all magnesium ions in the vicinity of the K-edge.

Photoionization of Ne8

The time-dependent close-coupling method based on the Dirac equation is used to calculate single and double photoionization cross sections for Ne8+ in support of planned FLASH/DESY measurements. The fully correlated ground state radial wavefunction is obtained by solving a time independent inhomogeneous set of close-coupled equations. The repulsive interaction between electrons includes both Coulomb and Gaunt interactions. A Bessel function expression is used to include both dipole and quadruple effects on the radiation field interaction. Propagation of the time-dependent close-coupled equations yields single and double photoionization cross sections for Ne8+ in reasonably good agreement with distorted-wave and R-matrix results.