Issue 
A&A
Volume 566, June 2014



Article Number  A104  
Number of page(s)  11  
Section  Atomic, molecular, and nuclear data  
DOI  https://doi.org/10.1051/00046361/201423864  
Published online  23 June 2014 
Rmatrix electronimpact excitation data for the Belike isoelectronic sequence ^{⋆}
^{1}
Department of PhysicsUniversity of Strathclyde,
Glasgow
G4 0NG, UK
email: luis.fernandezmenchero@strath.ac.uk
^{2}
Department of Applied Mathematics and Theoretical Physics,
University of Cambridge, Cambridge
CB3 0WA,
UK
Received: 21 March 2014
Accepted: 15 May 2014
Aims. Emission lines from ions in the Belike isoelectronic sequence can be used for reliable diagnostics of temperature and density of astrophysical and fusion plasmas over a wide range of temperatures. Surprisingly, interpolated data is all that is available for a number of astrophysically important ions.
Methods. We have carried out intermediate coupling frame transformation Rmatrix calculations which include a total of 238 finestructure levels in both the configuration interaction target and closecoupling collision expansions. These arise from the configurations 1s^{2} 2 {s, p} nl with n = 3−7, and l = 0−4 for n ≤ 5 and l = 0−2 for n = 6,7.
Results. We obtain ordinary collision strengths and Maxwellaveraged effective collision strengths for the electronimpact excitation of all the ions of the Belike sequence, from B^{+} to Zn^{26+}. We compare with previous Rmatrix calculations and interpolated values for some benchmark ions. We find good agreement for transitions n = 2−2 with previous Rmatrix calculations but some disagreements with interpolated values. We also find good agreement for the most intense transitions n = 2−3 which contribute via cascade to the (n = 2) diagnostic radiating levels.
Key words: atomic data / techniques: spectroscopic
These data are made available in the archives of APAP via http://www.apapnetwork.org and OPENADAS via http://open.adas.ac.uk
© ESO, 2014
1. Introduction
Emission lines from berylliumlike ions are used in astrophysics to study a variety of emission sources. For example, solar corona ultraviolet spectra (Vernazza & Reeves 1978; Sandlin et al. 1986) or solar flares (Neupert et al. 1967). Emission lines have been recorded by several solar missions, such as Skylab (Dere 1978). In the recent years, highresolution XUV spectroscopic observations by Chandra and XMMNewton satellites, have also shown that a vast number of astrophysical sources produce emission lines from Belike ions, such as Fe XXIII (Audard 2003).
Many emission lines from Belike ions have temperature or density sensitivity, so they can be used for diagnostics of astrophysical plasmas. In particular, the intensity ratios of the resonance versus the intercombination transitions in the Belike ions is an excellent temperature diagnostic. The ratio between the 2s 2p ^{1}P_{1}−2p^{2}^{1}D_{2} and the intercombination transition is also a good diagnostic, considering that the lines always fall close in wavelength. Indeed, this ratio has provided one of very few direct measurements of electron temperatures in the solar corona from SOHO (see, e.g., Wilhelm et al. 1998). Belike ion emission lines can be also used for diagnostics of fusion plasmas (Inoue et al. 2001; Summers et al. 1992).
Despite their importance, accurate electron impact excitation data for ions in this sequence are sparse. CoulombBornplusExchange intermediate coupling (IC) calculations to n = 3 were carried out by Sampson et al. (1984) for seventeen ions between Ne^{6+} and W^{70+}. Rmatrix calculations were carried out by Berrington et al. (1985) for C^{2+}, O^{4+}, Ne^{6+}, and Si^{10+} in LScoupling followed by algebraic recoupling of the reactance matrices, only for transitions among the n = 2 levels (which give rise to ten finestructure levels). The effective collision strengths of Berrington et al. (1985) were interpolated by Keenan et al. (1986) to provide data for N^{3+}, F^{5+}, Na^{7+}, Mg^{8+}, and Al^{9+}. With the addition of data for Ca^{16+} from Rmatrix calculations similar to Berrington et al. (1985) by Dufton et al. (1983), Keenan (1988) provided interpolated data for P^{11+}, S^{12+}, Cl^{13+}, Ar^{14+}, and K^{15+}. These two sets of interpolated rates have been widely used in the literature, and have been included in early versions of the CHIANTI database (Dere et al. 1997).
