Dielectronic Capture with Subsequent Two-Electron Emission in Electron-Impact Ionization ofC3+Ions

In electron-impact ionization of ${\mathrm{C}}^{3+}$ ions we found the first unambiguous experimental evidence for resonant-excitation auto-double-ionization processes. These involve capture of the projectile electron by the ion with simultaneous $K$-shell excitation and subsequent emission of two correlated electrons. The resonance strength measured for the $1s2s^{2}2p{}_{}{}^3{}_{}{}^{}P$ contribution is (2.0 ± 0.4) × ${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$ eV. In addition, we resolved individual thresholds of excitation autoionization via states $1s2s2l$ and identified other resonant contributions to net single ionization.