T. Bartsch

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.

Dielectronic recombination of lithiumlike Ni 2 5 + ions: High-resolution rate coefficients and influence of external crossed electric and magnetic fields

Absolute dielectronic recombination (DR) rates for lithiumlike

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

{\mathrm{Ni}}^{25+}{(1s}^{2}2s)

Dielectronic recombination of ground-state and metastable Li+ ions

ions were measured at high-energy resolution at the Heidelberg heavy-ion storage ring TSR. We studied the center-of-mass energy range 0\char21{}130 eV which covers all

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

\ensuremath{\Delta}n=0

Radiative recombination of bare Bi 83+ : Experiment versus theory

core excitations. The influence of external crossed electric (0\char21{}300 V/cm) and magnetic (41.8\char21{}80.1 mT) fields was investigated. For the measurement at near-zero electric field, resonance energies and strengths are given for Rydberg levels up to

Autoionizing high-Rydberg states of very heavy Be-like ions: A tool for precision spectroscopy

n=32;

Recent dielectronic recombination experiments

also Maxwellian plasma rate coefficients for the