Issue 
A&A
Volume 518, JulyAugust 2010
Herschel: the first science highlights



Article Number  A64  
Number of page(s)  20  
Section  Atomic, molecular, and nuclear data  
DOI  https://doi.org/10.1051/00046361/201014170  
Published online  03 September 2010 
Rmatrix electronimpact excitation data for the Nelike isoelectronic sequence^{}
G. Y. Liang  N. R. Badnell
Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK
Received 1 February 2010 / Accepted 7 May 2010
Abstract
We present results for the electronimpact excitation
of all Nelike ions from Na^{+} to Kr^{26+} obtained using the
intermediatecoupling frame transformation Rmatrix approach.
For each ion's calculation, the closecoupling expansion is taken
to be the 113 LS terms (209 levels) belonging to the
configurations
,
,
(
,
and
), and
(
,
and
). An additional configuration interaction effect arising
from configurations of
(
and )
was included in the target
expansion. A detailed comparison of the target structure has been
made for six specific ions (Si^{4+}, Ar^{8+}, Ca^{10+},
Fe^{16+}, Ni^{18+}, and Kr^{26+}) spanning the sequence to
assess the accuracy for the entire sequence. Effective collision
strengths (s) are presented at temperatures ranging from
K to
K (where q is the
residual charge of ions, i.e. Z10). Detailed comparisons for
the s are made with the results of previous calculations
for several ions, which span the sequence. Furthermore, we examine
the isoelectronic trends of effective collision strengths as a
function of temperature. The present results are the only
Rmatrix ones for the majority of the ions and the most
extensive and complete data for modelling todate.
Key words: atomic data  atomic processes  plasmas
1 Introduction
Because Nelike ions have a stable closed Lshell ground state, they show high abundance over a wide range of temperatures in ionization equilibrium for each isonuclear sequence (see Mazzotta et al. 1998; Bryans et al. 2009,2006). Thus, they attract extensive studies for spectral diagnostic and modelling in astrophysical and laboratory plasmas, and in particular iron, due to its high cosmic abundance. Xray lasers ( Mathews et al. 1985; Tomasel et al. 1997) based on Nelike ions are another significant area of interest. However, the atomic structure and electronimpact excitation (EIE) of Nelike ions are extremely complex, which results in there being large uncertainties in line intensity ratios ( vs , this is usually designated 3C/3D, as well as vs. 3C) between measurements or observations and predictions ( Beiersdorfer et al. 2002,2001; Gu et al. 2004). For example, even for iron, EIE of this ion has been investigated experimentally and theoretically for a long time ( Smith et al. 1985; Chen et al. 2003; Loch et al. 2006; Beiersdorfer et al. 2002,2001 and references therein).
Resonances in electronion impact excitation have been observed in laboratory measurements ( Brown et al. 2006). They play an important role in the spectral diagnostic and modelling of astrophysical and laboratory plasmas. The closecoupling (CC) approximation (e.g. Rmatrix, Hummer et al. 1993) satisfactorily reproduces and describes such resonances. Recently, there have been several works using this method for three ions in this isoelectronic sequence. Chen et al. (2003) performed (BPRM) BreitPauli Rmatrix ( Berrington et al. 1995) calculations for Fe^{16+} with an 89 finestructure level closecoupling expansion (to n=4). Loch et al. (2006) performed a fullyrelativistic larger scale Fe^{16+} calculation (139 finestructure levels, including an additional 50 levels of the configurations) using the Dirac atomic Rmatrix code ( DARC, Norrington & Grant 1987). Collisionalradiative (CR) modelling with their updated excitation data was also undertaken ( Chen 2008; Loch et al. 2006), the combination of which gives satisfactory agreement between measurements/observations and theory for the 3C/3D line ratio. A benchmark work performed by Del Zanna & Ishikawa (2009) revealed the data of Loch et al. (2006) to be reliable. Similar differences for other (noniron) ions in this isoelectronic sequence have been observed between measurements ( Beiersdorfer et al. 2001) and theoretical predictions based upon distortedwave (DW) excitation data. By making a semiempirical configurationinteraction (CI) correction to excitation data and taking CR effects into account, Fournier & Hansen (2005) brought the predictions into agreement with measurements for Nelike ions from Cr^{14+} to Ag^{37+}. This confirms again that accurate atomic data is essential for the reliable diagnostic modelling of astrophysical and laboratory plasmas. However, most excitation data in this isoelectronic sequence are from the DW approximation ( Zhang et al. 1987; Bhatia et al. 1985), except for Rmatrix calculations for three ions, viz Fe^{16+} (BPRM and DARC, as noted above), Ni^{18+}, and Kr^{26+} (both DARC). For Ni^{18+}, Aggarwal & Keenan (2008) performed an 89level CC (n=4, , , and ) calculation and Chen et al. (2006) a 125level ( , , and ) CC calculation. For Kr^{26+}, Griffin et al. (2008) used a 139level (n=5, , , ) CC expansion; they also demonstrated that the radiative damping of resonance contributions is a small effect.
Due to the advantage of high accuracy  see Griffin et al. (1998), Badnell & Griffin (1999), Berrington et al. (2005) and Liang et al. (2008)  and computational efficiency of the intermediatecoupling frame transformation (ICFT) Rmatrix methodology and associated codes, along with the high capability of parallel computer clusters, it is now feasible to provide excitation data for isoelectronic sequences across the entire range of astrophysical interest within the Rmatrix framework. Witthoeft et al. (2007) investigated the physics of electronimpact excitation along the Flike isoelectronic sequence (Ne^{+} to Kr^{27+}) and Liang et al. (2009a,b) also did an entire sequence calculation for Nalike ions (for both outer and innershell excitations) with Auger and radiativedamping included for the innershell excitations. Based upon the robustness of the current suite of Rmatrix codes, the Rmatrix calculation of effective collision strengths () currently can be performed automatically for each ion without manual intervention along an isoelectronic sequence after sufficiently accurate radial wave functions have been obtained and CI/CC expansions have been confirmed. This ensures that each calculation is performed uniformly and reliably, as well as that the calculation along the sequence is consistent. Careful analysis of the results for several specified ions spanning the sequence is still necessary so as to further validate the accuracy of the data along the sequence.
In this paper, we report on the electronimpact excitation of the Nelike isoelectronic sequence (from Na^{+} to Kr^{26+}), via the ICFT Rmatrix approach. In Sect. 2, we discuss details of the calculation method and pay particular attention on comparing our underlying atomic structure with previous results. The excitation results themselves are discussed in Sect. 3. Our work is a part of ongoing collaborative work  the UK Atomic Processes for Astrophysical Plasmas (APAP) network^{}, a broadening of scope of the original UK RmaX network.