However, the irregular contribution of resonances to effective collision strengths along an isoelectronic sequence (Witthoeft et al. 2007) places an unknown uncertainty on such interpolated data. For example, significant problems with the interpolated values were found by Del Zanna et al. (2008). Del Zanna et al. (2008) performed an explicit Rmatrix calculation for Mg^{8+} and compared the intensities of the main lines with those obtained with the interpolated values. Significant differences (up to 50%) were found. There is therefore a need for explicit Rmatrix calculations for all Belike ions of astrophysical interest. This is the aim of the present work, which is part of a larger program of work to use the Rmatrix method to calculate effective collision strengths for all ions, up to Zn, of all Lshell sequences as well as a start on the Mshell. The most recent sequence todate is the Blike by Liang et al. (2012) and which contains references to earlier work on other sequences.
Some further LSplusalgebraic recoupling Rmatrix calculations were carried out by Ramsbottom and coworkers up to 2s3d (12 terms) for N^{3+} (Ramsbottom et al. 1994b) and up to 2p3d (26 terms) for Ne^{6+} (Ramsbottom et al. 1994a; Ramsbottom et al. 1995). Zhang & Sampson (1992) obtained fullyrelativistic distorted wave collision strengths, for transitions within the n = 2 complex, for all of the Belike ions from O^{4+} to U^{88+}. More recent fullyrelativistic work includes distorted wave data (up to 2p4d and 2s5d) for Si^{10+} by Bhatia & Landi (2007) and Rmatrix results (up to n = 5) for S^{12+}, obtained using the Dirac Rmatrix code (DARC) by Li et al. (2013).
Because of their importance to astrophysics, extensive IC Rmatrix calculations for Belike iron and nickel were carried out by Chidichimo et al. (2005; 2003) within the Iron Project, following an early distorted wave study by Bhatia & Mason (1981). In Chidichimo et al. (2005), collision strengths and effective collision strength were obtained for Belike Fe for the configurations n = 2,3,4, including a total of 98 finestructure levels in the basis set. A quite fine mesh was used for the electron impact energies, so the resonances were well resolved. In Del Zanna & Mason (2005), these atomic data were benchmarked against observations, pointing out temperature and density diagnostics. Overall good agreement between predicted and observed intensities was found. We therefore adopt this work on Fe^{22+} as a benchmark of the whole sequence.
We also adopt the Del Zanna et al. (2008) results for Mg^{8+} as a benchmark. Del Zanna et al. (2008) adopted the same target as Chidichimo et al. (2005) to calculate the scattering data for Mg^{8+}. They also benchmarked the atomic data against SOHO spectroscopic observations of the solar corona, finding excellent agreement, thus resolving longstanding discrepancies between observed and predicted line intensities. The use of Rmatrix data resulted in significantly higher electron temperatures.
Finally, we also adopt as a benchmark the Rmatrix results for C^{2+} by Berrington et al. (1985, n = 2) and Berrington et al. (1989, n = 3). In this latter case (n = 3) no algebraic recoupling of the LScoupling reactance matrices was carried out. Rather, when these data were uploaded to the CHIANTI database, level resolved data were obtained by splitting the LScoupling effective collision strengths according to statistical weights (see Dere et al. 1997).
We note also that Mitnik et al. (2003) carried out an Rmatrix with pseudostates calculation for C^{2+}, but only in LScoupling. They found that inclusion of pseudostates reduced effective collision strengths for transitions n = 2−3 by typically 10% and those n = 2−4 by 20−30%. Furthermore, a similar Rmatrix with pseudostates calculation by Badnell et al. (2003) found reductions of up to a factor of two in effective collision strengths for transitions to n = 4 in B^{+}. The effect of coupling to the continuum diminishes rapidly with increasing charge state though and so we would expect only modest overestimates for our N^{3+} data.
In the present work we include states up to n = 7 in our configuration interaction (CI) expansion, for a total of 238 finestructure levels. This basis set includes more bound states than any other previous nonpseudostate work and so levels up to n = 4 are better represented. In addition, cascading effects following collisional excitation up to the n = 7 shell can be examined with this basis set expansion. We use the same basis set and methods for the whole isoelectronic sequence, from B^{+} to Zn^{26+}. The present data therefore includes significantly more transitions than the previous published works for ions in the same sequence.
