Search

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.

We have measured resonance strengths and energies for dielectronic recombination (DR) of beryllium-like Fe XXIII forming boron-like Fe XXII via N = 2 → N' = 2 and N = 2 → N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics (MPI-K) in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, perturbative, state-of-the-art theoretical techniques: the multiconfiguration Breit-Pauli (MCBP) method, the multiconfiguration Dirac-Fock (MCDF) method, and the Flexible Atomic Code (FAC). Overall reasonable agreement is found between our experimental results and these theoretical calculations. We have used our measurements to produce a Maxwellian-averaged DR rate coefficient for Fe XXIII. Our experimentally derived rate coefficient is estimated to be accurate to better that ≈20%. At temperatures where Fe XXIII is predicted to form in both photoionized and electron-ionized gas, we find mixed agreement between our experimental rate coefficient and previously published rate coefficients. We find good agreement at these temperatures between the experimentally derived rate coefficient and our MCBP, MCDF, and FAC results.

A brief overview is given of theoretical results for electron-impact ionization of atoms and ions. A description is given of the main theoretical methods, along with the databases where the data are archived. It is shown that for light species, ground and metastable ionization cross sections are in reasonable agreement with experiment when non-perturbative data are used for the near neutrals and distorted wave data are used for ions greater than a few times ionized. Some discrepancies between theory and experiment still remain for systems with open d and open p subshells. The sensitivity of ionization rate coefficients to the near threshold part of the ionization cross section is shown. The role of excited states in effective ionization rate coefficients is demonstrated and recent excited state ionization cross section results for H, He, He+, B2+ and Ne are presented.