2 Sequence calculation
The aim of this work is to perform Rmatrix calculations employing the ICFT method (see Griffin et al. 1998) for all Nelike ions from Na^{+} to Kr^{26+}. In our calculations we included the following 31 configuration basis set in our closecoupling expansion: , , ( and ) and ( and ), and an additional 33 correlation configuration  (l and and ) in our CI expansion. This results in 113 closecoupling LS terms and 209 finestructure levels. The CI expansion consists of 1337 LS terms and 2775 finestructure levels, which were determined to be important for improving the accuracy of the energy levels which we included in the closecoupling expansion.
2.1 Structure: energies
The target wave functions (1s7d) were obtained from AUTOSTRUCTURE (AS, Badnell 1986) using the ThomasFemiDiracAmaldi model potential. Relativistic effects were included perturbatively via the onebody BreitPauli operator (viz. massvelocity, spinorbit and Darwin) without valence electron twobody finestructure operators. This is consistent with the operators included in the standard BreitPauli Rmatrix suite of codes. The radial scaling parameters, (n= 2 and 3; and ), were obtained separately for each ion by a twostep optimization procedure with . In the first step, the energy of the ground level was minimized by varying the and scaling parameters. Then, the averageenergy of the finestructure levels of the 14 terms of the configuration was minimized by varying the scaling parameters. This optimization procedure was found to be the best common one that could be used for all ions over the sequence. Optimizing the nl (n=4,5,6 and 7) orbitals was found to give only a small improvement of the target level energies for several specified ions (Si^{4+}, Fe^{16+} and Kr^{26+}) spanning the sequence. In order to maintain consistency and so as not to introduce arbitrary changes along the sequence, the optimization procedure is done automatically in AUTOSTRUCTURE without any manual readjustment. The resultant scaling parameters are listed in Table 1.
Table 1: Radial scaling factors used in AUTOSTRUCTURE to minimize the total energies of (2s and 2p orbitals) and (3l orbitals) complexes, respectively  see text for details.
Table 2: The level energies (Ryd) of Si^{4+} from different calculations along with the compilation of NIST v3^{2}.
A comparison of level energies with previous calculations and data, derived semiempirically from experimental energies, available from the compilation of NIST v3^{} or observed values available in the CHIANTI v6 database and astrophysical modelling code ( Dere et al. 2009) was made for several specific ions (Si^{4+}, Ar^{8+}, Ca^{10+}, Fe^{16+}, Ni^{18+} and Kr^{26+}) spanning the sequence so as to assess the accuracy of our present AS calculations over the entire isoelectronic series. Tables 2, 4, 6, 8, 10, and 12 list various theoretical level energies along with NIST (v3) derived or CHIANTI (v6) observed ones for the 60 lowestlying levels. A fully relativistic calculation with the Flexible Atomic Code (FAC, Gu 2008) was also performed for these ions with only CI from included besides that of the CC configurations. This is because a correction of level energies has been carriedout by using the difference of average configuration energy obtained using a different orbital basis for each configuration and that obtained using the unique orbital basis required for multiconfiguration level structure  see Gu (2008) and the FAC manual for details. Such a procedure is not readily usable in an Rmatrix calculation. A complete comparison with available NIST experimentally derived or CHIANTI observed data is shown in Fig. 1. A complete set of level energies from the present AS calculation is available electronically^{}. Figure 1 and Tables 2, 4, 6, 8, 10, and 12 show that excellent agreement (within 0.5%) is obtained when compared with NIST derived or CHIANTI observed data except for a few energy levels. Moreover, better agreement (0.3%) is obtained for Si^{4+}, Ar^{8+}, Fe^{16+}, Ni^{18+}, etc.
For Si^{4+}, the results of Bhatia et al. (1985) currently used by the CHIANTI are lower than the NIST data by 1.5%2.3%. The results from multiconfiguration HartreeFock (MCHF) or multiconfiguration DiracFock (MCDF) method available from the MCHF/MCDF Collection^{} show excellent agreement with the NIST data. For Ar^{8+}, Ca^{10+} and Ni^{18+}, the calculation of Zhang et al. (1987) was adopted by CHIANTI, showing the same level of accuracy with the present AS calculation. For Fe^{16+}, the present AS data is systematically higher than that of Landi & Gu (2006) used by CHIANTI by 0.4%. However, both show a better level of accuracy (0.2%) when compared with NIST data. Calculations with the MCDF method have been done for highly charged ions, e.g. Fe^{16+} ( Aggarwal et al. 2003), Ni^{18+} ( Aggarwal & Keenan 2006) and Kr^{26+} ( Griffin et al. 2008) recently. When compared with them, the present AS data agrees also to within 0.4%. This means that our atomic structure is accurate, and the target expansion of 31 spectroscopic configurations and additional 33 correlation configurations in scattering calculation is reliable along the Nelike isoelectronic sequence.
2.2 Structure: weighted oscillator strength
A further test of our structure calculations is to compare weighted oscillator strengths ( g_{i}f_{i,j} for a given transition, where g_{i} is statistical weight of the initial level i and f is the oscillator strength of the transition) with those of other calculations. Tables 3, 5, 7, 9, 11, and 13 show a detailed comparison for a selection of gfvalues from the 5 lowestlying levels for six ions spanning the sequence: Si^{4+}, Ar^{8+}, Ca^{10+}, Fe^{16+}, Ni^{18+} and Kr^{26+}.Table 3: Comparison of the weighted oscillator strength gf between the AS and other calculations for Si^{4+}.
For Si^{4+}, around 68% of transitions available^{} from the CHIANTI v6 database ( Dere et al. 2009) show agreement to within 20% between the present AS calculations and the results of Bhatia et al. (1985). There are about 56% of alltype transitions (this refers to dipole and quadrupole) showing with (gf_{V} and gf_{L} are weighted oscillator strengths in velocity and length gauges, respectively). As shown in Table 3, the present AS calculation also shows good agreement with the results of our FAC calculations and the MCHF ones^{4}.
Table 4: The level energies (Ryd) of Ar^{8+} from different calculations along with the compilation of NIST v3 and CHIANTI v6.
Figure 1: Comparison of the level energies between the theoretical calculations and the ``experimental'' data available from NIST or CHIANTI databases. Level index refers to the ID number listed in Tables 2: Si^{4+}; 4: Ar^{8+}; 6: Ca^{10+}; 8: Fe^{16+}; 10: Ni^{18+}, and 12: Kr^{26+}. Labels in each panel corresponds to explanation in Tables of 2,4, 8, 10, and 12, respectively. [ Colour online] 

