Anomalous mixing of the w2Po3/2 and x6Po3/2 levels in Fe ii

The neighbouring lines a ⁶D_5/2– x ⁶P^o_3/2 (1272.617 Å) and a ⁶D_5/2– w ²P^o_3/2 (1272.657 Å) have been observed in the UV spectrum of χ Lupi to be of comparable intensity. The latter, Δ S  = 2, transition would be expected to be very weak. The two upper states should display negligible mixing. We give a detailed, quantitative discussion of how the two upper states are in fact strongly mixed through their mixing with 3d⁶(³D)4p ⁴P^o_3/2, and hence we explain the relative strengths of the two UV lines.     

Radiative decay data for highly excited Zr i levels

Radiative lifetimes of 17 excited levels in Zr  i , in the energy interval 29 000–40 974 cm⁻¹, have been investigated using the time-resolved laser-induced fluorescence method. The levels belong to the 4d²5s5p, 4d³5p and 4d5s²5p electronic configurations and were excited in a single-step process from either the ground term, 4d²5s² a ³F, or from the low-lying 4d²5s² a ³P and a ⁵F terms. For three levels, we confirm previous measurements while for 14 of the levels the lifetimes have been measured for the first time. The experimental results are compared to theoretical calculations performed with a multiconfiguration relativistic Hartree–Fock method including core-polarization effects. Theoretical transition probabilities of astrophysical interest, scaled by the experimental lifetimes, for the depopulating channels of the investigated levels are also presented.     

Core-polarization effects, oscillator strengths and radiative lifetimes of levels in Pb iii

Transition probabilities and oscillator strengths of 382 lines with astrophysical interest arising from 5d⁹6s²6p, 5d¹⁰6s n l, 5d¹⁰6s², 5d¹⁰6p², 5d¹⁰6p7s and 5d¹⁰6p6d configurations and some levels radiative lifetimes of Pb  iii have been calculated. These values were obtained in intermediate coupling (IC) and using relativistic Hartree–Fock calculations including core-polarization effects. We use for the IC calculations the standard method of least-square fitting from experimental energy levels by means of Cowan computer code. The inclusion in these calculations of the 5d¹⁰6p7s and 5d¹⁰6p6d configurations has facilitated us a complete assignment of the levels of energy in the Pb  iii . Transition probabilities, oscillator strengths and radiative lifetimes obtained are generally in good agreement with the experimental data.     

Effective collision strengths for fine-structure forbidden transitions among the 2s22p3 levels of Ne iv

Effective collision strengths for electron-impact excitation of the N-like ion Ne  iv are calculated in the close-coupling approximation using the multichannel R-matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 2s²2p³ ground-state configuration. The expansion of the total wavefunction incorporates the lowest 11 LS eigenstates of Ne  iv , consisting of eight n  = 2 terms with configurations 2s²2p³, 2s2p⁴ and 2p⁵, together with three n  = 3 states of configuration 2s²2p²3s. We present in graphical form the effective collision strengths obtained by thermally averaging the collision strengths over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log T (K) = 3.6 to log T (K) = 6.1 (the range appropriate for astrophysical applications). Comparisons are made with the earlier, less sophisticated close-coupling calculation of Giles, and excellent agreement is found in the limited temperature region where a comparison is possible [log T (K) = 3.7 to log T (K) = 4.3]. At higher temperatures the present data are the only reliable results currently available.     

Effective collision strengths for fine-structure forbidden transitions among the 2s22p3 levels of S x

Effective collision strengths for electron-impact excitation of the N-like ion S  x are calculated in the close-coupling approximation using the multichannel R -matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 2s²2p³ ground configuration. The total (e⁻+ion) wavefunction is expanded in terms of the 11 lowest LS eigenstates of S  x , and each eigenstate is represented by extensive configuration-interaction wavefunctions. The collision strengths obtained are thermally averaged over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log  T (K)=4.6–6.7 (the range appropriate for astrophysical applications). The present effective collision strengths are the only results currently available for these fine-structure transition rates.     

Electron impact excitation of the ground-state 3P fine-structure levels in atomic oxygen

Fine-structure collision strengths are calculated for transitions between the ground-state levels of atomic oxygen. The R -matrix method is used in which the (2p⁴) ³P, ¹D and ¹S terms are included as well as three pseudo-states chosen to represent the dipole polarizability of the ³P ground state. Very sophisticated configuration–interaction wavefunctions are used for the target states and a recoupling transformation to pair coupling is applied to the inner-region Hamiltonian matrix, with the ³P fine-structure energy levels being adjusted to match the observed splitting prior to diagonalization. Effective collision strengths are obtained by integrating over a Maxwellian distribution and the rate coefficient of the cooling of electron gas is determined. The cooling rates are significantly lower than those currently available, in confirmation of the result deduced from measurements of the Earth's ionosphere electron gas.     

