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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.

31st International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC XXXI, Caen, France, 23 Jul 2019 - 30 Jul 2019; Journal of physics / Conference series Conference series [...] 1412, 142015 (2020). doi:10.1088/1742-6596/1412/14/142015, Published by IOP Publ., Bristol

International audience

We have studied the K-shell excitation of He-like uranium (U90+) in relativistic collisions with hydrogen and argon atoms. Performing measurements with different targets, as well as with different collision energies, enabled us to explore the proton- (nucleus-) impact excitation as well as the electron-impact excitation process for the heaviest He-like ion. The large fine-structure splitting in uranium allowed us to partially resolve excitation into different L-shell levels. State-of-the-art relativistic calculations which include excitation mechanisms due to the interaction with both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the electron-impact excitation process. ISSN:1094-1622 ISSN:0556-2791 ISSN:1050-2947

International audience; In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe 54+ , Xe 52+ ) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70 fm.

We present measurements of the energy loss of relativistic highly charged uranium ions interacting with a target beam of near-liquid density hydrogen droplets at the experimental storage ring (ESR) at GSI. Our results reveal that a liquid droplet target beam virtually behaves like a homogeneous gas jet target with respect to both energy loss and ion beam cooling. We also provide first results on ion beam cooling efficiency at high hydrogen area target densities, which are consistent with numerical estimations based on a simple model of the cooling force.

The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.

New journal of physics 20(7), 073033 (2018). doi:10.1088/1367-2630/aad01d, Published by IOP, [London]

International audience; The two-photon 2E1 transition from the singlet 2s state of heliumlike tin was measured. An alternative experimental approach was adopted in the present investigation where the two-photon emission results from the decay of the 2s state by selective K-shell ionization of Li-like tin ions in relativistic collisions with a low-Z gaseous target, which allowed for a measurement of the undistorted two-photon spectral shape. The measured 2E1 energy distribution was compared with fully relativistic calculations, which predict a Z dependence of the distribution. The preliminary results indicate the best agreement with the relativistic many body calculations for tin, and thus confirm the sensitivity on Z. Detailed data analysis is still in progress.

Lifetimes for 10–50 MeV/u U28+ ions were measured for base vacuum conditions in the ESR storage ring at GSI-Darmstadt. The lifetimes are due to total electron loss from U28+ resulting from interactions with background gases in the ring. Lifetimes were also measured for interactions with H2 and N2 targets. These data provide information about the relative magnitudes and energy dependences of the stripping cross-sections resulting from interactions with H2 and N2, gases which represent the primary constituents in high and ultra-high vacuum environments.

Highly energetic projectiles (20 MeV/u E beam v beam (the ion beam velocity) and 2 v beam . Results obtained both at the Catania LNS superconducting cyclotron CS with 58 Ni 14+,19+ (23 and 45 MeV/u) and at Ganil with 78 Kr 32+,34+ (64 MeV/u) are presented. Absolute emission cross-sections and kinematic properties are discussed.

We report on an experiment aiming for a study of the radiative decay modes of the 1s(2s)2 level in Li-like uranium. The experiment was performed of initially Be-like uranium colliding with N2 molecules at an energy of 90 MeV/u. By measuring the x-ray production associated with K-shell ionization of the projectile, a high selectivity for the production of the 1s(2s)2 level is observed.

Radiative processes occurring in collision of decelerated bare uranium ions and molecular hydrogen are studied at the heavy-ion storage ring ESR. The combination of the deceleration technique and the narrow Compton profile of molecular hydrogen allowed us to resolve a multitude of REC transitions into the bound states of the projectile and to resolve unambiguously the tip region of primary bremsstahlung. For this purpose, a supersonic molecular hydrogen jet-target, precooled with liquid nitrogen and optimized for long-term stability, was applied.

In this contribution, we present an experimental study carried out at GSI Darmstadt devoted to investigation of quantum electrodynamical (QED) effects for the ground states in hydrogen- and helium-like uranium. In the experiment, X-ray spectra following radiative recombination of free electrons with bare and H-like uranium ions (U92+, U91+) were measured at the electron cooler of the ESR storage ring. Utilizing clean and favorable experimental conditions present at the electron cooler, we were able to obtain very accurate values for the ground-state binding energies from the observed X-ray transitions. When compared with theory, our results provide the most stringent test of bound-state QED for one- and two-electron systems in the strong-field regime.

The x-ray spectra produced in collisions of 98 MeV/u Li-like uranium ions with N2 molecules have been studied. By measuring the decay photons associated with the capture process, experimental information for the formation of the excited [1s22s2p3/2]1P1 and [1s22s2p3/2]3P2 states in Be-like uranium has been obtained. It was found that relative probability for production of the P-states is close to statistical population law 2J + 1.

