Electron‐Impact Ionization of Be‐like Ciii, Niv, and Ov

Abstract

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.

STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe11+FORMING Fe12+AND Fe13+

“…This calculation involves multiplying the ionization cross section by the electron-ion relative velocity and integrating over the Maxwellian velocity distribution. Following Fogle et al (2008), we truncate the integral over energy at E 0 + 6k B T e , where E 0 = 330.79 eV is the ionization potential of Fe 11+ , T e is the temperature at which α I (T e ) is to be calculated, and k B is the Boltzmann constant. The cross section was measured only up to E = 2300 eV.…”

Electron-impact-ionization measurements using hyperfine-assisted state preparation of ground-state berylliumlike sulfur

“…other systems [22]. Theoretical calculations for Be-like ions predict that the metastable EII cross section can be 1.3 to 2 times larger than the ground-state cross section [13,29]. Thus, hyperfine-assisted state preparation to remove metastables is critical in order to provide a benchmark at the current high level of accuracy.…”

“…However, it is difficult to reproducibly manipulate the metastable fraction. One can also determine the metastable fraction directly by passing the beam through a helium gas cell as a function of the gas density [13]. The ground-state and 3 P levels have different electron capture cross sections so the attenuation rates for the ground-state and metastable components of the beam are different, allowing the metastable fraction to be inferred.…”

“…However, it is not possible to quantify the population of the various J levels. This method is also limited to low-charge states where the ground-state and metastable capture cross sections differ significantly [13]. Thus, for EII measurements on most Be-like ions it is not possible to directly measure the metastable fraction.…”

Division Xii / Commission 14 / Working Group Collision Processes

“…Ionization and detachment have been studied for H − (Jung 2008); C 2+ , N 3+ , and O 4+ (Fogle et al 2008); Ne 4+ and Au 47+ (Pindzola et al 2008); and Si, Cl, and Ar isonuclear sequences .…”