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We describe a joint experimental and theoretical investigation on oxygen double photoionization—the emission of two electrons from atomic oxygen following single photon absorption. High-resolution experimental measurements were performed at the Advanced Light Source, revealing sharp resonance structure superimposed on the more familiar Wannier-like, nearly-linear background. These resonance features are attributed to ionization-plus-excitation Feshbach resonances embedded in the double ionization continuum, doubly-excited states that lie above the double-ionization threshold. Such features are absent in the double photoionization cross section of He, or other quasi-two-electron systems, for which the doubly-ionized atomic core remains inert. For a corresponding theoretical analysis, the R-matrix with pseudostates (RMPS) method was invoked by calculating final-state, two-electron resonances-plus-continua wavefunctions and corresponding single-photon absorption cross sections. Overall agreement is found in the direct, background double photoionization cross section. However, the RMPS method, using a small basis due to practical computational limitations, was unable to reproduce quantitatively the smooth background or the sharper resonance features observed in the measurements, showing instead large-scale oscillations about the experimental background, and characteristic pseudoresonance jitter, associated with an insufficient convergence of the pseudostate representation to the true two-electron infinite series of Feshbach resonances embedded in the two-electron continuum. The prominent resonance structure observed highlights the need to consider multiple excitation processes in atoms more complex than He or quasi-two-electron systems.

We describe the atomic database of the XSTAR spectral modeling code, summarizing the systematic upgrades carried out in the past twenty years to enable the modeling of K lines from chemical elements with atomic number $Z\leq 30$ and recent extensions to handle high-density plasmas. Such plasma environments are found, for instance, in the inner region of accretion disks round compact objects (neutron stars and black holes), which emit rich information about the system physical properties. Our intention is to offer a reliable modeling tool to take advantage of the outstanding spectral capabilities of the new generation of X-ray space telescopes (e.g., XRISM and ATHENA) to be launched in the coming years. Data curatorial aspects are discussed and an updated list of reference sources is compiled to improve the database provenance metadata. Two XSTAR spin-offs -- the ISMabs absorption model and the uaDB database -- are also described., Comment: 36 pages, 11 figures

STGF is a community code employed for outer-region R-matrix calculations, describing electron-impact collisional processes. It is widely recognised that the original version of STGF was written by M. J. Seaton in 1983, but through constant refinement over the next decades by worldwide contributors has evolved into its current form that more reflects modern coding practice and current computer architectures. Despite its current wide acceptance, it was never formally published. Therefore, we present an updated high-performance parallel version of PSTGF, that balances the requirements of small university clusters, yet can exploit the computational power of cutting edge supercomputers. There are many improvements over the original STGF, but most noticeably, the full introduction of MQDT options that provide subsequent integration with ICFT (Intermediate Coupling Frame Transformation) codes, and for either Breit-Pauli / DARC (Dirac Atomic R-matrix Codes), better load balancing, high levels of vectorisation and simplified output. Semantically, the program is full fortran 90 in conjunction with MPI (Message Passing Interface) though has CUDA fortran options for the most numerically intensive code sections., Comment: 35 pages, 5 figures, full manual for users

Inner-shell photodetachment of Na&minus, near the L-edge threshold was investigated using the R-matrix method. Significant structure was found in the cross section, and this structure is shown to be related to the complicated correlated electron dynamics endemic in negative ions. Comparison with experiment suggests that the absolute values of the measured cross section might be too small by a factor of two.

The modeling of X-ray spectra from photoionized astrophysical plasmas has been significantly improved due to recent advancements in the theoretical and numerical frameworks, as well as a consolidated and reliable atomic database of inner-shell transitions for all the relevant ions. We discuss these developments and the current state of X-ray spectral modeling in the context of oxygen cold absorption in the interstellar medium (ISM). Unconventionally, we use high-resolution astrophysical observations to accurately determine line positions, and adjust the theoretical models for a comprehensive interpretation of the observed X-ray spectra. This approach has brought to light standing discrepancies in the neutral oxygen absorption-line positions determined from observations and laboratory measurements. We give an overview of our current efforts to devise a definitive model of oxygen photoabsorption that can help to resolve the existing controversy regarding ISM atomic and molecular fractions.