Traditionally, astrophysics interest stops at Zn − for example, the Chianti and Cloudy modeling packages span H−Zn. This is because elemental abundance, e.g., solar, drops by over an order of magnitude at the next element onwards. However, we continue on up to Kr for magnetic fusion application. Thereon, relativistic effects may need to be included in the wave functions themselves, e.g., via use of the Dirac Rmatrix code. We leave no gaps because of the difficulty of deeming a priori those elements which will never be of interest to astrophysics or, especially, magnetic fusion. Any such would be small in number and so their omission not represent any significant saving of effort.
The paper is organised as follows. In Sect. 2 we give details of our description of the atomic structure and in Sect. 3 that of the Rmatrix calculation. In Sect. 4 we show some representative results and compare them with the previous data of other Rmatrix calculations or interpolated values. In Sect. 5 the main conclusions are discussed. Atomic units are used unless otherwise is specified.
2. Structure
To obtain the wave functions of the isolated target we used the autostructure program (Badnell 2011). autostructure calculates the wave functions by diagonalizing the BreitPauli Hamiltonian (Eissner et al. 1974), which includes the relativistic terms, massvelocity, spinorbit, and Darwin, as a perturbation. The electronic potential is included in terms of the ThomasFermiDiracAmaldi model, adjusting the scaling parameters λ through a variational method, minimizing the equallyweighted sum of all LS term energies. We included a total of 21 atomic orbitals in the basis set: 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 5g, 6s, 6p, 6d, 7s, 7p, 7d. In the configuration interaction we included all the configurations 1s^{2} 2s^{2}, 1s^{2} 2s 2p, 1s^{2} 2p^{2}, 1s^{2} 2s nl and 1s^{2} 2p nl, with nl all the orbitals previously mentioned with n ≥ 3, for a total of 39 configurations. The minimized values of the scaling parameters are shown in Table 3 for all the ions in the sequence. As the atomic number increases, λ for the 1s orbital increases far away from unity. This is due to the Darwin term becoming more important as the charge of the nucleus increases. This does not affect the actual atomic structure nor the values of the level energies. The values of λ for orbitals with high angular momentum d, f and g are also much larger than the unity, which is necessary to influence the wave function for these eccentric orbits.
Fig. 1 Comparative plot of oscillator strengths for C^{2+}, Mg^{8+}, and Fe^{22+}. xaxis, present work; yaxis, refers to: C^{2+}Tachiev & Fischer (1999), Mg^{8+}Chidichimo et al. (2005), Fe^{22+}Del Zanna et al. (2008); ° for n = 2 upper levels; □ for n = 3 upper levels; × for n = 4 upper levels. 

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For such a configuration list we get a total of 130 LS terms, which are split into 238 IC levels. The calculated target energies for the IC levels up to n = 4 of the sample ions C^{2+}, Mg^{8+}, and Fe^{22+} are shown in Tables 4–6, respectively. They are compared with the observed ones, taken from the National Institute of Standards and Technology (NIST^{1}) database (Moore 1993 for carbon, Martin & Zalubas 1980 for magnesium and Sugar & Corliss 1985 for iron) and previous theoretical works collected in the CHIANTI database (Berrington et al. 1985 for carbon n = 2, Berrington et al. 1989 for carbon n = 3, Del Zanna et al. 2008 for magnesium and Chidichimo et al. 2005 for iron). With a few exceptions in the lower excited singlet levels, the agreement with the observed values is within 1.5%. The deviation of the calculated energies respect the observed values is smaller in present work than in previous ones, only in the case of carbon it is larger than Berrington et al. (1985, 1989), this is due to their use of pseudoorbitals. We prefer to use a spectroscopic orbitals so as to avoid having to deal with pseudoresonances. In any case, our philosophy is to use the same approach to the structure along the entire sequence. The energy values for the rest of the levels and the other ions of the sequence not shown in Tables 4–6, can be found online. As with the previous sequences that we have considered, we use the calculated energies in the Rmatrix calculation.
Comparison of gf values for some selected transitions of the ion Fe^{22+}.
