ELECTRON-IMPACT EXCITATION OF Cr II: A THEORETICAL CALCULATION OF EFFECTIVE COLLISION STRENGTHS FOR OPTICALLY ALLOWED TRANSITIONS

Abstract

In this paper, we present electron-impact excitation collision strengths and Maxwellian averaged effective collision strengths for the complicated iron-peak ion Cr ii. We consider specifically the allowed lines for transitions from the 3d5 and 3d44s even parity configuration states to the 3d44p odd parity configuration levels. The parallel suite of R-Matrix packages, RMATRX II, which have recently been extended to allow for the inclusion of relativistic effects, were used to compute the collision cross sections. A total of 108 LSπ/280 Jπ levels from the basis configurations 3d5, 3d44s, and 3d44p were included in the wavefunction representation of the target including all doublet, quartet, and sextet terms. Configuration interaction and correlation effects were carefully considered by the inclusion of seven more configurations and a pseudo-corrector type orbital. The 10 configurations incorporated into the Cr ii model thus listed are 3d5, 3d44s, 3d44p, 3d34s2, 3d34p2, 3d34s4p, , , , and , constituting the largest Cr ii target model considered to date in a scattering calculation. The Maxwellian averaged effective collision strengths are computed for a wide range of electron temperatures 2000–100,000 K which are astrophysically significant. Care has been taken to ensure that the partial wave contributions to the collision strengths for these allowed lines have converged with "top-up" from the Burgess–Tully sum rule incorporated. Comparisons are made with the results of Bautista et al. and significant differences are found for some of the optically allowed lines considered.