208 research outputs found
Storage-ring measurement of the hyperfine induced 47Ti18+(2s 2p 3P0 -> 2s2 1S0) transition rate
The hyperfine induced 2s 2p 3P0 > 2s2 1S0 transition rate AHFI in
berylliumlike 47Ti18+ was measured. Resonant electron-ion recombination in a
heavy-ion storage ring was employed to monitor the time dependent population of
the 3P0 state. The experimental value AHFI=0.56(3)/s is almost 60% larger than
theoretically predicted.Comment: 4 pages. 3 figures, 1 table, accepted for publication in Physical
Review Letter
Absolute rate coefficients for photorecombination and electron-impact ionization of magnesium-like iron ions from measurements at a heavy-ion storage ring
Rate coefficients for photorecombination (PR) and cross sections for
electron-impact ionization (EII) of Fe forming Fe and
Fe, respectively, have been measured by employing the electron-ion
merged-beams technique at a heavy-ion storage ring. Rate coefficients for PR
and EII of Fe ions in a plasma are derived from the experimental
measurements. Simple parametrizations of the experimentally derived plasma rate
coefficients are provided for use in the modeling of photoionized and
collisionally ionized plasmas. In the temperature ranges where Fe is
expected to form in such plasmas the latest theoretical rate coefficients of
Altun et al. [Astron. Astrophys. 474, 1051 (2007)] for PR and of Dere [Astron.
Astrophys. 466, 771 (2007)] for EII agree with the experimental results to
within the experimental uncertainties. Common features in the PR and EII
resonance structures are identified and discussed.Comment: 12 pages, 6 figures, 3 tables, submitted for publication to Physical
Review
Absolute rate coefficients for photorecombination of berylliumlike and boronlike silicon ions
We report measured rate coefficients for electron-ion recombination for Si10+
forming Si9+ and for Si9+ forming Si8+, respectively. The measurements were
performed using the electron-ion merged-beams technique at a heavy-ion storage
ring. Electron-ion collision energies ranged from 0 to 50 eV for Si9+ and from
0 to 2000 eV for Si10+, thus, extending previous measurements for Si10+ [Orban
et al. 2010, Astrophys. J. 721, 1603] to much higher energies. Experimentally
derived rate coefficients for the recombination of Si9+ and Si10+ ions in a
plasma are presented along with simple parameterizations. These rate
coefficients are useful for the modeling of the charge balance of silicon in
photoionized plasmas (Si9+ and Si10+) and in collisionally ionized plasmas
(Si10+ only). In the corresponding temperature ranges, the experimentally
derived rate coefficients agree with the latest corresponding theoretical
results within the experimental uncertainties.Comment: 17 pages, 7 figures, 3 tables, 66 references, submitted to the J.
Phys. B special issue on atomic and molecular data for astrophysicist
Dissociative recombination measurements of HCl+ using an ion storage ring
We have measured dissociative recombination of HCl+ with electrons using a
merged beams configuration at the heavy-ion storage ring TSR located at the Max
Planck Institute for Nuclear Physics in Heidelberg, Germany. We present the
measured absolute merged beams recombination rate coefficient for collision
energies from 0 to 4.5 eV. We have also developed a new method for deriving the
cross section from the measurements. Our approach does not suffer from
approximations made by previously used methods. The cross section was
transformed to a plasma rate coefficient for the electron temperature range
from T=10 to 5000 K. We show that the previously used HCl+ DR data
underestimate the plasma rate coefficient by a factor of 1.5 at T=10 K and
overestimate it by a factor of 3.0 at T=300 K. We also find that the new data
may partly explain existing discrepancies between observed abundances of
chlorine-bearing molecules and their astrochemical models.Comment: Accepted for publication in ApJ (July 7, 2013
Dielectronic Recombination in Photoionized Gas. II. Laboratory Measurements for Fe XVIII and Fe XIX
In photoionized gases with cosmic abundances, dielectronic recombination (DR)
proceeds primarily via nlj --> nl'j' core excitations (Dn=0 DR). We have
measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe
XIX to Fe XVIII Dn=0 DR. Using our measurements, we have calculated the Fe
XVIII and Fe XIX Dn=0 DR DR rate coefficients. Significant discrepancies exist
between our inferred rates and those of published calculations. These
calculations overestimate the DR rates by factors of ~2 or underestimate it by
factors of ~2 to orders of magnitude, but none are in good agreement with our
results. Almost all published DR rates for modeling cosmic plasmas are computed
using the same theoretical techniques as the above-mentioned calculations.