To check the quality of the calculated wave functions of the target we compare the oscillator strengths (gf values) for selected transitions in Table 1 for Fe^{22+} with data from Chidichimo et al. (2005), which can be found on line in the CHIANTI database. Very good agreement is found, with the exception of the very weak transition 1−62: 2s^{2}^{1}S_{0}−2p 4s ^{3}P_{1}.
Figure 1 shows a global comparison of oscillator strengths gf for all the transitions between the levels shown in Tables 4–6, with the upper level with a configuration 2lnl′ with n ≤ 4, for the benchmark ions. We plot in the xaxis the present results, and in the yaxis the results of Tachiev & Fischer (1999) for carbon, Del Zanna et al. (2008) for magnesium, and Chidichimo et al. (2005) for iron. We note that the CHIANTI data for magnesium and iron are actually the results of separate structure calculations, and not those employed for the scattering target.
Points lying on the diagonal x = y in Fig. 1 mean a full agreement between our calculation and previous ones. On the graph we display about 1200 gf values and more than a 90% of them deviate less than a 5% from the diagonal. In carbon we appreciate four points far from the diagonal, they correspond to the transitions 2s^{2}^{1}S_{0}−2s 3p ^{3}P_{1}, 2p^{2}^{1}D_{2}−2s 3p ^{3}P_{1}, 2s 3s ^{1}S_{0}−2s 3p ^{3}P_{1} and 2p^{2}^{1}S_{0}−2s 3p ^{3}P_{1} (off the scale of the graph). Tachiev & Fischer (1999) used a multiconfiguration HartreeFock (MCHF) calculation followed by a configuration interaction (CI) calculation using the BreitPauli Hamiltonian. These transitions are forbidden ones as they are spinchanging, and the nonzero value of gf comes from state mixing, between the and the . Such E1transitions are very sensitive to the precise mixing. In carbon the nuclear charge is quite low, so the relativistic effects which can mix singlets and triplets are quite small. Repeating the autostructure calculation with different scaling parameters λ, we checked that the value of the gf for those transitions is very sensitive and it can vary up to six orders of magnitude, nevertheless, the values of the level energies remain stable.
Such extreme sensitivity has little physical consequence. The radiative lifetime of the 2s 3p ^{3}P_{1} is dominated the strong E1transition to 2s 3s ^{3}S_{1}. The corresponding electronimpact excitation transitions are mediated by the twobody electrostatic exchange operator. As such, the effective collision strengths will behave for the most of the temperature range of interest as a forbiden transition, tending to zero. Only at high temperatures, above 10^{6} K, will a dipole tail appear tending to a non zero value. Such temperatures are much above the ionization temperature of C^{2+}. This sensitivity in such transition probabilities will also be reduced as the charge of the nucleus increases because the relativistic effects become larger and the state mixing fractions become more stable.
For iron and magnesium the agreement shown in Fig. 1 is very good, all the points for n = 2,3 lie on the diagonal (less than a 5% of deviation) and about 90% of the n = 4 too, only the ones which correspond to weak transitions have a larger deviation. The points which lie far from the diagonal correspond to transitions between levels with configurations 4d and 4f, the last orbitals included in the basis sets of Chidichimo et al. (2005) and Del Zanna et al. (2008). As our basis set includes more bound orbitals, up to 7d, the description of these excited levels can vary respect the previous works and that is the likely reason for the discrepancy in the gf values for those transitions.
3. Scattering
We use the Rmatrix method (Hummer et al. 1993; Berrington et al. 1995) in combination with an intermediate coupling frame transformation (ICFT; Griffin et al. 1998; Badnell & Griffin 1999). The approach used is the same one as Chidichimo et al. (2005) and Del Zanna et al. (2008) for Belike Fe and Mg, but with a larger closecoupling expansion.
In the Rmatrix inner region, exchange effects were included for angular momentum up to 2J = 23, then extended using a nonexchange approximation for 2J up to 89, the contributions for higher J values were added using a topup with the Burgess sum rule (Burgess 1974) for dipole transitions and a geometric series for the nondipole transitions (Badnell & Griffin 2001). In the outer region we used two different meshes for the impact energy. A coarse mesh was applied for the non exchange calculation in the whole energy range and also for the exchange calculation for impact energies above the highest target level energy. This coarse mesh was around 10^{4}z^{2} Ry, with z the ion charge Z−4, being Z the atomic number.