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For Ar^{8+}, our AS agreement is within 20% when compared with that of Zhang et al. (1987) for 70% of their transitions. The percentage of alltype transition increases up to 78% with for this ion. As shown in Table 5, our AS results also show good agreement when compared with the results of FAC and MCHF calculations.
For Ca^{10+}, there are about 76% of transitions with a gfdifference within 20% when compared with the data of Zhang et al. (1987). The difference of the present AS gfvalues between the velocity and length gauges is also within 20% for 78% of alltype transitions. The comparison with results from the FAC and MCHF methods also shows good agreement, see Table 7.
For Fe^{16+}, there are about 80% of alltype transitions with . The percentage is 67% of all available transitions from CHIANTI v6 ( Dere et al. 2009) with a difference within 20% when compared with those of Landi & Gu (2006). In comparison with results of Aggarwal et al. (2003) from the MCDF method, the percentage is 65%. For the two key transition lines (3C and 3D), the present AS calculations (2.43 and ) are slightly lower than the results (2.49 and ) of Loch et al. 2006) by 2% and 7%, respectively, while those from Chen et al. (2003) obtained from SUPERSTRUCTURE (3C2.57, 3D ) and by Landi & Gu (2006) using FAC (3C2.52, 3D ) and the present FAC calculation are also within 7%. When pseudostates were included by Chen (2007) using GRASP2, it results in a slightly larger difference^{} of 12% (3C2.27, 3D ).
Table 5: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ar^{8+}.
For Ni^{18+}, there are about 81% of alltype transitions showing . When compared with results of Aggarwal & Keenan (2006), 68% of electricdipole transitions show agreement to within 20%. The present AUTOSTRUCTURE calculations show better agreement with those from FAC (83% of transitions) and the data of Zhang et al. (1987), as currently used by CHIANTI v6 (91% of transitions).
For Kr^{26+}, the present results also show good agreement with previous calculations obtained using the MCDF method: Griffin et al. (2008), Rice et al. (2000) and Zhang et al. (1987), see Table 13. The ratio between the present AS gf in length and velocity gauges is within 20% of unity for 72% of alltype transitions. For the 3C and 3Dlines, the present AS results are in close agreement Griffin et al's data (to within 3%).
Table 6: The level energies (Ryd) of Ca^{10+} from different calculations along with experimental data compiled in CHIANTI v6.
Thus, we believe that the atomic structure of the ions spanning the sequence is reliable, and expect uncertainty on collision strengths from target structure to be small.
3 Scattering
The scattering calculations were performed using a suite of parallel intermediatecoupling frame transformation Rmatrix codes ( Griffin et al. 1998). Due to the large size of the Rmatrix ``box'' (due to the 7d orbital included), we employed 60 basis orbitals to represent the (N+1)thelectron continuum per angular momentum for most ions over the sequence. For lower charged ions, the basis orbitals are increased, e.g. 65 for Si^{4+} and P^{5+}, 75 for Al^{3+}, 85 for Mg^{2+}and 95 for Na^{+}. All partial waves from J=1/2 to 81/2 were included explicitly and contributions from higher Jvalues were included using a ``topup'' procedure ( Burgess 1974; Badnell & Griffin 2001). The contributions from partial waves up to J=23/2 were included in the exchange Rmatrix calculation, while those from J=25/2 to 81/2 were included via a nonexchange Rmatrix calculation. For the exchange calculation, a fine energy mesh was used to resolve the dominant resonances below the highest excitation threshold, see Table 14. From just above the highest threshold excitation to a maximum energy of 3.0 times the ionization potential for each ion, a coarse energy mesh (2.0 Ryd, q=Z10 is the residual charge of ion) was employed. For the nonexchange calculation, a step of 2.0 Ryd was used over the entire energy range. Witthoeft et al. (2007) tested the convergence of the effective strengths () with respect to resonance resolutions for several ions spanning the Flike sequence  we adopt the recommended energy meshes of Witthoeft et al. (2007) or better ones, see Table 14.
We then used the infinite energy Born limits (nondipole allowed) and linestrengths (dipoleallowed) from AUTOSTRUCTURE so that higher energy reduced collision strengths (), as defined by Burgess & Tully (1992), can be found from interpolation in BurgessTully space for all additional higher energies. The effective collision strengths at 13 electron temperatures ranging from K to 2 K (q is the residual charge of the ion, that is Z10), are produced as the end product. The data were stored in the ADAS adf04 format ( Summers 2004).
4 Results and discussions
4.1 Comparison with previous calculations
We compare the present ICFT Rmatrix results with those of previous works (DW and/or Rmatrix) for three ions (Si^{4+}, Fe^{16+} and Kr^{26+}) which span the calculated data for this isoelectronic sequence. Here, we select the extensively studied transition line 3D as a sensitive test of the accuracy of the present ICFT Rmatrix calculation, and give special attention to the cosmic abundant ionFe^{16+}. (The 3C line is less sensitive to the collision method because its excitation is more strongly nonresonant, but we show a comparison with experiment for Fe^{16+} along with 3D.) An extensive comparison (all available excitation data from ground state ) between the present ICFT Rmatrix and previous calculations (with preference to data with resonances included) has been made for the three ions to test widespread of accuracy of the present ICFT Rmatrix data.
