Influence of long-lived metastable levels on the electron-impact single ionization ofC2+

Abstract

A joint theoretical and experimental investigation is made of the influence of long-lived metastable levels on the electron-impact single ionization of ${\mathrm{C}}^{2+}$. It is expected that our electron cyclotron resonance ion source produces a beam with 40% of the ${\mathrm{C}}^{2+}$ ions in the $1{s}^{2}2{s}^{2}\phantom{\rule{0.3em}{0ex}}^{1}S_{0}$ ground level and 60% in the $1{s}^{2}2s2p\phantom{\rule{0.3em}{0ex}}^{3}P_{0,2}$ excited levels. The comparison of nonperturbative close-coupling calculations with previous single-pass crossed beams and with our multiple-pass storage-ring measurements for the electron-impact ionization of ${\mathrm{C}}^{2+}$ is consistent with the predicted large metastable fraction. Reasonable agreement is found between the present time-dependent close-coupling, $R$-matrix with pseudostates, and converged-close-coupling ionization cross-section calculations for the ground and first excited configuration, and experimental measurement, assuming a 60% metastable fraction in the ion beam. Distorted-wave calculations are found to overestimate the ionization cross section from both the ground and metastable terms, compared with nonperturbative calculations, resulting in an overestimation of the resultant total cross section when compared with experiment. It is clear that collisional-radiative modeling of the evolution of atomic plasmas through the Be-like ionization stage will need to take into account the role of both ground and metastable levels.