J. Linkemann

Storage ring measurement of the C IV recombination rate coefficient

The low-energy C IV dielectronic recombination (DR) rate coefficient associated with 2s → 2p Δn = 0 excitations of this lithium-like ion has been measured with high-energy resolution at the heavy-ion storage ring TSR of the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany. The experimental procedure and especially the experimental detection probabilities for the high Rydberg states produced by the recombination of this ion are discussed in detail. From the experimental data a Maxwellian plasma rate coefficient is derived with ±15% systematic uncertainty and parameterized for ready use in plasma-modeling codes. Our experimental result especially benchmarks the plasma rate coefficient below 104 K where DR occurs predominantly via C III (1s22p4l) intermediate states and where existing theories differ by orders of magnitude. Furthermore, we find that, to within our systematic uncertainty of 15%, the total dielectronic and radiative C IV recombination can be represented by the incoherent sum of our DR rate coefficient and the radiative recombination rate coefficient of Pequignot and coworkers.

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 (*n \ 0 DR). We have measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe XIX to Fe XVIII *n \ 0 DR. Using our measurements, we have calculated the Fe XVIII and Fe XIX *n \ 0 DR rate coefficients. Signi—cant discrepancies exist between our inferred rates and those of published calculations. These calculations overestimate the DR rates by factors of D 2o r underestimate it by factors of D2 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 theo- retical techniques as the above-mentioned calculations. Hence, our measurements call into question all theoretical *n \ 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 D2 and over- estimate the Fe XIX DR rate by a factor of D1.6. We have carried out new multicon—guration Dirac- Fock and multicon—guration Breit-Pauli calculations which agree with our measured resonance strengths and rate coefficients to within typically better than We provide a —t to our inferred rate coeffi- (30%. cients for use in plasma modeling. Using our DR measurements, we infer a factor of D2 error in the Fe XX through Fe XXIV *n \ 0 DR rates. We investigate the eUects of this estimated error for the well- known thermal instability of photoionized gas. We —nd that errors in these rates cannot remove the instability, but they do dramatically aUect the range in parameter space over which it forms. Subject headings: atomic dataatomic processesgalaxies: activeinstabilitiesX-rays: general

Dielectronic Recombination in Photoionized Gas: The Importance of Fine-structure Core Excitations

At the low electron temperatures existing in photoionized gases with cosmic abundances, dielectronic recom- bination (DR) proceeds primarily via excitations of core electrons ( DR). At these temperatures, 0 nl r nl Dn 5 0 0 jj the dominant DR channel often involves fine-structure core excitations, which are not included in 2 p r 2 p 1/2 3/2 LS-coupling calculations or the Burgess formula. Using the heavy-ion storage ring at the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany, we have verified experimentally for Fexviii that DR proceeding via this channel can be significant in relation to other recombination rates, especially at the low temperatures characteristic of photoionized gases. At temperatures in photoionized gases near where Fe xviii peaks in fractional abundance, our measured Fe xviii to Fe xvii DR rate coefficient is a factor of »2 larger than predicted by existing Dn 5 0 theoretical calculations. We provide a fit to our measured rate coefficient for ionization equilibrium models. We have carried out new fully relativistic calculations using intermediate coupling, which include the channel and agree to within »30% with our measurements. DR via the channel may 2 p r 2 p

Recombination in electron coolers

Abstract An introduction to electron–ion recombination processes is given and recent measurements are described as examples, focusing on low collision energies. Discussed in particular are fine-structure-mediated dielectronic recombination of fluorine-like ions, the moderate recombination enhancement by factors of typically 1.5–4 found for most ion species at relative electron–ion energies below about 10 meV, and the much larger enhancement occurring for specific highly charged ions of complex electronic structure, apparently caused by low-energy dielectronic recombination resonances. Recent experiments revealing dielectronic resonances with very large natural width are also described.

Photorecombination of ions: search for interference effects, recombination at low energies and rate coefficient in plasmas

In the search for interference between radiative and dielectronic recombination (RR and DR), absolute recombination rate coefficients for Ar-like ions have been measured at the Heidelberg Test Storage Ring in the centre-of-mass energy range 0-80 eV. The dominant recombination channels beside RR is DR via the formation of (33d ) and (33d ) intermediate doubly excited states. The DR rate coefficient in plasmas is inferred from this measurement. It is about a factor of 3 lower than the previously available result based on a semi-empirical calculation. Asymmetric lineshapes due to quantum mechanical interference as recently predicted theoretically for isoelectronic ions have not been observed. The broad (3) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero centre-of-mass energy where an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed.

