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We study the electron-loss-to-continuum (ELC) cusp experimentally and theoretically by comparing the ionization of U 89 + projectiles in collisions with N 2 and Xe targets, at a beam energy of 75.91 MeV/u. The coincidence measurement between the singly ionized projectile and the energy of the emitted electron is used to compare the shape of the ELC cusp at weak and strong perturbations. A significant energy shift for the centroid of the electron cusp is observed for the heavy target of Xe as compared to the light target of N 2 . Our results provide a stringent test for fully relativistic calculations of double-differential cross sections performed in the first-order approximation and in the continuum-distorted-wave approach.

The electron capture process was studied for Xe$^{54+}$ colliding with H$_2$ molecules at the internal gas target of the ESR storage ring at GSI, Darmstadt. Cross section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogen-like ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 MeV/u and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron capture data was recorded at the intermediate and in particular the low collision energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data is found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.

We have measured the widths and energies of the 1s2s2p 2 P 1/2,3/2 → 1s 2 2s 2 S 1/2 transitions in lithiumlike sulfur and argon, as well as the energies of the forbidden 1s2s2p 4 P 5/2 → 1s 2 2s 2 S 1/2 M2 transition in both elements. All measurements were performed with a double-flat crystal spectrometer without the use of any reference line. The transition energy measurements have accuracies ranging from 2.3 ppm to 6.4 ppm depending on the element and line intensity. The widths and the intensity ratios of the 1s2s2p 2 P 1/2,3/2 → 1s 2 2s 2 S 1/2 lines have also been measured. These are the first reference-free measurements of transitions in core-excited lithiumlike ions, and have an accuracy comparable to the best relative measurements. We have also performed multi-configuration Dirac-Fock calculations of the widths, energies and intensity ratios. Extensive comparison between existing experimental results and theory is performed, and Bayesian techniques employed to extract the energy of the 1s 2p 2 4 P 1/2 → 1s 2 2p 2 P 1/2 transition in sulfur and identify contaminant transitions.

This research was supported in part by Projects No. PEstOE/FIS/UI0303/2011 and No. PTDC/FIS/117606/2010, and by research center Grant No. UID/FIS/04559/2013 (LIB-Phys), from FCT/MCTES/PIDDAC, Portugal. We acknowledge partial support from NIST (P.I.), from PESSOA Huber Curien Program No. 38028UD, and PAUILF Program No. 2017-C08. P.A., J.M., and M.G. acknowledge support from FCT under Contracts No. SFRH/BPD/92329/2013, No. SFRH/BD/52332/2013, and No. SFRH/BPD/92455/2013, respectively. Laboratoire Kastler Brossel (LKB) is "Unite Mixte de Recherche de Sorbonne Universite, de ENS-PSL Research University, du College de France et du CNRS No. 8552." P.I. is a member of the Allianz Program of the Helmholtz Association, Contract No. EMMI HA-216 "Extremes of Density and Temperature: Cosmic Matter in the Laboratory." The SIMPA ECRIS has been financed by grants from CNRS, MESR, and University Pierre and Marie Curie (now Sorbonne Universite). The experiment has been supported by BNM Grant No. 0130002 and ANR Grant No. ANR-06-BLAN-0223. We wish to thank Jean-Paul Desclaux for his help improving the MCDFGME code, and Dr. Martino Trassinelli (INSP) for valuable discussions and his help during early stages of the experiment.

The process of electron-loss to the continuum (ELC) has been studied for the collision systems ${\mathrm{U}}^{28+}+{\mathrm{H}}_{2}$ at a collision energy of 50 MeV/u, ${\mathrm{U}}^{28+}+{\mathrm{N}}_{2}$ at 30 MeV/u, and ${\mathrm{U}}^{28+}+\mathrm{Xe}$ at 50 MeV/u. The energy distributions of cusp electrons emitted at an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam were measured using a magnetic forward-angle electron spectrometer. For these collision systems far from equilibrium charge state, a significantly asymmetric cusp shape is observed. The experimental results are compared to calculations based on first-order perturbation theory, which predict an almost symmetric cusp shape. Some possible reasons for this discrepancy are discussed.

For the collision system ${\text{U}}^{88+}+{\text{N}}_{2}$ at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge ${Z}_{p}^{\mathrm{eff}}=88$ at a near-relativistic collision velocity of $\ensuremath{\beta}\ensuremath{\approx}0.41$ is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.