I met Oleg Zatsarinny in 2001, and he then worked with me at Western Michigan University for two years. From 2003 to 2013, we were coauthors of 15 papers on theoretical atomic physics, and maintained a friendly relationship over twenty years, meeting and socializing often at conferences. Further elaboration follows below.

As a part of the investigation of the entire Si isonuclear sequence for x-ray spectral diagnostics, we use the Li-like Si11+ ion, a stable component that thus features prominently in x-ray astrophysical spectra. We perform R-matrix calculations for the photoabsorption cross section of Si11+ including relativistic contributions via the use of a Breit-Pauli Hamiltonian. Relativistic effects — predominantly spin-orbit splitting of resonances and mass-velocity and one-body Darwin shifts of relative energies — are shown to be tractable and worthy of consideration in final x-ray spectral modeling data banks.

Endofullerene photoionization of open-shell N@C60 is investigated using a coupled-channel, correlated R-matrix approach with additional modifications for a finite spherical-shell fullerene potential. Particular focus is on the initial-state dependence of the photoionization cross section. Comparisons show significant differences between the 1s 22s 22p 3(4S) initial state and the metastable 2D and 2P states, as well as strong confinement oscillations.

Synopsis The effect of inner-shell excited channels on outer-shell photodetachment is investigated using a correlated and coupled-channel R-matrix method. The simplest multishell negative ion of Li− is treated near the 1s ionization threshold to determine strong”hole-state” continuum coupling with the direct photodetachment channels.

Synopsis Endofullerene photoionization of open-shell N@C60 is investigated using a coupled-channel, correlated R-matrix approach with additional modifications for a finite spherical-shell fullerene potential. Particular focus is on the initial-state dependence of the photoionization cross section. Comparisons show significant differences between the 1s 22s 22p 3(4S) initial state and the metastable 2D and 2P states, as well as strong confinement oscillatory structure.

We present a study of the C K-edge using high-resolution LETGS Chandra spectra of four novae during their super-soft-source (SSS) phase. We identified absorption lines due to C~ii K$\alpha$, C~iii K$\alpha$ and C~ iii K$\beta$ resonances. We used these astronomical observations to perform a benchmarking of the atomic data, which involves wavelength shifts of the resonances and photoionization cross-sections. We used improved atomic data to estimate the C~ii and C~iii column densities. The absence of physical shifts for the absorption lines, the consistence of the column densities between multiple observations and the high temperature required for the SSS nova atmosphere modeling support our conclusion about an ISM origin of the respective absorption lines. Assuming a collisional ionization equilibrium plasma the maximum temperature derived from the ratio of C~ii/C~iii column densities of the absorbers correspond to $T_{max}< 3.05\times10^{4}$ K., Comment: 7 pages, 7 figures

We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination rate to all levels is preserved. At intermediate densities, on the other hand, collisions can lead to ionization of higher-lying excited states, thereby suppressing the dielectronic recombination rate. The improved suppression factors presented here, although highly approximate, allow summed recombination rate coefficients to be used to intermediate densities. There have been several technical improvements to our previously presented fits. For H- through B-like ions the activation log densities have been adjusted to better reproduce existing data. For B-, C-, Al-, and Si-like ions secondary autoionization is now included. The treatment of density discontinuity in electron excitations out of ground state H-, He-, and Ne-like ions has been improved. These refined dielectronic recombination suppression factors are used in the most recent version of the plasma simulation code Cloudy. We show how the ionization and emission spectrum change when this physics is included. Although these suppression factors improve the treatment of intermediate densities, they are highly approximate and are not a substitution for a complete collisional-radiative model of the ionization balance.

The neutrino mass can be extracted from a high statistics, high resolution calorimetric spectrum of electron capture in ${}^{163}$Ho. In order to better understand the shape of the calorimetric electron capture spectrum, a second isotope was measured with a close to ideal absorber-source configuration. ${}^{193}$Pt was created by irradiating a ${}^{192}$Pt-enriched platinum foil in a nuclear reactor. This Pt-in-Pt absorber was designed to have a nearly ideal absorber-source configuration. The measured ${}^{193}$Pt calorimetric electron-capture spectrum provides an independent check on the corresponding theoretical calculations, which have thus far been compared only for ${}^{163}$Ho. The first experimental and theoretically-calculated spectra from this ${}^{193}$Pt-in-Pt absorber are presented and overlaid for preliminary comparison of theory with experiment., 17th International Workshop on Low Temperature Detectors

Synopsis A detailed investigation of Si-like dielectronic recombination is performed for the entire Si-like isoelectronic sequence using a perturbative multi-configurational Breit-Pauli method.