Effective collision strengths for forbidden transitions among the 3s23p3 fine-structure levels of Cl IIIIII

Effective collision strengths for the 10 astrophysically important fine-structure forbidden transitions among the ⁴S^o, ²D^o and ²P^o levels in the 3s²3p³ configuration of Cl  iii are presented. The calculation employs the multichannel R-matrix method to compute the electron-impact excitation collision strengths in a close-coupling expansion, which incorporates the lowest 23 LS target eigenstates of Cl  iii . These states are formed from the 3s²3p³, 3s3p⁴, 3s²3p²3d and 3s²3p²4s configurations. The Maxwellian-averaged effective collision strengths are presented graphically for all 10 fine-structure transitions over a wide range of electron temperatures appropriate for astrophysical applications [log  T (K)=3.3−log  T (K)=5.9]. Comparisons are made with the earlier seven-state close-coupling calculation of Butler & Zeippen, and in general excellent agreement is found in the low-temperature region where a comparison is possible [log  T (K)=3.3−log  T (K)=4.7]. However, discrepancies of up to 30 per cent are found to occur for the forbidden transitions which involve the ⁴S^o ground state level, particularly for the lowest temperatures considered. At the higher temperatures, the present data are the only reliable results currently available.     

3s23p–3s3p2 transitions in S iv

We use both the civ3 and mchf codes to calculate oscillator strengths of allowed and intercombination lines in the  3s²3p–3s3p²  multiplets. Valence, core–valence and some core–core correlation effects are included. The two approaches give results in excellent agreement. Core effects are particularly important for the intercombination lines, though relatively minor for allowed transitions.
We obtain a branching ratio of 1.42 with an estimated accuracy of 0.02 for the     transitions, compared with an experimental value of  1.12±0.1  . The A -values of the     intercombination lines are substantially different from those of previous calculations.     

Effective collision strengths for fine-structure forbidden transitions among the 3s23p4 levels of Cl ii

The ab initio R -matrix method is used to calculate effective collision strengths for electron-impact excitation of the sulphur-like ion Cl  ii in the close-coupling approximation. All 10 astrophysically important fine-structure forbidden transitions within the 3s²3p⁴     ,     ,     ground configuration levels are considered. The 12 lowest LS target states are included in the calculation. Effective collision strengths are obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities. Results are presented for electron temperatures in the range  log  T (K)=3.3  to  log  T (K)=5.5  , appropriate for astrophysical applications. These are the only effective collision strength data that are currently available.     

The 3s 2S–4p 2Po transition probability in Mg ii

The superposition of configurations (SOC) method has been used to calculate f -values for the Mg  ii doublet at 1240 Å, which has been observed in the interstellar medium near stars such as ζ Ophiuchi. Our best value for the multiplet oscillator strength is 0.00083. SOC calculations have also been undertaken for the stronger (3s−3p) doublet at 2800 Å giving a multiplet f -value of 0.92.     

Calculation of the structure and transition probabilities of low energy levels of Fe and Fe

We use analytical wave functions in intermediate coupling to make a complete relativistic calculation of the structure and transition probabilities for the n = 1, 2, 3 states of Fe⁺²⁴. Data for Fe⁺²⁵ are also given for comparison. The results have a high accuracy and those relating to the n = 3 state are given in detail for the first time here.     

Charge‐exchange X‐ray emission at hot/cool gas interfaces

Charge‐transfer (CT) X‐ray emission may occur at interfaces between a partially neutral gas and gas possessing high ions, provided there is a relative motion between those two phases. The CTX surface brightness from distant objects must be taken into account if it is not far below other “classical” emission sources, especially the thermal emission from the hot phase. I discuss those conditions and potential spectroscopic or photometric diagnostics. I also mention potential indirect effects of the CT reactions by means of pickup ion production, acceleration and subsequent modification of interface and plasma properties (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inner-shell photoionization of O iii

A 25-state R -matrix calculation is performed to obtain photoionization cross-sections for transitions from the 1s²2s²2p²³P ground state of the O  iii ion. Results are obtained for a range of photon energies, including those at which K-shell photoionization processes take place. We compare our results with those from previous calculations. Excellent agreement is obtained. We also consider resonances owing to transitions of a 1s electron excited into the 2p orbital and compare with a recent calculation.