 Si^{4+} To our best knowledge, there is no Rmatrix data available. The DW data of (Bhatia et al. 1985, with only ground and configurations included) was extensively used by current modelling codes, including CHIANTI v6. For the 3D line as shown in Fig. 2, the data from the DW calculations ( Bhatia et al. (1985) at eV, and the present FAC calculation) agrees with the background crosssection () of the present ICFT Rmatrix calculation to within 20%. Below the electron energy of eV, the data of FAC is slightly higher than the background of the present ICFT Rmatrix calculation. At low temperatures K, the present ICFT is higher than that of Bhatia et al. (1985) by 80%, however, it is in agreement with the FAC calculation. At the temperature ( K) with peak abundance in ionization equilibrium ( Mazzotta et al. 1998; Bryans et al. 2006); Bhatia et al.'s data is lower than the present ICFT data by 40%. The FAC result shows an excellent agreement with the present ICFT result. Above the temperature of K, the difference between the present and the result of Bhatia et al. (1985) is about 45%. This is higher than the general assessment criteria for the 3D transition as in cases of Fe^{16+} and Kr^{26+} discussed at following. This discrepancy at high temperatures is due to lower crosssections from the DW calculation at high energies where the line strength (S) dominates the crosssection. This is consistent with the difference of the collision strength limit 4gf/E_{ij}=4S/3 for this transition (AS: ; FAC: ; Bhatia et al. 1985: , MCHF^{4}: , see Table 3). The present AS calculation is within the range of the low (FAC) and high (MCHF) cases. Moreover, the excellent agreement of level energies between the present AUTOSTRUCTURE calculation and NIST data give more confidence to the present ICFT Rmatrix calculation.
Table 7: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ca^{10+}.
An extensive comparison with the results of Bhatia et al. (1985) is made in Fig. 3. At a low temperature of K, all available excitation data (DW) of Bhatia et al. (1985) from the ground state is lower than the present ICFT Rmatrix calculation, and only 4% of them are within 20%. At the temperature ( K) of peak fractional abundance in ionization equilibrium ( Mazzotta et al. 1998; Bryans et al. 2006), the percentage increases to 19%. At a high temperature of K, 50% of excitations from the ground state show agreement to within 20%.
Table 8: The level energies (Ryd) of Fe^{16+} from different calculations along with the compilation of NIST v3.
Table 9: Comparison of the weighted oscillator strength gf between the AS and other calculations for Fe^{16+}.
 Fe^{16+} Many calculations have been done with resonances taken into account, such as the serial work of Chen et al. (2003, 2006), Chen (2007), Aggarwal et al. (2003), Loch et al. (2006), and Landi & Gu (2006). In Fig. 4, we present the comparison of crosssections and effective collision strengths with previous available data, for the 3Dexcitation (123). In the work of Loch et al. (2006), a finer energy mesh of about 10 times present case was employed to test the convergence of the relative to the resolution of resonances. They concluded that the effect is quite small when compared with their results obtained with a coarser energy mesh (20 000 points in the resonance region, comparable to our present ICFT Rmatrix calculation). Good agreement is obtained between the present results and those of Loch et al. for the background crosssection (e.g. 10% at an electron energy of 1100 eV). The crosssection convoluted by a Gaussian profile (a width of 30 eV, comparable with resolution of present detectors in the laboratory) also shows agreement except for that around energies of 870 eV. At energies of 910 eV and 964 eV, the present ICFT Rmatrix results show a better agreement (6% and 19%) with laboratory measurement ( Brown et al. 2006) than results of Chen ( 2007, 24% and 28%) and Loch et al. (2006, 26% and 33%). This results in a slightly lower than previous results, see Fig. 4b. An isolated resonance approximation has been employed by Landi & Gu (2006) to take the resonances in electronimpact excitation into account. However, their at lower temperatures ( K) is far above that from the present calculation, by up to 30% around K. At higher temperatures, their results show good agreement with Chen's and Loch et al.'s data, as well as the present ICFT Rmatrix calculations (to within 10%). Landi & Gu (2006) data is currently used by the astrophysical modelling codeCHIANTI v6. Over the entire temperature range, the Dirac Rmatrix calculation of Loch et al. (2006) is slightly higher than the present ICFT Rmatrix calculation, by about 7%, which is consistent with the difference level of atomic structure, e.g. the gfvalue discussed above in Sect. 2.2.
Table 10: The level energies (Ryd) of Ni^{18+} from different calculations along with the compilation of NIST v3.
Table 11: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ni^{18+}.
Table 12: The level energies (Ryd) of Kr^{26+} from different calculations along with the compilation of NIST v3.
For the stronger 3C excitation (127), see Fig. 5, the present ICFT Rmatrix results agree well (better than 5%) with those from the DARC calculation performed by Loch et al. (2006) at the energies of 910 and 964 eV. Both are higher than the measurement ( Brown et al. 2006) by 35%. For the DARC calculation of Chen (2007), the difference drops to about 20% when compared with experimental data. This mirrors the reduction in his reported Avalues, and the weighted oscillator strengths shown above, due to the inclusion of target pseudostates  a similar effect was noted by Fournier & Hansen (2005). The present result is also in agreement (8%) with that reported by Aggarwal et al. (2003), see the point at eV. The resulting effective collision strengths also show good agreement (about 7%) between the present results and the BPRM of Chen et al. (2003) and DARC of Loch et al. (2006) over temperatures of equilibrium abundance for Fe^{16+}. With decreasing electron temperature, the difference between the present results and the DW plus isolated resonance results of Landi & Gu (2006) increases, but is still less than 20% at K. The DARC results of Chen (2007) are slightly lower than the present ones, by about 10%.
A complete set of data for Fe^{16+} for the work of Loch et al. (2006) is available from the Oak Ridge National Laboratory (ORNL) Controlled Fusion Atomic Data Center (CFADC)^{} and for Landi & Gu (2006) from CHIANTI v6. Thus, we make an extensive comparison (all excitation data from ground state ) with them at low ( K), intermediate (4.0 K) and high ( K) temperatures, see Fig. 6.
Figure 2: Comparison of the collision crosssection and of Si^{4+} for (3D) excitation between the present ICFT Rmatrix and previous calculations. Red smooth solid line is Gaussian convolution with width of 30 eV. [ Colour online] 

