Electron-impact excitation collision strengths for transitions between all singly excited levels up to the n = 4 shell of helium-like argon and the n = 4 and 5 shells of helium-like iron have been calculated using a radiation-damped R-matrix approach. The theoretical collision strengths have been examined and associated with their infinite-energy limit values to allow the preparation of Maxwell-averaged effective collision strengths. These are conservatively considered to be accurate to within 20% at all temperatures, 3×105-3×108 K for Ar16+ and 106-109 K for Fe24+. They have been compared with the results of previous studies, where possible, and we find a broad accord. The corresponding rate coefficients are required for use in the calculation of derived, collisional-radiative, effective emission coefficients for helium-like lines for diagnostic application to fusion and astrophysical plasmas. The uncertainties in the fundamental collision data have been used to provide a critical assessment of the expected resultant uncertainties in such derived data, including redistributive and cascade collisional-radiative effects. The consequential uncertainties in the parts of the effective emission coefficients driven by excitation from the ground levels for the key w, x, y and z lines vary between 5% and 10%. Our results remove an uncertainty in the reaction rates of a key class of atomic processes governing the spectral emission of helium-like ions in plasmas.

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

Double photoionization accompanied by loss of n C atoms (n = 0, 2, 4, 6) was investigated by merging beams of Xe@C + 60 ions and synchrotron radiation and measuring the yields of product ions. The giant 4d dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 ± 1.4 of the total Xe 4d oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe 4d photoelectron waves that may be transmitted or reflected by the spherical C + 60 molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic R-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry.

We present complete collisional-radiative modelling results for the soft x-ray emission lines of Fe16+ in the 15 Å–17 Å range. These lines have been the subject of much controversy in the astrophysical and laboratory plasma community. Radiative transition rates are generated from fully relativistic atomic structure calculations. Electron-impact excitation cross sections are determined using a fully relativistic R-matrix method employing 139 coupled atomic levels through n = 5. We find that, in all cases, using a simple ratio of the collisional rate coefficient times a radiative branching factor is not sufficient to model the widely used diagnostic line ratios. One has to include the effects of collisional-radiative cascades in a population model to achieve accurate line ratios. Our line ratio results agree well with several previous calculations and reasonably well with tokamak experimental measurements, assuming a Maxwellian electron-energy distribution. Our modelling results for four EBIT line ratios, assuming a narrow Gaussian electron-energy distribution, are in generally poor agreement with all four NIST measurements but are in better agreement with the two LLNL measurements. These results suggest the need for an investigation of the theoretical polarization calculations that are required to interpret the EBIT line ratio measurements.

The current design plans for the International Thermonuclear Experimental Reactor (ITER) call for tungsten to be employed for certain plasma facing components in the divertor region. Thus, accurate atomic collision data are needed for emission modelling of tungsten. Electron-impact excitation and radiative rates are of particular importance for Ni-like W, since this ion emits some of the most intense spectral lines of all ionization stages. We report on a fully relativistic 115-level R-matrix calculations of W 46+ , which includes the effects of radiation damping. Although radiation damping is very important in most highly ionized species, its effects are reduced in this case because of the closed-shell Ni-like ground state. The rates from these relativistic atomic calculations will be employed for collisional-radiative modelling of this ion.

A number of convergent close-coupling and R-matrix with pseudo-state (RMPS) calculations for H-like, He-like, Li-like and Be-like ions have demonstrated that coupling to the target continuum can have large effects on the electron-impact excitation cross sections of neutral and low-charge species. However, no one has yet attempted such advanced calculations on a system as complex as neutral neon. We report on a series of RMPS calculations of electron-impact excitation of Ne using recently developed parallel Breit-Pauli R-matrix programs. Our largest calculation included 235 spectroscopic and pseudo-state levels in the close-coupling expansion of the target. Although the results clearly reveal the importance of coupling to the target continuum in this atom, the pseudo-state expansion is not yet sufficiently complete to provide reliable cross sections for energies above the ionization limit. However, this is the largest intermediate-coupling calculation that can be performed with present computer resources. Thus, we have also carried out a series of RMPS calculations in LS coupling with different pseudo-state expansions. Comparisons of these results have allowed us to determine the approximate size of the pseudo-state expansion required to achieve convergence in future intermediate-coupling calculations for neon.