We study the x-ray emission following the collision of a ${\mathrm{Bi}}^{83+}$ ion with a neutral Xe atom at the projectile energy 70 MeV/u. The collisional and post-collisional processes are treated separately. The probabilities of various many-electron processes at the collision are calculated within a relativistic independent electron model using the coupled-channel approach with atomiclike Dirac-Fock-Sturm orbitals. The analysis of the post-collisional processes resulting in the x-ray emission is based on the fluorescence yields, the radiation, and Auger decay rates, and allows one to derive intensities of the x-ray emission and compare them with experimental data. A reasonable agreement between the theoretical results and the recent experimental data is observed. The role of the relativistic effects is investigated.

The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system ${\text{U}}^{88+}+{\text{N}}_{2}\ensuremath{\rightarrow}{\text{U}}^{88+}+{[{\text{N}}_{2}^{+}]}^{*}+{e}^{\ensuremath{-}}+\ensuremath{\gamma}$ at 90 MeV/$\mathrm{u}$. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of ${0}^{\ensuremath{\circ}}$ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.

We report an experimental study of the charge-transfer process in collisions of Xe${}^{q+}$ ions ($16\ensuremath{\le}q\ensuremath{\le}20$) with magnesium atoms at an energy of 5.5$q$ keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe${}^{q+}$-Mg collision also behaves very differently from ``traditional'' collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.

Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, $220\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/u$. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U${}^{91+}$). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum $j=1/2$ and $j=3/2$ states of the L shell is determined directly from the obser-ved yields of Ly${}_{\ensuremath{\alpha}1}$ and Ly${}_{\ensuremath{\alpha}2}$ radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.

We have performed a measurement of the spectral shape from the two-photon decay of the $1s2s{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ state in He-like uranium. The two-photon emission followed the ionization of initially Li-like uranium ions in collisions with a N${}_{2}$ gas-jet target. The measured shape of the two-photon energy distribution shows good agreement with results of the relativistic calculations that take into account the electron-electron interaction rigorously up to the first order in quantum electrodynamic perturbation expansion. From the full width at half maximum of the measured two-photon energy distribution, we confirm the theoretically predicted modification of the shape due to the relativistic effects.

The projectile excitation of high-Z ions has been investigated in relativistic ion-atoms collisions by observing the subsequent x-ray emission. The x-ray spectra from the projectile excitation have been separated from the x-ray emission following electron capture into the excited states using a novel anticoincidence technique. For the particular case of hydrogenlike Au{sup 78+} ions colliding with Ar atoms, Coulomb excitation from the ground state into the fine-structure-resolved n=2 levels as well as into levels with principal quantum number n{>=}3 has been measured with excellent statistics. The observed spectra agree well with simulated spectra that are based on Dirac's relativistic equation and the proper inclusion of the magnetic interaction into the amplitudes for projectile excitation. It is shown that a coherent inclusion of the magnetic part of the Lienard-Wiechert potential leads to the lowering of the excitation cross section by up to 35%. This effect is more pronounced for excitation into states with high angular momentum and is confirmed by our experimental data.

The Coulomb excitation of highly charged few-electron ions in relativistic collisions with protons and low-$Z$ atoms is studied within the framework of first-order perturbation theory and the multiconfiguration Dirac-Fock method. Apart from the computation of the total excitation cross sections, a detailed theoretical analysis has been performed for the magnetic sublevel population of the residual ions. To describe this population, general expressions are derived for the alignment parameters of the excited states of the ions, taking into account the relativistic and many-electron effects. Calculations are performed for the $K\ensuremath{\rightarrow}L$ and $K\ensuremath{\rightarrow}M$ excitation of helium- and lithiumlike uranium ions and for a wide range of projectile energies. It is shown that the alignment of heavy few-electron ions is sensitive to relativistic and magnetic-interaction effects and, hence, to the collision energies of the projectiles. The theoretical predictions are discussed in the context of recent measurements on the Coulomb excitation of heliumlike uranium ${\text{U}}^{90+}$ ions which were recently performed at the GSI storage ring in Darmstadt.