Inner-shell photodetachment from $\mathrm{N}{\mathrm{i}}^{\ensuremath{-}}([\mathrm{Ar}]\phantom{\rule{0.16em}{0ex}}3{d}^{9}4{s}^{2})$ leading to $\mathrm{N}{\mathrm{i}}^{+},\mathrm{N}{\mathrm{i}}^{2+}$, and $\mathrm{N}{\mathrm{i}}^{3+}$ ion production was studied near and above the $3p$ excitation region, in the 60--90 eV photon energy range, using a merged ion-photon beam technique. The absolute photodetachment cross section of $\mathrm{N}{\mathrm{i}}^{\ensuremath{-}}$ leading to $\mathrm{N}{\mathrm{i}}^{+}$ ion production was measured. The $3p\ensuremath{\rightarrow}3d$ photoexcitation in $\mathrm{N}{\mathrm{i}}^{\ensuremath{-}}$ gives rise to a giant Feshbach resonance. In the near-threshold region, a Fano profile, modified by a Wigner $s$-wave ($l=0$) threshold law, accurately fits the $\mathrm{N}{\mathrm{i}}^{\ensuremath{-}}$ single-photodetachment cross section. A lower-order $R$-matrix calculation shows overall agreement with essential features of the experimental data, confirming the nature of the strong, asymmetric Fano profile of the giant $3p\ensuremath{\rightarrow}3d$ photoexcitation-autodetachment resonance in $\mathrm{N}{\mathrm{i}}^{\ensuremath{-}}$.

Context.We aim to present a comprehensive theoretical investigation of dielectronic recombination (DR) of the silicon-like isoelectronic sequence and provide DR and radiative recombination (RR) data that can be used within a generalized collisional-radiative modelling framework.Aims.Total and final-state level-resolved DR and RR rate coefficients for the ground and metastable initial levels of 16 ions between P+and Zn16+are determined.Methods.We carried out multi-configurational Breit-Pauli DR calculations for silicon-like ions in the independent processes, isolated resonance, distorted wave approximation. Both Δnc= 0 and Δnc= 1 core excitations are included usingLSand intermediate coupling schemes.Results.Results are presented for a selected number of ions and compared to all other existing theoretical and experimental data. The total dielectronic and radiative recombination rate coefficients for the ground state are presented in tabulated form for easy implementation into spectral modelling codes. These data can also be accessed from the Atomic Data and Analysis Structure (ADAS) OPEN-ADAS database. This work is a part of an assembly of a dielectronic recombination database for the modelling of dynamic finite-density plasmas.

Many astrophysical and laboratory plasmas possess Maxwell–Boltzmann (MB) electron energy distributions (EEDs). Interpreting or predicting the properties of these plasmas requires accurate knowledge of atomic processes such as radiative lifetimes, electron impact excitation and de-excitation, electron impact ionization, radiative recombination, dielectronic recombination, and charge transfer, all for thousands of levels or more. Plasma models cannot include all of the needed levels and atomic data. Hence, approximations need to be made to make the models tractable. Here we report on an “analog” technique we have developed for simulating a Maxwellian EED using an electron beam ion trap and review some recent results using this method. A subset of the atomic data needed for modeling Maxwellian plasmas relates to calculating the ionization balance. Accurate fractional abundance calculations for the different ionization stages of the various elements in the plasma are needed to reliably interpret or predict the properties of the gas. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and are often highly suspect. Here we will also review our recent updating of the recommended atomic data for “digital’ computer simulations of MB plasmas in collisional ionization equilibrium (CIE), describe the changes relative to previously recommended CIE calculations, and discuss what further recombination and ionization data are needed to improve this latest set of recommended CIE calculations. PACS Nos.: 34.70.+e,34.80.Dp, 34.80.Kw, 34,80,Lx, 52.50.–j, 52.20.Fs, 52.20.Hv, 52.25.Jm, 52,72.+v, 52.75.–d, 95.30.Dr, 95.30.Ky, 98.38.Bn, 98.58.Bz