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Figure 3: An extensive comparison (all available excitations from the ground state) of effective collision strength for Si^{4+} between the present ICFT Rmatrix and previous available data (DW calculation of Bhatia et al. (1985), from CHIANTI v6.0 database) at low ( K), intermediate ( K, that of peak fraction in ionization equilibrium) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. `` '' symbols correspond to the 3D transition in Fig. 2. [ Colour online] 

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Figure 4: Comparison of the collision crosssection and of Fe^{16+} for (3D) excitation between the present ICFT Rmatrix and previous calculations. a) Top: the result of Loch et al. (2006) who used a finer energy mesh (around 10 times) than present calculation. The smoothlines are crosssections convoluted by Gaussian with a width of 30 eV (solid: Loch et al. 2006; dashed: Chen et al. 2003). Bottom: the present ICFT Rmatrix result along with Gaussian convolution (the width of 30 eV) and previous Dirac Rmatrix calculations ( Chen 2007, Gaussian convolution; Aggarwal et al. 2003, unconvoluted), as well as experimental measurements of Brown et al. (2006) at two energies. b) The effective collision strength from different Rmatrix calculations, and the DW plus isolated resonance approximation employed by Landi & Gu (2006). [ Colour online] 

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Figure 5: Comparison of the collision crosssection and of Fe^{16+} for s^{2}2 (3C) excitation between the present ICFT Rmatrix and previous calculations. The same figure caption as in Fig. 4. [ Colour online] 

