We report on ${\mathrm{C}}^{3+}(1s2\ell 2{\ell }^{\prime }{}^{2S+1}\mathrm{L})$-resolved cross sections of electron capture in collisions of swift ${\mathrm{C}}^{4+}(1s2s{}^3\mathrm{S})$ ions with helium and hydrogen. The study focuses on the formation of doubly excited triply open-shell ${\mathrm{C}}^{3+}(1s2s2p){}^4\mathrm{P}$ and ${{}^2\mathrm{P}}_{\pm }$ states with emphasis on the ratio $R$ of their cross sections as a measure of spin statistics. Using zero-degree Auger projectile spectroscopy and a three-electron close-coupling semiclassical approach, we resolve a long-standing puzzle and controversy on the value of $R$ and on the effect of cascades, to clarify the underlying physics. The present results invalidate the frozen core approximation generally used in the past when considering electron capture in multielectron multi-open-shell quantum systems. A distinctive screening effect due to the Pauli exclusion principle (Pauli shielding) is proposed to account for the value of $R$, consistent with our findings.

We present SIMION 8.1 Monte Carlo type simulations of the response function and detection solid angle for long lived Auger states (lifetime τ ∼ 10⁻⁹ − 10⁻⁵ s) recorded by a hemispherical spectrograph with injection lens and position sensitive detector used for high resolution Auger spectroscopy of ion beams. Also included in these simulations for the first time are kinematic effects particular to Auger emission from fast moving projectile ions such as line broadening and solid angle limitations allowing for a more accurate and realistic line shape modeling. Our results are found to be in excellent agreement with measured electron line shapes of both long lived 1s2s2p⁴P and prompt Auger projectile states formed by electron capture in collisions of 25.3 MeV F⁷⁺ with H₂ and 12.0 MeV C⁴⁺ with Ne recorded at 0^∘ to the beam direction. These results are important for the accurate evaluation of the 1s2s2p ⁴P/²P ratio of K-Auger cross sections whose observed non-statistical production by electron capture into He-like ions, recently a field of interesting interpretations, awaits further resolution.

We report on double and single differential cross section measurements of the $1s2s^2{}^{2}S$, $1s2s2p{}^{2}P_{\pm }$, and $1s2s2p{}^{4}P$ doubly excited states produced by single-electron capture in 6–15 MeV collisions of pre-excited ${\mathrm{C}}^{4+}\left(1s2s{}^{3}S\right)$ ions with ${\mathrm{H}}_2$, He, Ne, and Ar targets. Ab initio cross section calculations based on a three active electrons atomic orbital close-coupling approach are compared to the measurements for collisions with the lighter ${\mathrm{H}}_2$ and He targets, showing an overall good agreement. Theoretical details concerning Auger electron angular distributions, partial cross sections, impact parameter dependencies, and radiative cascade feeding effects are discussed. The populations of the $1s2s2p{}^{4}P$, ${}^{2}P_{-}$, ${}^{2}P_{+}$ multiplets concerning the spin symmetry are finally presented for the four collision systems and discussed following the comparisons between theory and experiment.

For more than 40 years since the first ion-atom collision investigations of the two-electron process of electron transfer with excitation and its resonant and nonresonant features, a satisfactory quantum mechanical treatment has been lacking. We present now such a comprehensive transfer excitation treatment using a three-electron atomic orbital close-coupling approach within a full configuration interaction formalism, exemplified by ${\mathrm{C}}^{4+}\left(1s^2\right)+\mathrm{He}$ collisions at 0.5–18 MeV impact energies. The calculated cross sections for the production of ${\mathrm{C}}^{3+}\left(1s2p^2{}^{2}D\right)$, which shows the strongest resonance signature among the KLL Auger transitions, are found to be in excellent agreement with zero-degree Auger projectile spectroscopy measurements covering the maximum of the resonance peak and its high energy wing. At the lower energies, the theoretical results show a second maximum which is interpreted through a nonresonant one-step transfer excitation mechanism, never considered to date. The present, fully coherent, many-body treatment provides an important advance in the modeling and understanding of multielectronic processes in quantum systems under strong and ultrafast perturbations.

The process of the capture of a target electron into the projectile continuum (ECC) is investigated both experimentally and theoretically for collisions of 1.25–6.00 MeV deuterons with multielectron gas targets. Double-differential cross sections (DDCS), $d^2\sigma /d\mathrm{\Omega }dE$, of the ECC cusp-shaped electron peak involving He, Ne, and Ar gas targets were measured at the emission angle of zero degrees with respect to the ion-beam velocity. Corresponding DDCS calculations, obtained using continuum distorted-wave (CDW) and continuum distorted-wave eikonal initial-state (CDW-EIS) theories, were critically compared to the measurements. The role of each atomic subshell of the multielectron Ne and Ar targets is examined employing the CDW-EIS theory with numerical target wave functions, which is found to best agree with the measurements.