The characteristic scattering energy increases as a factor z^{2} with the charge of the ion, nevertheless the width of the resonances remains constant. In order to maintain the resolution of the resonances over the sequence, we should use a constant fine energy step, thus increasing the number of grid points by a factor z^{2}. This is computationally impractical for all but small calculations. In practice, we have found (Witthoeft et al. 2007) that increasing the number of grid points by a factor z samples and converges the resonance structure satisfactorily. Thus, we use a fine energy mesh step which varies continuously versus the ionic charge, from 6.4 × 10^{5} for B^{+} up to 2.2 × 10^{6} for Zn^{26+}.
We convoluted the collision strengths Ω(i − j) with a Maxwellian distribution for the energies of the plasma electrons to form integrated effective collision strengths Υ(i − j): (1)where u = E/kT and E is the energy of the scattered electron, T the electron temperature and k the Boltzmann constant. We calculated the effective collision strengths for a wide range of temperatures from 1.6 × 10^{4} to 1.6 × 10^{8} K, which covers the whole range of interest for astrophysical and fusion plasmas.
For this integration, the collision strengths were extended to high energies by interpolation using the appropriate infiniteenergy limits in the Burgess & Tully (1992) scaled domain. The infiniteenergy limits were calculated with autostructure depending on the transition type: for the dipoleallowed transitions the results are given by 4S/3, where S is the line strength, and for the nondipoleallowed transitions by the Born approximation as described in Burgess et al. (1997). This infinite energy point can also be used to compare the present atomic structure with the previous ones. In Table 2 we show a comparison between the values of the collision strengths for infinite impact energy with the ones calculated by Chidichimo et al. (2005). Agreement below the 5% is found in most cases, with larger discrepancies present for the higher n = 4 levels.
Comparison of (scaled) infinite energy limit points for some dipole (4S/3) and allowed (Born) transitions in Fe^{22+}.
ThomasFermiDiracAmaldi potential scaling factors used in autostructure calculation.
C^{2+} target levels.
Mg^{8+} target levels.
Fe^{22+} target levels.
4. Results
We calculated the collision strengths Ω(i − j) and effective collision strengths Υ(i − j) for the electron impact excitation of ions in the Belike isoelectronic sequence, from B^{+} to Zn^{26+}, for all transitions between the first 238 fine structure levels. This results in a total of 28 203 transitions for each ion.
The effective collision strengths Υ(i − j) have been stored as an Atomic Data Format file adf04. These files also contain the full set of onephoton allowed transition Avalues calculated with autostructure. These data can be used for diagnostic of temperature and density of astrophysical and fusion plasmas. Nevertheless, for non Maxwellian velocity distributions in plasma, these adf04 files can not be used and the collision strengths Ω should be used directly.
As a sample of the results, we show in Fig. 2 the collision strengths for some important transitions within the n = 2 complex (see Del Zanna et al. 2008) for the benchmark ions in the Belike sequence. We show four different types of transitions: dipoleallowed (1 −5), dipoleallowed through spinorbit mixing (1−3), a doubleelectronjump Born transition (1−9), and a forbidden one (1−4). The collision strengths present the usual structure, a resonance region for the energies which correspond to transitions between the calculated levels, and a regular background. For dipoleallowed transitions, the collision strength diverges logarithmically as the energy tends to infinity, while for non dipoleallowed transitions it tends to a constant and for forbidden transitions the collision strength tends to zero as E^{2} in the infinite energy limit.
Looking down the columns of Fig. 2 we can follow the isoelectronic trend, or lack thereof, of a transition. The resonance structure and background varies differently as the ion charge increases. The resonance widths remain constant while the impact energy increases as a factor z^{2}. The height of the resonances increases as a factor z^{2} too, with respect to the background. The relative strength of the background can also increase with increasing charge due to increased spinorbit mixing, for example, in singlettriplet mixing. This effect is clearly seen in the transition 1−3. The spinorbit mixing of ^{3}P with ^{1}P turns this transition into a dipoleallowed one for iron, with corresponding asymptotic behavior, while in carbon (with a much lower nuclear charge) it behaves as a forbidden transition still.