Hence, our measurements call into question all theoretical Dn=0 DR rates used
for ionization balance calculations of cosmic plasmas. At temperatures where
the Fe XVIII and Fe XIX fractional abundances are predicted to peak in
photoionized gases of cosmic abundances, the theoretical rates underestimate
the Fe XVIII DR rate by a factor of ~2 and overestimate the Fe XIX DR rate by a
factor of ~1.6. We have carried out new multiconfiguration Dirac-Fock and
multiconfiguration Breit-Pauli calculations which agree with our measured
resonance strengths and rate coefficients to within typically better than
<~30%. We provide a fit to our inferred rate coefficients for use in plasma
modeling. Using our DR measurements, we infer a factor of ~2 error in the Fe XX
through Fe XXIV Dn=0 DR rates. We investigate the effects of this estimated
error for the well-known thermal instability of photoionized gas. We find that
errors in these rates cannot remove the instability, but they do dramatically
affect the range in parameter space over which it forms.Comment: To appear in ApJS, 44 pages with 13 figures, AASTeX with postsript
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Electron-ion recombination of Fe12+ forming Fe11+ : laboratory measurements and theoretical calculations
We have measured dielectronic recombination (DR) for Fe12 + forming Fe11 + using the heavy ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Using our results, we have calculated a plasma rate coefficient from these data that can be used for modeling astrophysical and laboratory plasmas. For the low temperatures characteristic of photoionized plasmas, the experimentally derived rate coefficient is orders of magnitude larger than the previously recommended atomic data. The existing atomic data were also about 40% smaller than our measurements at temperatures relevant for collisionally ionized plasmas. Recent state-of-the-art theory has difficulty reproducing the detailed energy dependence of the DR spectrum. However, for the Maxwellian plasma rate coefficient, recent theoretical results agree with our measurements to within about 30% for both photoionized and collisionally ionized plasmas
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Recombination and Ionization Measurements at the Heidelberg Heavy Ion Storage Ring TSR
Knowledge of the charge state distribution (CSD) of astrophysical plasmas is important for the interpretation of spectroscopic data. To accurately calculate CSDs, reliable rate coefficients are needed for dielectronic recombination (DR), which is the dominant electron-ion recombination mechanism for most ions, and for electron impact ionization (EII). We are carrying out DR and EII measurements of astrophysically important ions using the TSR storage ring at the Max-Plank-Institute for Nuclear Physics in Heidelberg, Germany. Storage ring measurements are largely free of the metastable contamination found in other experimental geometries, resulting in more unambiguous DR and EII reaction rate measurements. The measured data can be used in plasma modelling as well as for benchmarking theoretical atomic calculations
Dielectronic recombination of xenonlike tungsten ions
Dielectronic recombination (DR) of xenonlike W20+ forming W19+ has been studied experimentally at a heavy-ion storage ring. A merged-beams method has been employed for obtaining absolute rate coefficients for electron-ion recombination in the collision-energy range 0â140 eV. The measured rate coefficient is dominated by strong DR resonances even at the lowest experimental energies. At plasma temperatures where the fractional abundance of W20+ is expected to peak in a fusion plasma, the experimentally derived plasma recombination rate coefficient is over a factor of 4 larger than the theoretically calculated rate coefficient which is currently used in fusion plasma modeling. The largest part of this discrepancy stems most probably from the neglect in the theoretical calculations of DR associated with fine-structure excitations of the W20+([Kr]4d10â4f8) ion core
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