$\mathrm{Double}-K-\mathrm{vacancy}$ states in electron-impact single ionization of metastable two-electron $N{}^{5+}(1s2s$ ${}^3{S}_1$) ions

The role of hollow states intermediately produced in electron-impact ionization of metastable He-like $N{}^{5+}\left(1s2s{}^3{S}_1\right)$ ions has been investigated in detail. A crossed-beam setup and suitable experimental techniques were employed for the measurement of accurate absolute cross sections and precise energy-scan data. Fine structures arising from $K$-shell excitations and associated resonances have been observed for this two-electron ion with less than $\pm$0.5 eV uncertainty on the energy scale. Fine details, such as interference of the reaction pathways of direct ionization and excitation with capture of the incident electron followed by double-Auger decay, could be revealed. Ab initio calculations based on the convergent close coupling (CCC) approach are in good agreement with the experiment.

Figure 1

Cross section for electron-impact ionization of N${}^{5+}$ ions. The full circles show the results of absolute measurements together with their total error bars. The data were obtained with an ion beam containing both metastable (ms) $N{}^{5+}\left(1s2s{}^3{S}_1\right)$ and ground-state (gs) $N{}^{5+}\left(1s^2{}^1{S}_0\right)$ ions. The solid (olive) line is the result of the present CCC calculation, assuming a fraction of 6.025% ms ions in the parent beam. The cross section for the ms fraction is the thin solid (red) line. The smooth dashed line represents the gs ionization contribution, which is subtracted from the experimental data to obtain the ms cross section.

Figure 2

Cross section for electron-impact ionization of metastable $N{}^{5+}\left(1s2s{}^3{S}_1\right)$ ions. The data points with statistical error bars are derived from the present experiment. The solid (red) line is the result of the present CCC calculation convoluted with a Gaussian of 1.7 eV width. The inset shows the thresholds (vertical bars) of the metastable terms in comparison with the present data.

Figure 3

Detail of the cross section for electron-impact ionization of metastable $N{}^{5+}\left(1s2s{}^3{S}_1\right)$ ions in the energy range of READI resonances. The solid (red) line shows excursions predicted by the CCC calculation from direct single (knock-off) ionization, which is represented by the dashed (blue) line.

Figure 4

Cross section for electron-impact ionization of metastable $N{}^{5+}\left(1s2s{}^3{S}_1\right)$ ions: experiment–open circles with statistical error bars; convoluted CCC results–solid (red) line. The most prominent features are associated with levels, terms, and configurations. The abbreviation EA stands for excitation autoionization [23].