For the case of Fe^{22+}, we show also in Fig. 2 a comparison with the distorted wave results of Bhatia & Mason (1981). While there is good agreement between the distorted wave collision strengths and the background Rmatrix ones, the omission of resonances by the former method can give rise to significant differences in Maxwellian rate coefficients for some transitions. Chidichimo et al. (1999, 2005) compared their Rmatrix results for groundstate transitions to levels of n = 2 and 3 with the distorted wave ones of Bhatia & Mason (1981) and found differences of up to a factor of two and ~30%, respectively, at 10^{7} K.
Figure 3 shows our Maxwell integrated effective collision strengths for the same transitions as shown in Fig. 2. The figure also shows a comparison with the previous benchmark calculations: Berrington et al. (1985); Del Zanna et al. (2008); Chidichimo et al. (2005); Mitnik et al. (2003). The Mitnik et al. (2003) calculation included Laguerre pseudostates in the closecoupling expansion. It was performed in LScoupling and a direct comparison without further recoupling can only be made for transitions which involve a singlet state (or an Sstate), following a generalization of Burgess et al. (1970), Eq. (99) etc. For the singlet–triplet transitions, level resolution can be determined simply by multiplying the effective collision strength by the fractional statistical weight of the level. The inclusion of pseudostates gives a difference of less than 10% compared to calculations without them.
Fig. 2 Electronimpact excitation collision strengths versus the impact energy for some selected transitions within the n = 2 complex for the benchmark ions. Full line: present Rmatrix work; □: distorted wave results of Bhatia & Mason (1981). 

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Fig. 3 Electronimpact excitation effective collision strengths versus the electron temperature for some selected transitions and targets, as in Fig. 2. Full line: present work; dashed line: C^{2+}Berrington et al. (1989), Mg^{8+}Del Zanna et al. (2008), and Fe^{22+}Chidichimo et al. (2005); dotted line: C^{2+}Mitnik et al. (2003). 

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For low temperatures, the center of the Maxwellian envelope lies on the resonance region, so such temperatures are quite sensitive to the good description of the resonances, if the impact energy mesh is fine enough. Thus, we have carried out a convergence study of the effective collision strengths at low temperature and we have checked that the fine mesh step used is sufficient for the ions under consideration. Overall, excellent agreement with previous calculations is found. This indicates that resonance excitation due to the extra configurations in our extended target does not produce significant enhancements for the n = 2 transitions. The transition (1−3) shows when the spin orbit gives an important contribution. For carbon and magnesium this transition behaves as forbidden for temperatures of physical interest, but for iron it shows dipole behavior.
The main population mechanism to states which radiate the important lines for plasma diagnosis (electric dipole 2p^{2}−2s2p) is direct excitation from the ground state. A secondary population mechanism lies in the direct excitation from the ground state to n = 3 and n = 4 levels and afterwards cascade to these 2p^{2} levels. The most intense transitions to n = 3 calculated in the present work mainly agree with previous Rmatrix calculations and also with the interpolated data, but some discrepancies were found in weaker transitions, double electron jumps or forbidden ones.
Effective collision strengths for weak transitions can have a considerable contribution from resonances at lower temperatures. The present closecoupling expansion is larger than those used in previous (nonpseudostate) Rmatrix works, especially those which only expanded up to n = 3. Consequently, we expect a larger resonance enhancement compared to those works, and we illustrate a case in Fig. 4. There we compare our effective collision strengths for the 2s^{2}^{1}S_{0}−2s3p ^{3}P_{1} transition in Ne^{6+} with the LSplusalgebraic recoupling Rmatrix results of Ramsbottom et al. (1995). At low temperatures our results display a much larger resonance enhancement, compared to those of Ramsbottom et al. (1995), while at high temperatures we see the influence of spinorbit mixing turning the highenergy/temperature tail from a forbidden to weak dipoleallowed one.
Fig. 4 Electronimpact excitation effective collision strengths versus the electron temperature for transition 1−152s^{2}^{1}S_{0}−2s3p ^{3}P_{1} of Ne^{6+}. Full line: present work; dashed line: Data from Ramsbottom et al. (1995). 