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Figure 6: An extensive comparison (all excitations from the ground state) of effective collision strength of Fe^{16+} between the present ICFT Rmatrix and Dirac Rmatrix ( Loch et al. 2006)^{7}calculations, as well as results of Landi & Gu (2006) using an isolated resonance approximation, at low ( K) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. ``'' symbols correspond to the 3D transition in Fig. 4. [Colour online] 

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Figure 7: Comparison of the collision crosssection and of Kr^{26+} for (3D) excitation between the present ICFT Rmatrix and previous calculations including Dirac Rmatrix results of Griffin et al. (2008) with and without radiative damping, BreitPauli Rmatrix results of Gupta et al. (2000) and DW crosssections of Bhatia et al. (1985) at eV. Red smooth solid line is Gaussian convolution with width of 30 eV. [Colour online] 

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 Kr^{26+} Griffin et al. (2008) performed a 139level Rmatrix calculations using the Dirac method. Two separate calculations were done: one with radiation damping and one without. Figure 7 shows the crosssection (original and a Gaussian convolution with a width of 30 eV) and a comparison of between our present ICFT Rmatrix result and that of Griffin et al. (2008). Our original and convoluted crosssection show good agreement with data of Griffin et al. (2008), see Fig. 2c in their work. The background agrees well with the DW calculation from Bhatia et al. (1985)  see the point at eV. The 27level ( and ) BPRM calculation of Gupta et al. (2000) has no resonances above eV. Strong resonances attached to the and configurations appear, as demonstrated in the work of Griffin et al. (2008). The crosssection at eV (derived by us from the collision strength given at Ryd) of Gupta et al. (2000) agrees well with the background of present ICFT Rmatrix calculation. The present resultant is also consistent with the data of Griffin et al. (2008) both with and without radiative damping, being within 3% over the entire temperature range. Good agreement is also found when compared with Gupta et al.'s data.
Since a complete dataset of of Dirac Rmatrix data ( Griffin et al. 2008) is available from the CFADC^{7}, we make an extensive comparison of between the two different Rmatrix datasets for Kr^{26+}, as shown in Fig. 8. At the low temperature K, 75% of excitations from ground state show agreement of 20%. The percentage increases up to 88% at the high temperature of K.
Figure 8: An extensive comparison (all excitations from the ground state) of effective collision strength for Kr^{26+} between the present ICFT Rmatrix and Dirac Rmatrix (Griffin et al. 2008)^{7} calculations at low ( K) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. ``'' symbols correspond to the 3D transition in Fig. 7. [ Colour online] 

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Figure 9: The level ordering with the original level index (ID) relative to the ordering of Fe^{16+} by mapping according to the good quantum number  configuration, total angular momentum J and energy ordering for ions spanning the entire sequence. The spikes and dips are due to the shift of a given level, for example, (2837) levels in Fe^{16+} move to levels above 120 in Si^{4+}. [ Colour online] 

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Griffin et al. (2008) made a statistical analysis of over temperatures from K to K for 9591 transitions among 139 levels, and found the average difference between the with and without damping to be 1.58%. As we know, radiative rates have a dependence of q^{4} (where q is residual charge) for transitions. In their Nalike isoelectronic sequence Rmatrix calculation, Liang et al. (2009b) tested that the radiative damping becomes dominant with increasing of ionic charge. So, the radiative damping effect for the present ions of the Nelike isoelectronic sequence will be negligible. The present ICFT Rmatrix calculations without radiative damping are accurate over the sequence in this respect.
From the above comparison for the three specified ions (Si^{4+}, Fe^{16+} and Kr^{26+}) spanning the sequence, we believe that the present ICFT Rmatrix results ( and ) have the comparable level of accuracy with other Rmatrix calculations, including both Dirac and BreitPauli Rmatrix methods. Except for Fe, Ni and Kr, the present results are the only Rmatrix ones, todate. For ions near neutral (below Si^{4+}), Rmatrix with pseudostates calculations are likely needed to model ionization loss, but the present are the best data available, todate.
4.2 Isoelectronic trends of s
As noted in the work of Witthoeft et al. (2007), the level mixing effect for higher excited levels strongly affects the behaviour of the along the sequence. Similar levelordering cross was identified by Liang et al. (2009b) in Rmatrix EIE calculation of Nalike isoelectronic sequence. Witthoeft & Badnell (2008) and Liang et al. (2009b) noticed that taking configuration, total angular momentum J and energy ordering as good quantum number is a better choice for level matching in comparison between two different calculations and investigation of along the isoelectronic sequence. We find this to be true again, and map all ions relative to the level ordering of Fe^{16+} in the following discussion, see Fig. 9. This satisfactorily eliminates uncertainty originating from the noncontinuity of levelordering along the sequence. The choice of reference ion, Fe here, is of course irrelevant.
In Fig. 10, we show effective collision strength at , 10^{4}(q+1)^{2} and 10^{5}(q+1)^{2} K along the sequence for four dominant and strong transition lines in Nelike ions: , (see Fig. 10a) and and (see Fig. 10b). At the low temperature of 10^{3}(q+1)^{2} K, spikes and/or dips are observed along the sequence for the transitions. However,there are no clear spikes and/or dips for transitions. As pointed out by Witthoeft et al. (2007), such spikes/dips along the isoelectronic sequence at low temperature are due to the steady shifting of groups of resonances. This indirectly indicates that resonances are more important for the transitions than for the transitions. With increasing temperature, the spikes and/or dips disappear, as expected, because the resonance contribution becomes weaker and eventually negligible. For the 3D transition line, the increases again below Z=15 at the high temperature of 10^{5}(q+1)^{2} K. This is due to the highenergy collision strengths that are proportional to , as discussed for Si^{4+} for this transition line.
Figure 10: Effective collision strength () at temperatures of K (here q=Z10) along the isoelectronic sequence. a) and ^{1}P_{1} (3F) transitions; b) and ^{3}D_{1} (3D) transitions. [ Colour online] 

