Inner-shell photodetachment of na-using r-matrix methods

Inner-shell photodetachment of Na-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. © 2020 by the authors

Calculation of Atomic Data for NASA Missions

The interpretation of cosmic spectra relies on a vast sea of atomic data which are not readily obtainable from analytic expressions or simple calculations. Rather, their evaluation typically requires state-of-the-art atomic physics calculations, with the inclusion of weaker effects (spin-orbit and configuration interactions, relaxation, Auger broadening, etc.), to achieve the level of accuracy needed for use by astrophysicists. Our NASA-supported research program is focused on calculating data for three important atomic processes, 1) dielectronic recombination (DR), 2) inner-shell photoabsorption, and 3) fluorescence and Auger decay of inner-shell vacancy states. Some additional details and examples of our recent findings are given below

Photoionization of atomic sodium near threshold

R-matrix with pseudostates (RMPS) calculations have been carried out for photoionization of atomic sodium near threshold. The large RMPS atomic orbital and configuration basis allows for very accurate computations of low-energy photoionization cross sections up to ≈30 eV, the energy range for which the RMPS calculations were optimized. Consistency checks for accuracy include, first, the excellent agreement found between length- and velocity-gauge theoretical results, a necessary but not sufficient requirement for having a converged wave function. A second accuracy quantification is the excellent prediction of the position of the Cooper minimum compared to experimental results. Particular attention is paid to the Cooper minimum occurring just above threshold, and the spin-orbit splitting of minima, resulting in a nonzero total cross section. Our RMPS results away from the minimum are found to be lower than the experimental data, and we make the case that the experimental magnitudes are an overestimate. A third important affirmation of the present accuracy is the continuity found between the 3s→np bound-bound discrete oscillator strength density below threshold - see Wiese et al. [W. L. Wiese, M. W. Smith, and B. M. Miles, Atomic Transition Probabilities, Vol. 2: Sodium Through Calcium; A Critical Data Compilation (US Government Printing Office, Washington, DC, 1969)] - and the 3s→ϵp bound-continuum RMPS oscillator strength density above threshold. These three somewhat independent tests of the accuracy of the computed cross sections add confidence to our recommending the present RMPS results as the most reliable extant data for low-energy Na photoionization (and the earlier Wiese et al. results for the discrete states)

Silicon ISM X-ray absorption: The gaseous component

We present a detailed analysis of the gaseous component of the Si K edge using high-resolution Chandra spectra of low-mass X-ray binaries. We fit the spectra with a modified version of the ISMabs model, including new photoabsorption cross-sections computed for all Si ionic species. We estimate column densities for Si I, Si II, Si III, Si XII, and Si XIII, which trace the warm, intermediate temperature, and hot phases of the Galactic interstellar medium. We find that the ionic fractions of the first two phases are similar. This may be due to the physical state of the plasma determined by the temperature or due to the presence of absorber material in the close vicinity of the sources. Our findings highlight the need for accurate modelling of the gaseous component before attempting to address the solid component. © 2020 Oxford University Press. All rights reserved