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Figure 5 shows the effective collision strengths for some selected transitions of P^{11+}. Belike P^{11+} has not been calculated before using the Rmatrix method or a DW method, and the data currently used for diagnostic modeling within the CHIANTI database are interpolated ones from Keenan (1988). In this figure we show the same set of transitions as in Fig. 3. The doubleelectronjump (1−9) shows differences between the Rmatrix calculations and the interpolated data, and in the spinchanging transition (1−3) the discrepancy is quite large. Asymptotically, the transition 1−3 behaves as a dipole one through spinorbit mixing, as discussed earlier. But, algebraic recoupling only of LScoupling data does not include such mixing and it (1−3) behaves as a forbidden one still. Thus, neither the original data nor the interpolated data are valid for such transitions, at these energies.
We close this section with a note of caution: we have shown only a selection of transitions and when the totality of excitations pluscascade are modeled then Del Zanna et al. (2008) has shown that significant problems can arise on using interpolated data.
5. Conclusions
We have presented a complete data set of ICFT RMatrix calculations of electronimpact excitation of all ions in the Belike isoelectronic sequence from B^{+} to Zn^{26+}. We have shown a selected set of collision strengths and effective collision strengths for some important n = 2 transitions and ions, finding good agreement with previous similar calculations. The present work expands the previous ones Del Zanna et al. (2008); Chidichimo et al. (2003, 2005) for Belike Mg, Fe and Ni, by significantly increasing the orbitals in the basis set.
The present data set constitutes a significant improvement over previous available data for many ions in the Belike sequence, which was based upon interpolated data. With our basis set emission lines including from cascade effects from levels up to n = 7 can be predicted. With the present data, emission lines from Belike ions can reliably be used for diagnostics of temperature and density of astrophysical and fusion plasmas. The atomic data are made available at our APAP network web page^{2}. They will also be uploaded into the OPENADAS^{3} and CHIANTI^{4} databases.
Work is in progress to expand the method to other isoelectronic sequences, in particular, the Mglike, which is similar to this one in the sense that it consist of a closed nshell plus two electrons.
Fig. 5 Electronimpact excitation effective collision strengths versus the electron temperature for some selected transitions of P^{11+}. Full line: present work; dashed line: interpolated data from Keenan (1988). 

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Acknowledgments
The present work was funded by STFC (UK) through the University of Strathclyde UK APAP network grant ST/J000892/1 and the University of Cambridge DAMTP astrophysics grant.
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All Tables
Comparison of (scaled) infinite energy limit points for some dipole (4S/3) and allowed (Born) transitions in Fe^{22+}.
ThomasFermiDiracAmaldi potential scaling factors used in autostructure calculation.
All Figures
Fig. 1 Comparative plot of oscillator strengths for C^{2+}, Mg^{8+}, and Fe^{22+}. xaxis, present work; yaxis, refers to: C^{2+}Tachiev & Fischer (1999), Mg^{8+}Chidichimo et al. (2005), Fe^{22+}Del Zanna et al. (2008); ° for n = 2 upper levels; □ for n = 3 upper levels; × for n = 4 upper levels. 

Open with DEXTER  
In the text 
Fig. 2 Electronimpact excitation collision strengths versus the impact energy for some selected transitions within the n = 2 complex for the benchmark ions. Full line: present Rmatrix work; □: distorted wave results of Bhatia & Mason (1981). 

Open with DEXTER  
In the text 
Fig. 3 Electronimpact excitation effective collision strengths versus the electron temperature for some selected transitions and targets, as in Fig. 2. Full line: present work; dashed line: C^{2+}Berrington et al. (1989), Mg^{8+}Del Zanna et al. (2008), and Fe^{22+}Chidichimo et al. (2005); dotted line: C^{2+}Mitnik et al. (2003). 

Open with DEXTER  
In the text 
Fig. 4 Electronimpact excitation effective collision strengths versus the electron temperature for transition 1−152s^{2}^{1}S_{0}−2s3p ^{3}P_{1} of Ne^{6+}. Full line: present work; dashed line: Data from Ramsbottom et al. (1995). 

Open with DEXTER  
In the text 
Fig. 5 Electronimpact excitation effective collision strengths versus the electron temperature for some selected transitions of P^{11+}. Full line: present work; dashed line: interpolated data from Keenan (1988). 

Open with DEXTER  
In the text 
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