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5 Summary
We have performed 209level ICFT Rmatrix calculations of electron impact excitations with extensive configuration interaction (1337 LS terms or 2775 finestructure levels) for all ions of the Nelike isoelectronic sequence from Na^{+} to Kr^{26+}. The present work is the most extensive and complete Rmatrix data for modelling, todate.
Good agreement with the available NIST v3 experimentally derived or CHIANTI v6 observed data and the results of others for level energies and gfvalues for six specific ions (Si^{4+}, Ar^{8+}, Ca^{10+}, Fe^{16+}, Ni^{18+} and Kr^{26+}) spanning the isoelectronic sequence supports the reliability of the present Rmatrix excitation data. This was confirmed specifically, by detailed comparisons of / and for Si^{4+}, Fe^{16+} and Kr^{26+}.
The comparison (in the cases of Fe^{16+} and Kr^{26+}) with calculations using fully relativistic Dirac Rmatrix method reveals that present excitation data from ICFT Rmatrix shows the same level of accuracy. Excellent agreement of atomic structure for lower charged ions, e.g. Si^{4+}, gives us insight that the present excitation data is better than previous data (from the DW approximation) extensively used by the astrophysical and spectroscopic communities. It is noted that the isolated resonance approach appears to underestimate the resonant enhancement of for the majority of excitations in the case of Fe^{16+}.
By excluding the level crossing effects on the , we examined the isoelectronic trends of the effective collision strengths. A complicated pattern of spikes and dips of at low temperatures was noted again along the sequence, which precludes interpolation in Z. With increasing temperature, the difference between the present ICFT Rmatrix and previous DW results decreases as expected.
The data are made available through archives of the APAP website 1 in the ADAS adf04 format ( Summers 2004), OPENADAS 3 and CHIANTI^{}.
Table 13: Comparison of the weighted oscillator strength gf between the AS and other calculations for Kr^{26+}.
Table 14: The energy meshes (in unit of q^{2}, residual charge of ion) used for each ion.
In conclusion, we have generated an extensive set of reliable excitation data utilizing the ICFT Rmatrix method for spectroscopy/diagnostic research within the astrophysical and fusion communities. This will replace data from DW and isolated resonance approaches presently used by these communities, for most ions, and its use can be expected to identify new lines and may overcome some shortcomings in present astrophysical modelling, as seen previously for Mg^{8+} ( Del Zanna 2008), Fe^{6+} and Fe^{7+} ( Del Zanna 2009b,a), and Si^{9+} (Liang et al. 2009c).
AcknowledgementsThe work of the UK APAP Network is funded by the UK STFC under grant no. PP/E001254/1 with the University of Strathclyde. G.Y.L. thanks H. E. Mason, G. Del Zanna, P. J. Storey and K. A. Berrington for some helpful discussions.
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Footnotes
 ... sequence^{}
 These data are made available in the archives of APAP via http://www.apapnetwork.org, OPENADAS via http://open.adas.ac.uk as well as anonymous ftp to cdsarc.ustrasbg.fr (130.79.128.5) or via http://cdsweb.ustrasbg.fr/cgibin/qcat?J/A+A/518/A64
 ... network^{}
 http://www.apapnetwork.org
 ... NIST v3^{}
 http://physics.nist.gov/PhysRefData/ASD/index.html
 ... electronically^{}
 http://open.adas.ac.uk/
 ... Collection^{}
 http://atoms.vuse.vanderbilt.edu/
 ... available^{}
 The percentage refers to the fraction of transitions from the 5 lowestlying levels to all upper states contained within the cited references.
 ... difference^{}
 The MCDF level energies of Chen et al. (2003) and the Avalues of Chen (2007) are used to derive the gfvalues listed here.
 ... (CFADC)^{}
 http://wwwcfadc.phy.ornl.gov/
 ... CHIANTI^{}
 http://www.chianti.rl.ac.uk/
All Tables
Table 1: Radial scaling factors used in AUTOSTRUCTURE to minimize the total energies of (2s and 2p orbitals) and (3l orbitals) complexes, respectively  see text for details.
Table 2: The level energies (Ryd) of Si^{4+} from different calculations along with the compilation of NIST v3^{2}.
Table 3: Comparison of the weighted oscillator strength gf between the AS and other calculations for Si^{4+}.
Table 4: The level energies (Ryd) of Ar^{8+} from different calculations along with the compilation of NIST v3 and CHIANTI v6.
Table 5: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ar^{8+}.
Table 6: The level energies (Ryd) of Ca^{10+} from different calculations along with experimental data compiled in CHIANTI v6.
Table 7: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ca^{10+}.
Table 8: The level energies (Ryd) of Fe^{16+} from different calculations along with the compilation of NIST v3.
Table 9: Comparison of the weighted oscillator strength gf between the AS and other calculations for Fe^{16+}.
Table 10: The level energies (Ryd) of Ni^{18+} from different calculations along with the compilation of NIST v3.
Table 11: Comparison of the weighted oscillator strength gf between the AS and other calculations for Ni^{18+}.
Table 12: The level energies (Ryd) of Kr^{26+} from different calculations along with the compilation of NIST v3.
Table 13: Comparison of the weighted oscillator strength gf between the AS and other calculations for Kr^{26+}.
Table 14: The energy meshes (in unit of q^{2}, residual charge of ion) used for each ion.
All Figures
Figure 1: Comparison of the level energies between the theoretical calculations and the ``experimental'' data available from NIST or CHIANTI databases. Level index refers to the ID number listed in Tables 2: Si^{4+}; 4: Ar^{8+}; 6: Ca^{10+}; 8: Fe^{16+}; 10: Ni^{18+}, and 12: Kr^{26+}. Labels in each panel corresponds to explanation in Tables of 2,4, 8, 10, and 12, respectively. [ Colour online] 