As an improvement to an earlier study [T.W. Gorczyca, M.S. Pindzola, F. Robicheaux, N.R. Badnell, Phys. Rev. A 56 (1997) 47421, we have calculated dielectronic recombination rate coefficient spectra for Sc3+ and Ti4+ ions as a test case toward the assembly of a database [http://homepages.wmich.edu/similar to gorczyca/drdata/] for the Ar-like isoelectronic sequence required for modeling of dynamic finite-density plasmas [N.R. Badnell et al., AA S. Schippers et al., Phys. Rev. A 65 (2002) 042723]. In order to compare Maxwellian-averaged rate coefficients, which are of main interest to the astrophysics community, we have developed an iterative deconvolution procedure that enables us to extract the cross-section from storage ring data. After folding the resultant cross-section with a Maxwellian electron velocity distribution, theoretical and experimental rate coefficient spectra agree better than similar to 18% subject to field reionization effects via high Rydberg states in Sc2+ and Ti3+ ions. (c) 2007 Elsevier B.V. All rights reserved.

Dielectronic recombination rate coefficients are computed for the Si-like isoelectronic sequence, focusing on the near-threshold resonances of S2+.

Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and are often highly suspect. This translates directly into the reliability of the collisional ionization equilibrium (CIE) calculations. We make use of state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He up to and including Zn. We also make use of state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H through to Zn. Here we present improved CIE calculations for temperatures from $10^4$ to $10^9$ K using our data and the recommended electron impact ionization data of \citet{Mazz98a} for elements up to and including Ni and Mazzotta (private communication) for Cu and Zn. DR and RR data for ionization stages that have not been updated are also taken from these two additional sources. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. for all elements from H to Ni. The differences in peak fractional abundance are up to 60%. We also compare with the fractional ionic abundances for Mg, Si, S, Ar, Ca, Fe, and Ni derived from the modern DR calculations of \citet{Gu03a,Gu04a} for the H-like through Na-like ions, and the RR calculations of \citet{Gu03b} for the bare through F-like ions. These results are in better agreement with our work, with differences in peak fractional abundance of less than 10%., Comment: 83 pages, 38 figures, 41 tables Accepted to ApJS

Using the five-electron K-shell vacancy 1s2s22p2 configuration as an example, we show that the fluorescence yields of the eight LSJ states of the configuration exhibit a dramatic dependence on LSJ. These results demonstrate that, in general, configuration-average fluorescence data are inappropriate for astrophysical modeling.

Partial and total dielectronic recombination (DR) rate coefficients for fluorine- like ions forming neon-like systems have been calculated as part of the assembly of a final-state level-resolved DR database necessary for the modelling of dynamic finite-density plasmas (Badnell et al. 2003). Calculations have been performed for DR of both ground and metastable initial states for Ne+ to Zn21+, as well as for Kr27+, Mo33+, and Xe45+. Results for a selection of ions are presented and discussed. We find that low-temperature DR, via 2 -> 2 core excitations involving no change in the principal quantum number of the core electron, does not scale smoothly with nuclear charge Z due to resonances straddling the ionization limit of the recombined system, thereby making explicit calculations for each ion necessary. Most of the earlier calculations neglected contributions from the fine-structure 2p(3/2)-2p(1/2) excitation which has been shown to be very important for low-temperature DR coefficients. The DR data are suitable for modelling of solar and cosmic plasmas under conditions of collisional ionization equilibrium, photoionization equilibrium, and non-equilibrium ionization.

Dielectronic recombination (DR) and radiative recombination (RR) data for neon-like ions forming sodium-like systems has been calculated as part of the assembly of a DR database necessary for modelling of dynamic and/or finite-density plasmas (Badnell et al. 2003). Dielectronic recombination coefficients for neon-like ions from Na + to Zn 20+ , as well as Kr 26+ , Mo 32+ , Cd 38+ , and Xe 44+ , are presented and the results discussed.

