| 1. |
- Fogle, M., et al.
(författare)
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Electron-impact ionization of Be-like CIII, NIV, and OV
- 2008
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 175:2, s. 543-556
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Tidskriftsartikel (refereegranskat)abstract
- We present recent measurements of absolute electron-impact ionization cross sections for Be-like C III, N IV, and O V forming Li- like C IV, N V, and O VI. The measurements were taken using the crossed-beams apparatus at Oak Ridge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in the ion beams. The measured ionization cross sections were compared with calculations using the R-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion.
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| 2. |
- Fogle, M, et al.
(författare)
-
Electron-ion recombination of Be-like C, N, and O
- 2005
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 442:2, s. 757-766
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Tidskriftsartikel (refereegranskat)abstract
- The absolute total recombination reaction rate coefficients for Be-like C, N, and O have been measured using the CRYRING storage ring and compared with the results from distorted-wave theory. For the theory results, it is found that shifts to NIST energy values for the core excited energies of the recombining system are not sufficient to accurately match all of the resonance positions and heights at lower energies. These theory results represent the quality of most archived theory DR data. The accurate calculation of these low energy resonances still presents a significant challenge to theory. In addition, trielectronic recombination resonances, associated with the formation of triply excited states during recombination, have been observed in the total recombination reaction rate coefficient spectra of N3+ and O4+. Finally, we construct a dielectronic recombination Maxwellian rate coefficient from the experimental results for low n resonances, and from the theoretical results for high n resonances. In the case of O4+, the trielectronic recombination resonances have a strong influence on the low temperature Maxwellian rate coefficient. Our best hybrid Maxwellian rate coefficient is compared with archived distorted-wave theory data, and is found to be in reasonable agreement, even at the low temperatures.
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| 3. |
- Loch, S.D., et al.
(författare)
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Influence of long-lived metastable levels on the electron-impact single ionization of C2
- 2005
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 71:1, s. 012716-1
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Tidskriftsartikel (refereegranskat)abstract
- A joint theoretical and experimental investigation is made of the influence of long-lived metastable levels on the electron-impact single ionization of C2+. It is expected that our electron cyclotron resonance ion source produces a beam with 40% of the C2+ ions in the 1s(2)2s(2) S-1(0) ground level and 60% in the 1s(2)2s2p P-3(0,2) excited levels. The comparison of nonperturbative close-coupling calculations with previous single-pass crossed beams and with our multiple-pass storage-ring measurements for the electron-impact ionization of C2+ is consistent with the predicted large metastable fraction. Reasonable agreement is found between the present time-dependent close-coupling, R-matrix with pseudostates, and converged-close-coupling ionization cross-section calculations for the ground and first excited configurations and experimental measurement, assuming a 60% metastable fraction in the ion beam. Distorted-wave calculations are found to overestimate the ionization cross section from both the ground and metastable terms, compared with nonperturbative calculations, resulting in an overestimation of the resultant total cross section when compared with experiment. It is clear that collisional-radiative modeling of the evolution of atomic plasmas through the Be-like ionization stage will need to take into account the role of both ground and metastable levels.
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| 4. |
- Loch, S.D., et al.
(författare)
-
The influence of long-lived metastable levels on the electron-impact single ionization of C2
- 2005
-
Ingår i: Physical Review A. ; 71:1
-
Tidskriftsartikel (refereegranskat)abstract
- A joint theoretical and experimental investigation is made of the influence of long-lived metastable levels on the electron-impact single ionization of C2+. It is expected that our electron cyclotron resonance ion source produces a beam with 40% of the C2+ ions in the 1s22s2 1S0 ground level and 60% in the 1s22s2p 3P0,2 excited levels. The comparison of nonperturbative close-coupling calculations with previous single-pass crossed beams and with our multiple-pass storage-ring measurements for the electron-impact ionization of C2+ is consistent with the predicted large metastable fraction. Reasonable agreement is found between the present time-dependent close-coupling, R-matrix with pseudostates, and converged-close-coupling ionization cross-section calculations for the ground and first excited configuration, and experimental measurement, assuming a 60% metastable fraction in the ion beam. Distorted-wave calculations are found to overestimate the ionization cross section from both the ground and metastable terms, compared with nonperturbative calculations, resulting in an overestimation of the resultant total cross section when compared with experiment. It is clear that collisional-radiative modeling of the evolution of atomic plasmas through the Be-like ionization stage will need to take into account the role of both ground and metastable levels.
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