Open with DEXTER  
In the text 
Figure 2: Comparison of the collision crosssection and of Si^{4+} for (3D) excitation between the present ICFT Rmatrix and previous calculations. Red smooth solid line is Gaussian convolution with width of 30 eV. [ Colour online] 

Open with DEXTER  
In the text 
Figure 3: An extensive comparison (all available excitations from the ground state) of effective collision strength for Si^{4+} between the present ICFT Rmatrix and previous available data (DW calculation of Bhatia et al. (1985), from CHIANTI v6.0 database) at low ( K), intermediate ( K, that of peak fraction in ionization equilibrium) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. `` '' symbols correspond to the 3D transition in Fig. 2. [ Colour online] 

Open with DEXTER  
In the text 
Figure 4: Comparison of the collision crosssection and of Fe^{16+} for (3D) excitation between the present ICFT Rmatrix and previous calculations. a) Top: the result of Loch et al. (2006) who used a finer energy mesh (around 10 times) than present calculation. The smoothlines are crosssections convoluted by Gaussian with a width of 30 eV (solid: Loch et al. 2006; dashed: Chen et al. 2003). Bottom: the present ICFT Rmatrix result along with Gaussian convolution (the width of 30 eV) and previous Dirac Rmatrix calculations ( Chen 2007, Gaussian convolution; Aggarwal et al. 2003, unconvoluted), as well as experimental measurements of Brown et al. (2006) at two energies. b) The effective collision strength from different Rmatrix calculations, and the DW plus isolated resonance approximation employed by Landi & Gu (2006). [ Colour online] 

Open with DEXTER  
In the text 
Figure 5: Comparison of the collision crosssection and of Fe^{16+} for s^{2}2 (3C) excitation between the present ICFT Rmatrix and previous calculations. The same figure caption as in Fig. 4. [ Colour online] 

Open with DEXTER  
In the text 
Figure 6: An extensive comparison (all excitations from the ground state) of effective collision strength of Fe^{16+} between the present ICFT Rmatrix and Dirac Rmatrix ( Loch et al. 2006)^{7}calculations, as well as results of Landi & Gu (2006) using an isolated resonance approximation, at low ( K) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. ``'' symbols correspond to the 3D transition in Fig. 4. [Colour online] 

Open with DEXTER  
In the text 
Figure 7: Comparison of the collision crosssection and of Kr^{26+} for (3D) excitation between the present ICFT Rmatrix and previous calculations including Dirac Rmatrix results of Griffin et al. (2008) with and without radiative damping, BreitPauli Rmatrix results of Gupta et al. (2000) and DW crosssections of Bhatia et al. (1985) at eV. Red smooth solid line is Gaussian convolution with width of 30 eV. [Colour online] 

Open with DEXTER  
In the text 
Figure 8: An extensive comparison (all excitations from the ground state) of effective collision strength for Kr^{26+} between the present ICFT Rmatrix and Dirac Rmatrix (Griffin et al. 2008)^{7} calculations at low ( K) and high ( K) temperatures. Horizontal dashed lines denote agreement of 20%. ``'' symbols correspond to the 3D transition in Fig. 7. [ Colour online] 

Open with DEXTER  
In the text 
Figure 9: The level ordering with the original level index (ID) relative to the ordering of Fe^{16+} by mapping according to the good quantum number  configuration, total angular momentum J and energy ordering for ions spanning the entire sequence. The spikes and dips are due to the shift of a given level, for example, (2837) levels in Fe^{16+} move to levels above 120 in Si^{4+}. [ Colour online] 

Open with DEXTER  
In the text 
Figure 10: Effective collision strength () at temperatures of K (here q=Z10) along the isoelectronic sequence. a) and ^{1}P_{1} (3F) transitions; b) and ^{3}D_{1} (3D) transitions. [ Colour online] 

Open with DEXTER  
In the text 
Copyright ESO 2010
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