We present extensive calculations of radiative transition rates and electron impact collision strengths for Fe II. The data sets involve 52 levels from the 3d 7 , 3d 6 4s, and 3d 5 4s 2 configurations. Computations of A-values are carried out with a combination of state-of-the-art multiconfiguration approaches, namely the relativistic Hartree-Fock, Thomas-Fermi-Dirac potential, and Dirac-Fock methods;while the R-matrix plus intermediate coupling frame transformation, Breit-Pauli R-matrix and Dirac R-matrix packages are used to obtain collision strengths. We examine the advantages and shortcomings of each of these methods, and estimate rate uncertainties from the resulting data dispersion. We proceed to construct excitation balance spectral models, and compare the predictions from each data set with observed spectra from various astronomical objects. We are thus able to establish benchmarks in the spectral modeling of [Fe II] emission in the IR and optical regions as well as in the UV Fe II absorption spectra. Finally, we provide diagnostic line ratios and line emissivities for emission spectroscopy as well as column densities for absorption spectroscopy. All atomic data and models are available online and through the AtomPy atomic data curation environment. Subject headings: atomic data -quasars: absorption lines -quasars: individual (QSO 2359-1241) -ISM: individual objects (HH 202) -ISM: individual objects (Orion) -ISM: individual objects (ESO-Hα 574, Par-Lup 3-4) -ISM: jets and outflows -ISM: lines and bands -stars: pre-main sequence reliable scattering calculations. For this work we use a combination of numerical methods: the pseudo-relativistic Hartree-Fock (hfr) code of Cowan (1981); the Multiconfiguration Dirac-Fock (mcdf) code (Dyall et al. 1989), and the scaled Thomas-Fermi-Dirac central-field potential as implemented in autostructure (Badnell 1997). hfr calculationshfr uses a superposition of configurations approach to account for configuration interactions. The code solves the Hartree-Fock equations for each electronic configuration.Relativistic corrections are also included in this set of equations. The radial parts of the multi-electron Hamiltonian can be adjusted empirically to reproduce the spectroscopic energy levels in a least-squares fit procedure. These semi-empirical corrections are used to account for the contributions from higher order correlations in the atomic state functions.The following configurations were explicitly included in the physical model: 3d 6 4s, 3d 7 , 3d 5 4s 2 , 3d 6 5s, 3d 6 4d, 3d 6 5d, 3d 5 4p 2 , 3d 5 4d 2 , 3d 5 4s4d, 3s3p 6 3d 7 4s, 3s3p 6 3d 8 , and 3s3p 6 3d 6 4s 2 . This configuration expansion extends the one used in the previous hfr calculation by Quinet et al. (1996) by including 3d 5 4d 2 and 3s3p 6 3d 6 4s 2 . In order to minimize the discrepancies between computed and experimental energy levels, the hfr technique was used in combination with a well-known least-squares optimization of the radial parameters. The fitting procedure was applied to 3d 6 4s, 3d 7 , ...

Electron-impact excitation collision strengths for transitions among doubly excited levels up to the n = 3 shell (excluding the 1s 3l3l' configurations) of lithium-like argon and iron have been calculated using a radiation- and Auger-damped, intermediate-coupling frame transformation, R-matrix approach. Collision strengths have also been calculated for transitions between all singly excited levels up to the n = 5 shell for the same systems. The Maxwell-averaged effective collision strengths are estimated to be accurate to within 20% at temperatures 5 × 104–5 × 108 K for Ar15+ and 105–109 K for Fe23+. These results are of substantially improved precision compared to previous studies. The data relate to the analysis of soft x-ray helium-like spectra in both astrophysical and fusion thermal plasmas. We summarize the sensitivity to the new data of the spectral simulations which are matched to experiment in current spectral analysis procedures. Also, we present some brief results of modelling using the presented data.