Recent theoretical and experimental work on the photodetachment of C−, B−, and Li− is reviewed, with emphasis on the dominant shape resonance behavior above the K-shell thresholds. Of particular importance, the interaction between low-energy photoelectrons and Auger electrons is found to deplete the measured yield of positive ions near threshold for Li− (although not for C− and B−). A fully quantum mechanical modification of the R-matrix method is introduced to include the recapture of slow photoelectrons following Auger decay of an inner-shell vacancy state and is applied to the photodetachment of Li− above the 1s−1 threshold. The result show excellent agreement with experiment and resolve a large discrepancy in the cross section near the 1s threshold between theory and experiment.

We have measured dielectronic recombination (DR) resonance strengths and energies for carbon-like Fe XXI forming Fe XX and for boron-like Fe XXII forming Fe XXI via N = 2 → N' = 2 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, state-of-the-art perturbative techniques: a multiconfiguration Breit-Pauli (MCBP) method using the code AUTOSTRUCTURE, a multiconfiguration Dirac-Fock (MCDF) method, and a relativistic configuration interaction method using the Flexible Atomic Code (FAC). Overall, reasonable agreement is found between our experimental results and our theoretical calculations. The most notable discrepancies tend to occur for relative collision energies 3 eV. We have used our measured 2 → 2 results to produce Maxwellian-averaged rate coefficients for Fe XXI and Fe XXII. Our experimentally derived rate coefficients are estimated to be accurate to better than ≈20% both for Fe XXI at kBTe > 0.5 eV and for Fe XXII at kBTe > 0.001 eV. For these results, we provide fits that are accurate to better than 0.5% for Fe XXI at 0.001 eV ≤ kBTe ≤ 10,000 eV and for Fe XXII at 0.02 eV kBTe ≤ 10,000 eV. Our fitted rate coefficients are suitable for ionization balance calculations involving Fe XXI and Fe XXII in photoionized plasmas. Previous published Burgess formula and LS-coupling calculations are in poor agreement with our experimentally derived rate coefficients. None of these published calculations reliably reproduce the magnitude or temperature dependence of our experimental results. Our previously published Fe XXI MCDF results are in good agreement with our experimental results for kBTe 0.07 eV. For both ions in this temperature range our new MCBP, MCDF, and FAC results are in excellent agreement with our experimentally derived rate coefficient.

Many new resonances in neon have been discovered by virtue of their LS-forbidden mirroring partial cross section profiles. In particular, all six of the previously undetected 2s22p4(3P)3s([2,4]P)3p(3[S, P, D] 1) resonances have been observed. These results confirm our earlier prediction (Canton-Rogan et al 2000 Phys. Rev. Lett. 85 3113) that mirroring triplet resonances are observable in partial cross sections where continuum spin–orbit effects exist, and suggest that future experimental studies of other atoms will also reveal new resonances.

Recent developments in quasi-free electron scattering from highly charged ions are reported. Results using a high-efficiency zero-degree Auger projectile spectroscopy setup are presented primarily for collisions of H- and He-like boron beams with H2 targets. A novel technique for the in situ control and determination of the metastable 1s 2s 3S beam fraction of He-like boron allowed for an accurate determination of absolute double differential scattering cross sections for 180° resonant and non-resonant electron scattering from both ground-(1s2) and mixed-(1s2, 1s 2s 3S) B3+ states. The observed richly structured B2+ (1s 2l nl′) Rydberg series is produced by resonant excitation (RE) from the 1s2 ground state, while the triply excited B2+ (2s 2l′ 2l′′) hollow ionic states are produced by RE from the 1s 2s 3S metastable state. The consistency of the method was checked by quasi-free electron scattering from H-like B4+ ions giving rise to the 2l nl′ Rydberg series. Differential R-matrix calculations within the electron scattering model for electron–ion scattering from both H-like and ground state He-like boron were found to be in excellent overall agreement. The production of triply excited (2s 2l′ 2l′′) states by RE was also studied for the Z=5–9 isoelectronic sequence. These results convincingly demonstrate that large-angle differential scattering cross section measurements, practically impossible to perform with present day crossed or merged electron–ion beam techniques, are amenable to measurement through high resolution electron spectroscopy of ion–atom collisions, providing some of the most sensitive tests of atomic structure and electron–ion scattering calculations.

