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.

The use of mixed-state ionic beams in collision dynamics investigations is examined. Using high resolution Auger projectile spectroscopy involving He-like ( $1s^2{}^{1}S,1s2s{}^{3,1}S$ ) mixed-state beams, the spectrum contributions of the $1s2s{}^{3}S$ metastable beam component is effectively separated and clearly identified. This is performed with a technique that exploits two independent spectrum measurements under the same collision conditions, but with ions having quite different metastable fractions, judiciously selected by varying the ion beam charge-stripping conditions. Details of the technique are presented together with characteristic examples. In collisions of 4 MeV B ${}^{3+}$ with H ${}_2$ targets, the Auger electron spectrum of the separated $1s2s{}^{3}S$ boron beam component allows for a detailed analysis of the formation of the $1s2s{(}^{3}S)nl{}^{2}L$ states by direct $nl$ transfer. In addition, the production of hollow $2s2p{}^{1,3}P$ doubly- and $2s2p^2{}^{2}D$ triply-excited states, by direct excitation and transfer-excitation processes, respectively, can also be independently studied. In similar mixed-state beam collisions of 15 MeV C ${}^{4+}$ with H ${}_2$ , He, Ne and Ar targets, the contributions of the $1s^2$ , $1s2s{}^{3,1}S$ beam components to the formation of the $2s2p{}^{3,1}P$ states by double-excitation, $1s\to 2p$ excitation and transfer-loss processes can be clearly identified, facilitating comparisons with theoretical calculations.

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.

We present an experimental and theoretical study of symmetric ${\text{Xe}}^{54+}+\text{Xe}$ collisions at 50, 30, and 15 MeV/u, corresponding to strong perturbations with $v_K/v_{\text{p}}=1.20$, 1.55, and 2.20, respectively ($v_K$ is the classical $K$-shell orbital velocity and $v_{\text{p}}$ is the projectile velocity), as well as ${\text{Xe}}^{53+}+\text{Xe}$ collisions at 15 MeV/u. For each of these systems, x-ray spectra are measured under a forward angle of ${35}^{\circ }$ with respect to the projectile beam. Target satellite and hypersatellite radiation $K{\alpha }_{2,1}^{\mathrm{s}}$ and $K{\alpha }_{2,1}^{\mathrm{hs}}$, respectively, are analyzed and used to derive cross-section ratios for double-to-single target $K$-shell vacancy production. We compare our experimental results to relativistic time-dependent two-center calculations.