Electron impact excitation of N³⁺using theB-splineR-matrix method: importance of the target structure description and the size of the close-coupling expansion

Abstract

Abstract.There are major discrepancies between recent ICFT (Intermediate Coupling Frame Transformation) and DARC (Dirac Atomic R-matrix Code) calculations (Fernández-Menchero et al. 2014, Astron. Astroph. 566 A104, Aggarwal et al. 2016 Mon. Not. R. Astr. Soc. 461 3997) regarding electron-impact excitation rates for transitions in several Be-like ions, as well as claims that the DARC calculations are much more accurate and the ICFT results might even be wrong. To identify possible reasons for these discrepancies and to estimate the accuracy of the various results, we carried out independent B-Spline R-Matrix (BSR) calculations for electron-impact excitation of the Be-like ion N 3+ . Our close-coupling expansions contain the same target states (238 levels overall) as the previous ICFT and DARC calculations, but the representation of the target wave functions is completely different. We find close agreement among all calculations for the strong transitions between low-lying states, whereas there remain serious discrepancies for the weak transitions as well as for transitions to highly excited states. The differences in the final results for the collision strengths are mainly due to differences in the structure description, specifically the inclusion of correlation effects, rather than the treatment of relativistic effects or problems with the validity of the three methods to describe the collision. Hence there is no indication that one approach is superior to another, until the convergence of both the target configuration and the close-coupling expansions have been fully established.

Discrepancies in Atomic Data and Suggestions for Their Resolutions

“…It is encouraging to note that some efforts have already been made to resolve discrepancies in Υ data. For example, very recently Fernández-Menchero et al [80] have performed yet another calculation for N IV by adopting the BSR (B-spline R-matrix) method. They have also highlighted a few reasons for the (possible) inaccuracies in the ICFT calculations, particularly for the weak spin changing transitions.…”

“…4 and 5. Like our calculations with DARC, Fernández-Menchero et al [80] have also adopted a large energy range to avoid this possible source of inaccuracy. We hope if similarly large energy range is adopted in the ICFT calculations then the discrepancies in Υ results may not be as striking as noted and demonstrated in this paper.…”

Benchmarking Atomic Data for Astrophysics: Be-like Ions between B ii and Ne vii

“…For example, the excitation energies of Fernández-Menchero et al (2014), Aggarwal et al (2016), andFernández-Menchero et al (2017) for N IV depart from the experimental energies compiled in the Atomic Spectra Database (ASD) of the National Institute of Standards and Technology (NIST) (Kramida et al 2016) by up to 5.8%, 5.7%, and 2.4%, respectively. These uncertainties are too large for identification and deblending of new observations from different space missions.…”

“…The AUTOSTRUCTURE calculations of Fernández-Menchero et al (2014) were performed for the n > 3 levels of Be-like ions with Z ≤ 10. Data for the n > 3 states of N IV were provided by Aggarwal et al (2016) using the GRASP0 code (Grant et al 1980), and by Fernández-Menchero et al (2017) using the B-spline box-based close-coupling method (Zatsarinny & Froese Fischer 2009).…”

“…Among the different studies involving the n > 3 levels the one by Savukov & Markhotok (2006) for Ne VII provides energy data of high accuracy. In comparison, although complete sets of data including transition rates are provided, the studies by Fernández-Menchero et al (2014), for the ions with Z = 5 − 10, and by Aggarwal et al (2016) and Fernández-Menchero et al (2017), for N IV, are quite inaccurate because of limited configuration interaction effects included in the calculations.…”