Dielectronic recombination of ground-state and metastable Li+ ions

Dielectronic recombination has been investigated for -n=1 resonances of ground-state Li+(1s2) and for -n=0 resonances of metastable Li+(1s2s 3S). The ground-state spectrum shows three prominent transitions between 53 and 64 eV, while the metastable spectrum exhibits many transitions with energies <3.2 eV. Reasonably good agreement of R-matrix, LS coupling calculations with the measured recombination rate coefficient is obtained. The time dependence of the recombination rate yields a radiative lifetime of 52.2±5.0 s for the 23S level of Li+.

Recombination measurements at low energies with Au49+,50+,51+ at the TSR

Recombination of Au49+, Au50+, and Au51+ ions has been studied at the TSR. With Au50+ ions a storage lifetime of only 2 to 4 s was observed with the magnetically expanded electron beam of the cooler at a density of ne = 107 cm-3. This short storage time is a consequence of the highest recombination rate coefficient ever observed with an atomic ion (1.8·10-6 cm3 s-1 at zero relative energy Erel = 0 between electrons and ions). At about 30 meV a huge dielectronic recombination resonance is found with a record small width of only about 15 meV. Such resonances fortuitously occurring near Erel=0 are probably the main reason for the enhanced recombination rates observed with Au50+, with Pb53+ (in a recent experiment at LEAR) as well as with other complex ions. For Au49+ and Au51+ the recombination rates are smaller by an order of magnitude.

Recent dielectronic recombination experiments

New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments.

Dielectronic Recombination of Fe XXIII Forming Fe XXII: Laboratory Measurements and Theoretical Calculations: Data

We havemeasured resonance strengths and energies for dielectronic recombination (DR) of Mg-like Fe xv forming Al-like Fe xiv viaN 1⁄4 3 ! N 0 1⁄4 3 core excitations in the electron-ion collision energy range 0Y45 eV. All measurements were carried out using the heavy-ion test storage ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also carried out new multiconfiguration Breit-Pauli (MCBP) calculations using the AUTOSTRUCTURE code. For electron-ion collision energies P25 eV we find poor agreement between our experimental and theoretical resonance energies and strengths. From 25 to 42 eV we find good agreement between the two for resonance energies. But in this energy range the theoretical resonance strengths are 31% larger than the experimental results. This is larger than our estimated total experimental uncertainty in this energy range of 26% (at a 90% confidence level). Above 42 eV the difference in the shape between the calculated and measured 3s3p( P1)nl DR series limit we attribute partly to the nl dependence of the detection probabilities of high Rydberg states in the experiment. We have used our measurements, supplemented by our AUTOSTRUCTURE calculations, to produce a Maxwellian-averaged 3 ! 3 DR rate coefficient for Fe xv forming Fe xiv. The resulting rate coefficient is estimated to be accurate to better than 29% (at a 90% confidence level) for kBTe 1 eV. At temperatures of kBTe 2:5Y15 eV, where Fe xv is predicted to form in photoionized plasmas, significant discrepancies are found between our experimentally derived rate coefficient and previously published theoretical results. Our newMCBP plasma rate coefficient is 19%Y28% smaller than our experimental results over this temperature range. Subject headinggs: atomic data — atomic processes — galaxies: active — galaxies: nuclei — plasmas — X-rays: galaxies

Ion storage ring measurements of dielectronic recombination for astrophysically relevant Feq+ ions

Iron ions provide many valuable plasma diagnostics for cosmic plasmas. The accuracy of these diagnostics, however, often depends on an accurate understanding of the ionization structure of the emitting gas. Dielectronic recombination (DR) is the dominant electron-ion recombination mechanism for most iron ions in cosmic plasmas. Using the heavy-ion storage ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany, we have measured the low temperature DR rates for Feq+ where q=15, 17, 18, and 19. These rates are important for photoionized gases which form in the media surrounding active galactic nuclei, X-ray binaries, and cataclysmic variables. Our results demonstrate that commonly used theoretical approximations for calculating low temperature DR rates can easily under- or over-estimate the DR rate by a factor of ∼2 or more. As essentially all DR rates used for modeling photoionized gases are calculated using these approximations, our results indicate that new DR rates are needed for alm...