Context. Absorption or emission lines of Cr ii are observed in a wide variety of astrophysical spectra and accurate atomic data are urgently needed to interpret these lines. Many of these data are impossible to measure experimentally and a full theoretical treatment is the only means by which these data can be obtained.Aims. In this paper, we present collision strengths and effective collision strengths for electron-impact excitation of Cr ii for forbidden transitions among the lowest-lying 74 fine-structure levels. Effective collision strengths have been computed for 18 individual electron temperatures of astrophysical importance, ranging from 2000-100 000 K. Methods. The parallel suite of R-matrix packages, RMATRX II, which has recently been extended to allow for the inclusion of relativistic effects, has been used in the present work to compute the collision strengths and effective collision strengths for electronimpact excitation of Cr ii. We concentrate in this publication on low-lying forbidden lines among the lowest 74 jj fine-structure levels with configurations 3d 5 and 3d 4 4s, although atomic data has been evaluated for all 39 060 transitions among the 280 jj levels of configurations 3d 5 , 3d 4 4s and 3d 4 4p. This work constitutes the largest evaluation ever performed for this ion involving 1932 coupled channels.Results. Collision and effective collision strengths are presented for all transitions among the lowest 74 Jπ states of Cr ii and comparisons made with the work of Bautista et al. (2009). While the effective collision strengths agree well for some transitions, significant discrepancies exist for others. We believe that the present atomic data represents the most accurate, most sophisticated and most complete data set for electron-impact excitation of Cr ii and we would recommend them to astrophysicists and plasma physicists in their application work. We would expect that the effective collision strengths presented for the important low-lying forbidden lines are accurate to within 15%.
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 3d 5 and 3d 4 4s even parity configuration states to the 3d 4 4p 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 3d 5 , 3d 4 4s, and 3d 4 4p 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 4d type orbital. The 10 configurations incorporated into the Cr ii model thus listed are 3d 5 , 3d 4 4s, 3d 4 4p, 3d 3 4s 2 , 3d 3 4p 2 , 3d 3 4s4p, 3d 4 4d, 3d 3 4s4d, 3d 3 4p4d, and 3d 3 4d 2 , 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.
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