The low energy C IV dielectronic recombination (DR) rate coefficient associated with 2s→2p ∆n = 0 excitations of this lithiumlike ion has been measured with high energy-resolution at the heavy-ion storage-ring tsr of the Max-Planck-Institut für 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 10 4 K where DR occurs predominantly via C III(1s 2 2p4l) intermediate states and where existing theories differ by orders of magnitude. Furthermore, we find that the total dielectronic and radiative C IV recombination can be represented by the incoherent sum of our DR rate coefficient and the RR rate coefficient of Pequignot et al. (1991, Astron. Astrophys., 251, 680).

At the low electron temperatures existing in photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via excitations of core electrons ( DR). At these temperatures, nl r nl Dn ϭ 0 j j

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. 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 ∆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 -2 -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 ∆n = 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.

Electric-dipole intercombination and forbidden transitions have been optically observed with ions circulating in a storage ring, and atomic lifetimes determined. For the 2s2p 3 P o 1 level in the Be-like ion B + , the measured lifetime of (97.65 ± 0.5) ms corresponds to a transition probability of (10.24 ± 0.05) s −1 . This result ties in with experimental work on the neighbouring ion C 2+ and with recent calculations. For the corresponding level 3s3p 3 P o 1 in the Mg-like ion Al + , a lifetime of (305±10) µs (transition rate (3280±100) s −1 ) has been measured, corroborating earlier experiments but not the more recent calculations. The transition probability of the M1/E2 transition between the fine-structure levels of the 2s 2 2p 5 2 P o ground state in F-like Sc 12+ has been determined as (1000 ± 30) s −1 , in agreement with the results of semi-empirically corrected multi-configuration Dirac-Fock calculations.

We report the first electron-impact ionization experiment employing an ion storage ring. Absolute cross sections for Na-like Fe'~+ ions were measured at energies between 450 and 1030 eV with a precision and energy resolution unprecedented for highly charged ions. Breit-Pauli distorted-wave calculations accompanying the measurements and previous Dirac-Fock distorted-wave data are in substantial agreement with the magnitude and the general appearance of the measured cross sections but do not reproduce all details of the rich structure seen in the experiment.

In the search for interference between radiative and dielectronic recombination (RR and DR), absolute recombination rate coefficients for Ar-like Ti 4+ 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 n = 0 DR via the formation of Ti 3+ (3s 2 3p 5 3d 1 P n ) and Ti 3+ (3s 2 3p 5 3d 3 P n ) intermediate doubly excited states. The n = 0 Ti 4+ 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 Sc 3+ ions have not been observed. The broad Ti 3+ (3s 2 3p 5 3d 2 2 F) DR resonance expected on the basis of multiconfiguration 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.

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We have measured resonance strengths and energies for dielectronic recombination (DR) of Fe xix forming Fe xviii via N ¼ 2 ! N 0 ¼ 2 and N ¼ 2 ! N 0 ¼ 3 core excitations. 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 calculated these resonance strengths and energies using two independent, state-of-theart techniques: the perturbative multiconfiguration Breit-Pauli (MCBP) and multiconfiguration Dirac-Fock (MCDF) methods. Overall, reasonable agreement is found between our experimental results and theoretical calculations. The most notable discrepancies are for the 3l3l 0 resonances. The calculated MCBP and MCDF resonance strengths for the n ¼ 3 complex lie, respectively, %47% and %31% above the measured values. These discrepancies are larger than the estimated d20% total experimental uncertainty in our measurements. We have used our measured 2 ! 2 and 2 ! 3 results to produce a Maxwellian-averaged rate coefficient for DR of Fe xix. Our experimentally derived rate coefficient is estimated to be good to better than %20% for k B T e ! 1 eV. Fe xix is predicted to form in photoionized and collisionally ionized cosmic plasmas at k B T e 41 eV. Hence, our rate coefficient is suitable for use in ionization balance calculations of these plasmas. Previously published theoretical DR rate coefficients are in poor agreement with our experimental results. None of these published calculations reliably reproduce the magnitude or temperature dependence of the experimentally derived rate coefficient. Our MCBP and MCDF results agree with our experimental rate coefficient to within %20%. Subject heading: atomic data-atomic processes-methods: laboratory