Electron impact excitation rate coefficients for P-like Ni XIV

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Abstract

We have calculated the atomic data including electron impact excitations and radiative decays among the lowest 143 fine-structure levels arising from 3s23p3, 3s3p4, 3s23p23d, 3p5, 3s3p33d, and 3s23p3d2 configurations in P-like Ni XIV. Direct excitation collision strengths are calculated employing the relativistic distorted-wave method. Resonances are included via the isolated resonance approximation using distorted-waves. Resonance contributions from S-like [3s23p3, 3s3p4, 3s23p23d,3p5, 3s3p33d,3s23p3d2, 3p43d,3s3p23d2,3s23d3]nl complex series are taken into account. Effective collision strengths are reported over an electron temperature range of 1.0×105–1.0×108 K.

Highlights

► Radiative and collisional atomic data are presented for the lowest 143 fine-structure levels in P-like Ni XIV. ► Calculations are performed using the FAC package. ► Resonances enhance significantly a large amount of transitions. ► Resonances play an important role of level population and line intensity ratios.

Introduction

Iron group elements (Sc–Zn) are becoming increasingly important in the study of astrophysical plasmas, as many of their emission lines are frequently observed from different ionization stages [1]. These observations provide a wealth of data about the plasma characteristics, such as temperature, density, and chemical composition. Additionally, iron group elements as impurities in fusion reactors are often used for diagnostic purposes [2], [3], [4], [5], [6]. Atomic data (namely energy levels, oscillator strengths or radiative decay rates, and excitation rates) are required for many ions.

Landi and Bhatia [7] have recently performed a distorted-wave (DW) calculation on electron impact excitation (EIE) in Ni XIV, employing the widely used Flexible Atomic Code (FAC) [8], [9]. They have provided a complete set of level energies, oscillator strengths (f), radiative transition rates (Ar), and collision strengths (Ω) for the lowest 143 fine-structure levels arising from 3s23p3, 3s3p4, 3s23p23d, 3p5, 3s3p33d, and 3s23p3d2 configurations. Interactions among all the configurations belonging to the n=3 complex have been considered. These results represent the first complete and self-consistent dataset for the calculation of the Ni XIV line intensities published in the literature. Unfortunately, resonance contributions were not included in [7]. The previous works for other P-like ions, such as K V [10], Ca VI [11] and Fe XII [12], [13], [14], have shown that resonances enhance the effective collision strengths (Υ) by up to an order of magnitude (or even more), depending upon the transition and/or electron temperature. Landi and Bhatia [7] pointed out that the omission of resonance contributions constituted the main limitation of their results and highly recommended that further calculations with the inclusion of resonance contributions could be carried out.

Recently, we performed the preliminary calculations on Υ with resonance contributions for P-like Ni XIV [15], also employing the FAC code. Both the Dirac R-matrix (DRM) theory [16], [17] and the relativistic distorted-wave method in conjunction with independent process and isolated resonance approximation (denoted the IPIRDW approximation) [18] were adopted for the calculations. A rather small target including only the lowest 41 levels arising from the 3s23p3, 3s3p4 and 3s23p23d configurations is involved. It is found that resonances enlarge significantly the effective collision strengths for many transitions, even by over two orders of magnitude. Resonances also have large effects on the spectral line intensity, changing significantly the line-intensity ratios (for example, see Figure 8 in [15]). The detailed comparisons between the DRM and IPIRDW results show that the IPIRDW approximation can describe the near threshold resonances reasonably well for most transitions in Ni XIV [15]. The channel-coupling effects are found to be important for weak transitions, such as the two-electron excitations in 3s3p4–3s23p23d transition array.

However, there is still large limitation in the work presented in [15]. The target included in [15] is too small to include the strong core-mixings of 3s2+3p2+3s3d+3d2. In many studies for M-shell ions with two or more 3l electrons [14], [19], [20], [21], [22], [23], [24], [25], [26], it has been shown that these mixings are critical for the determination of reliable energy levels and the values of Ar, Ω and Υ. Hence, the work in [15] can be improved significantly by including more configurations (especially those in the 3l5 complex) in both the configuration-interaction (CI) expansion of the target and the subsequent scattering calculations. To do this, here we perform a more extensive calculation. As done in [7], all the configurations in the 3l5 complex are included in the target description. We also provide atomic data for the lowest 143 fine-structure levels arising from 3s23p3, 3s3p4, 3s23p23d, 3p5, 3s3p33d, and 3s23p3d2 configurations of P-like Ni XIV. Electron impact direct excitation (DE) is dealt with relativistic distorted-wave (RDW) approximation. Resonance excitation (RE) is treated using the IPIRDW approximation rather than the DRM theory due to the limitation of computation resource. Contributions from the resonances attached to the levels in the above six configurations as well as those in the 3p43d, 3s3p23d2, and 3s23d3 configurations are included. Comparing to the previously reported works [7], [15], the atomic data presented here should be more accurate.

Section snippets

Calculation procedure, tables and discussions

The latest available version of FAC code [8], [9], namely FAC 1.1.1, is throughout used in the present work. FAC is a fully relativistic program computing both structure and scattering data. It uses a modified Dirac–Fock–Slater central-field potential which includes an approximate treatment of the exchange interaction. The orbitals are optimized in a self-consistent-field (SCF) iterative procedure during which the average energy of a fictitious mean configuration with fractional orbital

Conclusions

In conclusion, we have calculated the collision strengths for electron impact excitations among the lowest 143 levels arising from 3s23p3, 3s3p4, 3s23p23d, 3p5, 3s3p33d, and 3s23p3d2 configurations in Ni XIV. Effective collision strengths are reported over electron temperature range of 1.0×105–1.0×108 K. The relativistic distorted-wave method in conjunction with isolated resonance approximation are employed. Comparing to the previously reported works, we present here one more comprehensive

Acknowledgments

The authors would like to thank an anonymous referee for suggestions that improved the manuscript significantly. This work is supported by NSAF under Grant number 11076009 and by the Chinese Association of Atomic and Molecular Data. It is also partially supported by the Chinese National Fusion Project for ITER under Grant number 2009GB106001, and by the Shanghai Leading Academic Discipline Project under Grant number B107. One of the authors (JLZ) gratefully acknowledges the support from the

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