Dielectronic Recombination Calculations Research Articles

Calculations of Dielectronic Recombination and Electron-impact Excitation Rate Coefficients of Highly Charged Sulfur Ions

Abstract Connected to the experiment performed at the Stockholm electron beam ion trap, a systematic relativistic configuration-interaction calculation is carried out to compare with the experimental spectra. In particular, separate rate coefficients are calculated for dominant recombination and excitation processes in the range of the impact electron energy determined by the experiment. By means of the relevant experimental parameters, the presently calculated dielectronic recombination rate coefficients for and S14+ ions and electron-impact excitation ones for S15+, S14+, and S13+ ions are employed further to synthesize overall theoretical spectra for comparison with the experimentally measured spectra. Overall, very good agreements with the experimental results are found except for one missing excitation peak around the impact electron energy 2.52 keV, which cannot be explained by the present calculations and thus remains open.

Collisional Ionization Equilibrium for Optically Thin Plasmas. I. Updated Recombination Rate Coefficients for Bare through Sodium‐like Ions

Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and are often highly suspect. This translates directly into the reliability of the collisional ionization equilibrium (CIE) calculations. We make use of state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He up to and including Zn. Where measurements exist, these published theoretical DR data agree with recent laboratory work to within typically 35% or better at the temperatures relevant for CIE. We also make use of state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H through to Zn. Here we present improved CIE calculations for temperatures from 104 to 109 K using our data and the recommended electron impact ionization data of Mazzotta et al. for elements up to and including Ni and Mazzotta for Cu and Zn. DR and RR data for ionization stages that have not been updated are also taken from these two additional sources. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. for all elements from H to Ni. The differences in peak fractional abundance are up to 60%. We also compare with the fractional ionic abundances for Mg, Si, S, Ar, Ca, Fe, and Ni derived from the modern DR calculations of Gu for the H-like through Na-like ions, and the RR calculations of Gu for the bare through F-like ions. These results are in better agreement with our work, with differences in peak fractional abundance of less than 10%.

New Collisional Ionization Equilibrium Calculations for Optically Thin Plasmas

Reliably interpreting spectra from electron-ionized laboratory and cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have carried out state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Mg-like ions of all elements from He to Zn as well as for Al-like to Ar-like ions of Fe. We have also carried out state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using Current address: NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA. Current address: Department of Physics, Stanford University, Stanford, CA 94305, USA. -C-5-

SUMER Observations of the Quiet‐Sun Transition Region

We present new results from observations of the quiet Sun taken with the SUMER spectrograph on the SOHO spacecraft. This study includes line and continuum emission covering a wide range of tem- peratures, and the data have been used to create a number of intensity maps with a —eld of view of D60@@ ) 300@@ and a spatial resolution of D1A. These maps reveal bright features that extend from D6000 Kt oD2 MK, indicating that much of the observed transition region emission arises in an atmospheric interface between the chromosphere and corona. The emission-line intensities over our —eld of view vary by up to 2 orders of magnitude and have lognormal distributions suggestive of small-scale magnetic frag- mentation. We —nd empirical evidence for a power-law relationship between line emission and electron density, with an exponent in the range D0.3¨1.0. Possible temperature dependence of the exponent is re—ected in the slope of the emission measure curve, which appears to change slightly with varying elec- tron density. However, there is no major variation in the shape of the lower transition region emission measure distribution with either the intensity or the size of the area studied. This indicates that ensem- bles of ii cool loops ˇˇ (loops physically distinct from those structures which reach coronal temperatures) are unlikely to be the source of this emission unless the size of the structures is considerably less than 1A. We also —nd that the Li-sequence N V j1240 emission is higher than expected, in agreement with several previous studies. It seems that this eUect diminishes with intensity, suggesting that it is unlikely to be due to the omission of density dependence in dielectronic recombination calculations. Subject headings: Sun: transition regionSun: UV radiation

Absolute measurements and calculations of dielectronic recombination with metastable He-like N, F, and Si ions.

Measurements and calculations of the rate coefficient 〈v\ensuremath{\sigma}〉 for dielectronic recombination associated with He-like ions of nitrogen, fluorine, and silicon are reported for electron energies between 0 and 27 eV. In this energy region, no resonances are associated with excitation of the 1${\mathit{s}}^{2}$ ground state, however, the metastable states 1s2s${(}^{1}$S) and 1s2s${(}^{3}$S) contribute with very large rate coefficients. The population of the 1s2s${(}^{3}$S) metastable state was determined by an independent x-ray measurement. We performed calculations in the LS-coupling and intermediate-coupling approximations for dielectronic recombination from the initial $^{3}$S and $^{1}$S states. For the $^{3}$S initial state, a comparison on an absolute scale between theory and experiment is presented. We find good agreement with LS-coupling calculations for ${\mathrm{N}}^{5+}$${(}^{3}$S), one exception being a structure observed below the $^{1}$S and $^{3}$P threshold. For ${\mathrm{Si}}^{12+}$${(}^{3}$S), the intermediate-coupling calculation reproduces the data fairly well. In the case of ${\mathrm{F}}^{7+}$${(}^{3}$S), we find discrepancies with both the LS- and intermediate-coupling calculations at low energy and just below the $^{1}$S and $^{3}$P thresholds. The contribution from the initial 1s2s${(}^{1}$S) state is important only for ${\mathrm{N}}^{5+}$. Here, the experimental data are in excellent agreement with the theory if the $^{1}$S metastable fraction is assumed to be about 1%.

Distorted-wave calculations of dielectronic recombination as a function of electric field strength.

The effects of external electric fields on dielectronic recombination have been studied in the distorted-wave approximation. The eigenvectors for the doubly excited Rydberg states were determined by diagonalizing a Hamiltonian matrix which includes the internal electrostatic and spin-orbit terms, as well as the Stark matrix elements. Calculations were performed for the dielectronic recombination transitions associated with the 2s..-->..2p excitation in the Li-like ions B/sup 2 +/, C/sup 3 +/, and O/sup 5 +/; and the 3s..-->..3p excitation in the Na-like ions P/sup 4 +/, S/sup 5 +/, and Cl/sup 6 +/. The results for the Na-like ions are compared with recent merged-beam measurements, and the differences between the two sequences with respect to the enhancement of dielectronic recombination as a function of electric field strength are discussed.