International audience; In this contribution, we present an experimental and theoretical study of the proton- and electron- impact excitation effects in helium-like uranium ions in relativistic collisions with different gaseous targets. The experiment was carried out at the experimental storage ring at GSI Darmstadt. Our experimental results are in agreement with predictions of the state-of-the art theory which treats both processes within the relativistic framework.

Beam lifetimes of stored ${\mathrm{U}}^{28+}$ ions with kinetic energies of 30 and $50\text{ }\mathrm{MeV}/\mathrm{u}$, respectively, were measured in the experimental storage ring of the GSI accelerator facility. By using the internal gas target station of the experimental storage ring, it was possible to obtain total projectile electron loss cross sections for collisions with several gaseous targets ranging from hydrogen to krypton from the beam lifetime data. The resulting experimental cross sections are compared to predictions by two theoretical approaches, namely the CTMC method and a combination of the DEPOSIT code and the RICODE program.

International audience; The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.

International audience; We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U91+ ions and 13 MeV/u Pb81+ ions channeled in silicon crystals. This collective response (wake effect) induces a shift of the continuum energy level by more than 100 eV, which is observed by means of radiative electron capture into the K- and L-shells of the projectiles. We also observe an increase of the REC probability by 20–50% relative to the probability in a non-perturbed electron gas. The energy shift is in agreement with calculations using the linear response theory, whereas the local electron density enhancement is much smaller than predicted by the same model. This shows that, for the small values of the adiabaticity parameter achieved in our experiments, the density fluctuations are not strongly localized in the vicinity of the heavy ions.

The X-ray spectra produced by K-shell ionization in collisions of Li-like (U89+) uranium ions at an energy of 397 MeV/u with gaseous N2 target have been measured. The very high level selectivity during such collision has been found. The effect allows us to measure undistorted two-photon energy distribution from a decay of the 1s2s1S0 state in He-like uranium.

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.

Radiative double electron capture is a fundamental atomic process which should be observed in collisions of bare ions with atoms, albeit with a much smaller cross-section than single radiative electron capture. A new experiment—to observe this rare process under single-collision conditions—has been performed at the internal gas jet target of the experimental storage ring at GSI in Darmstadt. X-ray spectra associated with single and double charge exchange have been observed in 30 MeV u −1 collisions of bare chromium ions (Cr 24+ ) with helium and nitrogen target gases.

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.

International audience; The K shell excitation of H-like uranium (U91+) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.

16th International Conference on the Physics of Highly Charged Ions (HCI), Ruprecht Karls Univ, Heidelberg, GERMANY, SEP 02-07, 2012; International audience; In recent years for the fundamental theory of quantum electrodynamics, considerable progress in the evaluation of higher order corrections has been achieved-not only for hydrogen-but also for helium-like systems-up to very heavy nuclei. We were aiming at a more precise determination of the lifetime of the metastable 2 P-3(0) state in He-like uranium which has a calculated value of 57.3 ps [1, 2]. From the lifetime it is possible to derive the energy of the state. In October 2011 we were able to perform a first test experiment at GSI, Darmstadt to study the feasibility of a new experimental detection technique. This advanced set-up consists of two state-of-the-art energy-, time-and position-sensitive germanium detectors [3] in combination with collimators in a Soller-slit like assembly. A beam of U91+-ions at an energy of 290MeV u(-1) is passed through a thin nickel foil in the interaction chamber. From the decrease in intensity as a function of the target distance one may extract a decay curve from which the lifetime can be derived. The advantages of this new set-up, in comparison to former experiments [4, 5] will be discussed and the results of a preliminary data analysis will be presented.

Double-differential cross sections (DDCS{close_quote}s) for the production of binary encounter electrons (BEE{close_quote}s) were measured for collisions of 30-MeV O{sup {ital q}+} projectiles with H{sub 2}, He, O{sub 2}, Ne, and Ar targets with {ital q}=4{endash}8 and an electron ejection angle of {theta}=0{degree} with respect to the beam direction. Particular interest focused on (a) the evaluation of the contributions of the different electron subshells of the multielectron targets, O{sub 2}, Ne, and Ar; (b) the study of the well-known enhancement of the BEE DDCS{close_quote}s with decreasing projectile charge-state {ital q}; here this dependence was tested for higher collision energies and new targets; (c) the study of the dependence of the BEE {ital peak} {ital energy} on the particular target and projectile charge state. Results were analyzed in terms of the impulse approximation, in which target electrons in the projectile frame undergo 180{degree} elastic scattering in the field of the projectile ion. The electron scattering calculations were performed in a partial-wave treatment using the Hartree-Fock model. Good agreement with the data was found for the H{sub 2} and He targets, while for the multielectron targets O{sub 2}, Ne, and Ar only electrons whose velocity was lower than the projectile velocity needed tomore » be included for good agreement. All measured BEE DDCS{close_quote}s were found to increase with decreasing projectile charge state, in agreement with other recent BEE results. The BEE peak energies were found to be independent of the projectile charge state for all targets utilized. {copyright} {ital 1996 The American Physical Society.}« less