Photodetachment cross sections of the He− 1s2s2p 4Po metastable state in the region of the 1s threshold (38–52 eV) have been calculated using a modified R-matrix method based on a B-spline representation of the scattering orbitals. An accurate representation of initial state and target state wavefunctions has been obtained using a basis of non-orthogonal orbitals. The 17 bound (1s2l, 1s3l) and autoionizing (2l2l', 2l3l') states of He have been included in the close-coupling (CC) expansion. The convergence of the CC expansion has been checked by inclusion of additional 1s4l, 1s5l and 3l3l' target states. Close agreement was found with recent high-resolution K-shell photodetachment measurements of He− giving rise to He+ ions (Berrah N, Bozek J D, Turri G, Akerman G, Rude B, Zhou H L and Manson S T 2002 Phys. Rev. Lett. 88 93 001), except for the threshold maximum above the first 1s detachment threshold 2s2p 3Po at 38.88 eV, where the theoretical cross section is a factor of two larger than experiment. Our results show 1s photodetachment cross sections with numerous structures which have been analysed in detail. A set of triply excited resonances is also found, and their energy positions, widths and decay branching ratios are presented.

We have measured resonance strengths and energies for dielectronic recombination (DR) of Fe XIX forming Fe XVIII via N = 2 → N' = 2 and N = 2 → N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using two independent, state-of-the-art techniques: the perturbative multiconfiguration Breit-Pauli (MCBP) and multiconfiguration Dirac-Fock (MCDF) methods. Overall, reasonable agreement is found between our experimental results and theoretical calculations. The most notable discrepancies are for the 3l3l' resonances. The calculated MCBP and MCDF resonance strengths for the n = 3 complex lie, respectively, ≈47% and ≈31% above the measured values. These discrepancies are larger than the estimated 20% total experimental uncertainty in our measurements. We have used our measured 2 → 2 and 2 → 3 results to produce a Maxwellian-averaged rate coefficient for DR of Fe XIX. Our experimentally derived rate coefficient is estimated to be good to better than ≈20% for kBTe ≥ 1 eV. Fe XIX is predicted to form in photoionized and collisionally ionized cosmic plasmas at kBTe 1 eV. Hence, our rate coefficient is suitable for use in ionization balance calculations of these plasmas. Previously published theoretical DR rate coefficients are in poor agreement with our experimental results. None of these published calculations reliably reproduce the magnitude or temperature dependence of the experimentally derived rate coefficient. Our MCBP and MCDF results agree with our experimental rate coefficient to within ≈20%.

The triplet character of doubly excited 2lnl' resonances close to the He+ (N = 2) ionization threshold has been seen to affect the angular distribution of photoelectrons associated with the He+ (1s) state: a peak has been observed at a photon energy just below the N = 2 threshold when the photoelectrons are detected at 90° with respect to the photon polarization direction. This is forbidden in pure LS-coupling and therefore the observation immediately indicates the presence of triplet configurations due to spin–orbit interactions. Such weak interaction effects are usually overwhelmed by LS-allowed processes, particularly in such a low-Z atom, but the choice of observation angle in this case desensitizes the measurements to the dominant ionization processes. The results have been reproduced by R-matrix MQDT calculations which also indicate that all seven relativistically allowed Rydberg series are present.

We have measured the resonance strengths and energies for dielectronic recombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) core excitations. We have also calculated the DR resonance strengths and energies using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different state-of-the-art theoretical techniques. On average the theoretical resonance strengths agree to within ~30% which is significantly larger than the estimated relative experimental uncertainty of ~1 eV, which includes the predicted formation temperatures for Fe XX in an optically thin, low-density photoionized plasma with cosmic abundances, our experimental and theoretical results are in good agreement. We have also used our R-matrix results, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculate the rate coefficient for RR of Fe XX. Our RR results are in good agreement with previously published calculations.