A state-selective investigation of radiative recombination (RR) at threshold energies has been performed by x-ray detection at the electron cooler of the experimental storage ring (ESR) at Gesellschaft f\"ur Schwerionenforschung (GSI) for decelerated bare and hydrogenlike uranium ions. The application of the deceleration mode enabled us to observe state-selective RR into the $K$ and $L$ shells of bare and H-like uranium. The observed relative RR intensities into these low-lying bound states exhibit a good agreement with rigorous relativistic predictions, whereas the nonrelativistic approach fails to reproduce the data. Furthermore, the scaled relativistic one-electron treatment seems to be insufficient for the state-selective description of RR into H-like species at extremely low energies, suggesting the importance of electron-electron interactions for the RR process at the threshold.

The formation of the [1s2s] S states in heliumlike uranium (U{sup 90+}) has been studied in relativistic collisions of initially lithiumlike uranium (U{sup 89+}) ions with N{sub 2} target molecules. By measuring projectile x-ray emission in coincidence with projectile ionization, a strong selectivity for the formation of the [1s2s] S states in heliumlike uranium is observed. This selectivity is found to be unaffected by the subsequent rearrangement of the atomic orbitals involved. By measuring the photon emission associated with the decay of the [1s2s] {sup 1}S{sub 0} and the [1s2s] {sup 3}S{sub 1} substates, we obtain for their relative population probabilities a ratio of close to 1. This finding deviates considerably from the assumption of a statistical distribution 2J+1.

Absolute, total, single- and multiple-electron-loss cross sections are measured for $1.4\text{\ensuremath{-}}\mathrm{MeV}∕\mathrm{u}\phantom{\rule{0.3em}{0ex}}{\mathrm{U}}^{4,6,10+}$ ions colliding with neon and argon atoms and nitrogen molecules. It is found that the cross sections all have the same dependence on the number of electrons lost and that multiplying the cross sections by the initial number of electrons in the $6s$, $6p$, and $5f$ shells yields good agreement between the different projectiles. By combining the present data with previous measurements made at the same velocity, it is shown that the scaled cross sections slowly decrease in magnitude for incoming charge states between 1 and 10, whereas the cross sections for higher-charge-state ions fall off much more rapidly.

An experiment has been performed at the Gesellschaft f\"ur Schwerionenforschung accelerator facility using a beam-foil time-of-flight technique with a newly developed chemical vapor deposition diamond particle detector to measure the lifetime of the $2{}^{3}{P}_{0}$ state, $\ensuremath{\tau}(2{}^{3}{P}_{0}),$ of the two-electron ion ${}^{197}{\mathrm{Au}}^{77+}$ (He-like gold) with the result $\ensuremath{\tau}(2{}^{3}{P}_{0})=22.16(0.81)$ ps. The mechanism for the decay of this state is by a hyperfine-induced radiative transition to the $1{}^{1}{S}_{0}$ ground state (hyperfine quenching). The lifetime is therefore determined by a number of fundamental atomic and nuclear parameters not normally involved in radiative decay of allowed transitions. The experimental result is compared to several theoretical calculations.

Absolute total-, single-, and multiple-electron-loss cross sections are measured for $({\mathrm{Ar}}^{+}\ensuremath{-},$ ${\mathrm{Ar}}^{2+}\ensuremath{-},$ ${\mathrm{Xe}}^{3+})\ensuremath{-}(\mathrm{Ne},$ ${\mathrm{N}}_{2},$ Ar) collisions at 0.74 and 1.4 MeV/u. In addition, a many-body classical trajectory Monte Carlo model was used to calculate total- and multiple-electron-loss cross sections for ${\mathrm{Ar}}^{+}$ impact. For ${\mathrm{N}}_{2}$ and Ar targets, excellent agreement between the measured and calculated cross sections is found; for the Ne target the experimental data are approximately 40% smaller than the theoretical predictions. The experimental data are also used to examine cross-section scaling characteristics for electron loss from fast, low-charge-state, heavy ions. It is shown that multiple electron loss increased the mean charge states of the outgoing argon and xenon ions by 2 and 3 respectively. The cross sections decreased with increasing number of electrons lost and scaled roughly as the inverse of the sum of the ionization potentials required to sequentially remove the most weakly bound, next most weakly bound, etc., electrons. This scaling was found to be independent of projectile, incoming charge state, and target. In addition, the experimental total loss cross sections are found to be nearly constant as a function of initial projectile charge state. As a function of impact energy, the theoretical predictions yield an ${E}^{\ensuremath{-}1/3}$ behavior between 0.5 and 30 MeV/u for the total loss cross sections. Within error bars, the data are consistent with this energy dependence but are also consistent with an ${E}^{\ensuremath{-}1/2}$ energy dependence.