Breit–Pauli R-matrixcalculations for electron impact excitation of Fe XVII: a benchmark study

Abstract

We have studied low-energy electron impact excitation of Ne-like Fe16+with emphasis on relativistic and resonance effects using a Breit–PauliR-matrixcode. Processes with one electron excited into the M shell, i.e. involvingthe closed-shell ground state and 36 more fine-structure levels (37CC),are compared with those from a target extended to the N shell(n = 4,89CC—CC indicating a close coupling approach to a partial wave expansion). Incontrast to previous work the 37CC and the 89CC collision strengths exhibit farricher structure. All N-shell levels give rise to Rydberg resonant states dippingright to all M-shell thresholds. This will modify all 37CC collision strengthsdown to every excitation threshold, not merely over the extended targetenergy range, thus significantly affecting the collision strengths for theprimary x-ray and EUV transitions among the first 37 levels. Extensivestudy of other effects on the collision strength is also reported: (i) electricand magnetic multipole type transitions E1, E2, E3 and M1, M2, (ii)J-partialwave convergence of dipole and non-dipole transitions, (iii) high-energy behaviourcompared to other approximations. Theoretical results are benchmarked againstexperiment to resolve long-standing discrepancies—collision strengths for thethree prominent x-ray lines 3C, 3D and 3E at 15.015, 15.262 and 15.450 Å are ingood agreement with two independent measurements on electron beam ion traps(EBITs). Finally, line ratios from a collisional–radiative model using the newcollisional rates are compared with observations from stellar coronae and EBITsto illustrate potential applications in laboratory and astrophysical plasmas.