Chandra high-resolution spectra toward eight low-mass Galactic binaries have been analyzed with a photoionization model that is capable of determining the physical state of the interstellar medium. Particular attention is given to the accuracy of the atomic data. Hydrogen column densities are derived with a broadband fit that takes into account pileup effects, and in general are in good agreement with previous results. The dominant features in the oxygen-edge region are O I and O II K$\alpha$ absorption lines whose simultaneous fits lead to average values of the ionization parameter of $\log\xi=-2.90$ and oxygen abundance of $A_{\rm O}=0.70$. The latter is relative to the standard by Grevesse & Sauval (1998), but a rescaling with the revision by Asplund et al. (2009) would lead to an average abundance value fairly close to solar. The low average oxygen column density ($N_{\rm O}=9.2 \times 10^{17}$ cm$^{-2}$) suggests a correlation with the low ionization parameters, the latter also being in evidence in the column density ratios OII/OI and OIII/OI that are estimated to be less than 0.1. We do not find conclusive evidence for absorption by any other compound but atomic oxygen., Comment: 32 pages, 7 figures, Accepted for publication in the Astrophysical Journal

Photoionization of atomic oxygen near the 1s-1 threshold is reinvestigated by including a more extensive inner-shell configuration interaction description, and by using an optical potential to account for autoionization to the infinite number of singly excited O+ continua. These new theoretical results remove the earlier discrepancies between theory and experiment.

We address the validity and practicality of using multi-channel quantum defect theory (MQDT) to treat electron-ion collision problems for cases where some channel solutions propagate with extremely negative, classically forbidden, energies. We discuss theoretically and demonstrate numerically, within an unrealistically sensitive model-problem study, how MQDT results can be obtained which are in agreement with those obtained from the more standard non-MQDT method at all energies.

We demonstrate that much simpler techniques can reproduce the results of the full Breit-Pauli R-matrix method to include relativistic effects in photoionization calculations. To allow for the fine-structure splitting of channels in the photoionization of ${\mathrm{Fe}}^{16+},$ we have performed three sets of calculations. The first combined an $\mathrm{LS}$ R-matrix calculation with an $\mathrm{LS}\ensuremath{-}\mathrm{JK}$ frame transformation, using multichannel quantum defect theory. The second used a relativistic random phase approximation based on the Dirac equation. Both methods give resonant photoionization results nearly identical to those from a third calculation using the full Breit-Pauli R-matrix method. An accurate treatment of fine-structure splitting in ${\mathrm{Fe}}^{16+}$ is necessary to realistically include the ${2p}^{5}{(}^{2}{P}_{1/2})nl$ resonances which dominate the low-energy photoionization cross section; consequently, in the inverse process of photorecombination, the low-temperature rate coefficient is dominated by the ${2p}^{5}{(}^{2}{P}_{1/2})nl$ dielectronic recombination resonances.

Using multichannel quantum-defect theoretical methods, we have performed highly correlated R-matrix calculations for the photoionization excitation of neon in the (50{endash}53)-eV photon energy range, where numerous doubly excited resonances are found. By incorporating spin-orbit effects through an intermediate-coupling frame transformation, we have reproduced experimental observance of the significant breakdown of LS coupling in resonant angular distributions and fine-structure partial cross sections. Extreme sensitivity to the target structure in this weak process and strong interactions between neighboring Rydberg series of resonances make a detailed comparison for every resonance difficult, but many of the interesting spin-orbit features, beyond fine-structure splitting of series, are revealed. This method permits a quantitative analysis of LS-forbidden contributions to complex photoionization spectra and should prove useful for studying spin-orbit effects in light atomic systems. {copyright} {ital 1998} {ital The American Physical Society}

We present an experimental and theoretical study of photoexcitation and decay mechanisms of doubly excited resonances in neon. New spin-orbit induced effects are identified in the angle-resolved and photoelectron-resolved differential cross sections for a parity-unfavored transition. These observations are facilitated using a two-dimensional imaging technique, and highlight the importance of considering spin-orbit induced effects when studying detailed spectra in atomic systems as light as neon. {copyright} {ital 1998} {ital The American Physical Society}

We have implemented a multi-channel quantum defect theory approach within the asymptotic R-matrix codes (STGF and STGBF) which treats all closed channels as open. Furthermore, making use of this approach, we have implemented an analytic preconvolution of undamped R-matrix photoionization and photorecombination cross sections. This removes any doubt whatsoever as to whether the resonances are fully resolved or not. We show that fully resolved undamped R-matrix photorecombination cross sections are in very close agreement with undamped perturbative cross sections. We demonstrate, with explicit examples, the large and widespread effect of radiation damping on low-lying photorecombination resonances for H-like through Ni-like ions. This refutes the recent claims made by Pradhan and Zhang and validates the original work of Robicheaux et al and Gorczyca and Badnell with regard to the importance of radiation damping.

The ${e}^{\ensuremath{-}}+{\mathrm{Sc}}^{3+}$ recombination cross section exhibits a strong interference between the radiative and dielectronic recombination pathways at low energies. The resonances associated with the ${3p}^{5}{3d}^{22}F$ term in Sc${}^{2+}$ are found to have unusually large autoionization widths. Their strong interference with the background radiative recombination results in quite distinct asymmetric resonance profiles. Current ion storage ring experiments should be able to verify the strength of the interference effects.

We use the radiation damped Breit-Pauli R -matrix method that was introduced by Robicheaux {ital etthinspthinspal.}thinspthinsp[Phys.thinspthinspRev.thinspthinspA {bold 52}, 1319 (1995)] to study photorecombination of Fe{sup 24+} . Results are compared to perturbative and experimental results, and agreement is excellent in all cases. It is found that the inclusion of radiation damping is extremely important for this system, whereas resonance interference effects are negligible. These findings are in complete contrast to those of a recent study by Zhang and Pradhan [Phys.thinspthinspRev.thinspthinspLett.thinspthinsp{bold 78}, 195 (1997)]. {copyright} {ital 1997} {ital The American Physical Society}

We have developed an alternative R-matrix with pseudostates (RMPS) method for incorporating a two-electron continuum description into the wavefunction of electron - ion collision calculations. This method is similar in spirit to various recent treatments of the pseudostate expansion, most notably the R-matrix approaches of Meyer and Greene, and Bartschat et al. Our approach differs in that we: (1) utilize a direct sum of several bases: the physical target orbitals, additional MCHF pseudo-orbitals for the He ground state, R-matrix continuum orbitals, and, on occasion, Laguerre orbitals, and (2) rely on a different approach for the creation of an orthonormal basis. Since we use the Belfast codes that are based on a Wigner - Eisenbud R-matrix treatment, we also need to introduce a modified Buttle correction. The method is first tested on the s-wave (Temkin - Poet) model for electron-impact excitation and ionization of . Then it is applied to the calculation of cross sections and angular distributions for the photoionization - excitation process (n = 2,3). We find that these results are greatly improved by the inclusion of the two-electron continuum description, which is more important for the ground state, but plays a role in the final photoionized states as well. A highly correlated multiconfiguration Hartree - Fock expansion is used to represent the ground state, and from the configuration interaction coefficients, asymptotic ratios are determined and compared with other theoretical results. Results are also compared with recent high-energy measurements for the n = 2,3 cross section ratios and angular distribution parameters. This pseudostate expansion allows us to predict photo - double-ionization cross sections, . Of particular significance, we find good agreement between our length and velocity gauge results, indicating that our ground-state correlation is sufficiently converged for the present system.

Resonant transfer excitation (RTE) involving L{sup {minus}1}Mn (n{ge}M) resonant states has been investigated for Na-like and Ne-like Cu{sup q+}+H{sub 2} collisions (q=18 and 19). The M- to L-shell x-ray production cross sections (RTEX{close_quote}s) of these resonance states are studied by x-ray projectile ion coincidences. Previous measurements of L-shell RTEX for Nb{sup q+} (q=28{endash}31) ions showed the measured cross sections to be nearly a factor of 2 smaller than the calculated ones. For Cu{sup 18+} the present results show the position and width of the measured RTEX maximum cross section to be in agreement with the calculations; however, the measured absolute cross sections are about 60{percent} higher than the predicted ones. In the case of Ne-like Cu{sup 19+} projectiles, the metastable component in the beam made it impossible to observe RTEX{close_quote}s. {copyright} {ital 1997} {ital The American Physical Society}