The projectile excitation of high-Z ions has been investigated in relativistic ion-atoms collisions by observing the subsequent x-ray emission. Within this experiment information about the population of the magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. In addition, we have observed for the first time the effect of electron-impact excitation in a heavy highly charged ion undergoing a collision with a neutral atom.

The x-rays emitted in the process of radiative recombination (RR) of quasi-free electrons into excited state of hydrogenlike uranium ion were studied experimentally. Both the RR x-ray and the subsequently emitted x-ray were detected in time-coincidences. The angular distribution of the x-rays varied as a function of the RR x-ray emission direction. This observation revealed the coherent population of magnetic sublevels of the state in the hydrogenlike uranium ion.

We have measured the x-rays following 116.15 MeV u−1 collisions with H2 at 35°, 90°, 120° and 150° observation angles with regard to the ion beam direction. From our experimental spectra combined with radiative electron capture calculations, we obtain angular distribution of characteristic x-rays (L K) following the resonance transfer and excitation. Our result shows a good qualitative agreement with theoretical predictions.

The angular distribution, as well as the energy distribution of [delta] electrons produced in collisions of 1.0-MeV/u F[sup [ital q]+] ([ital q]=4,6,8,9) ions with molecular hydrogen, have been studied for laboratory observation angles ([theta][sub [ital L]]) from 0[degree] to 70[degree] with respect to the beam direction. The measurements are in fair agreement with the impulse approximation calculations which use the quantal elastic electron-ion differential scattering cross sections folded with the Compton profile of the target electrons. We observe that the energy of the centroid of the binary-encounter-electron (BEE) peak is projectile charge state, [ital q], and laboratory angle, [theta][sub [ital L]], dependent. Moreover, at 0[degree], an enhancement of the ratio of the observed double differential cross section for nonbare projectiles to that for the bare ion projectiles, [sigma]([ital q]+)/[sigma](9+), is observed, contrary to the [ital q][sup 2] scaling predicted by a first Born calculation for ionization. This ratio [sigma]([ital q]+)/[sigma](9+) decreases nonotonically with increasing [theta][sub [ital L]], and becomes smaller than one for [theta][sub [ital L]][ge]30[degree].

Using recoil-ion momentum spectroscopy (RIMS), the authors have measured differential cross sections for the production of highly charged Ne ions in collisions with 10 MeV F8+ as a function of the recoil-ion transverse momentum and charge state. In addition, the outgoing projectiles were charge-state analysed and detected in coincidence with the Ne recoil-ions. Thus different reaction channels like direct ionization, single- and double-electron capture by the projectile could be separated. The recoil-ion transverse momentum Pperpendicular to R was determined with an accuracy comparable to a projectile scattering angle resolution of 2.5*10-5 rad. The experimental data are compared to results of classical many-particle (nCTMC) and semiclassical quantum-statistical calculations.

We have measured the alignment of the $L$-shell magnetic substates following the $K$-shell excitation of hydrogen- and helium-like uranium in relativistic collisions with a low-$Z$ gaseous target. Within this experiment, the population distribution for the $L$-shell magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent deexcitation process. The results show a very distinctive behavior for the H- and He-like heavy systems. In particular, for $K\ensuremath{\rightarrow}L$ excitation of He-like uranium, a considerable alignment of the $L$-shell levels was observed. A comparison of our experimental findings with recent rigorous relativistic predictions provides a good qualitative and a reasonable quantitative agreement, emphasizing the importance of the magnetic-interaction and many-body effects in the strong-field domain of high-$Z$ ions.

The two-photon decay of the 2S state to the ground state in dressed atoms and one- or two-electron ions has been studied for several decades. Relativistic calculations have shown an Z-dependence of the spectral shape of this two-photon transition in one- or two-electron ions. We have measured the spectral distribution of the 1s2s \(^1\!S_0\) → 1s 2 \(^1\!S_0\) two-photon transition in He-like tin at the ESR storage ring using a new approach for such experiments. In this method, relativistic collisions of initially Li-like projectiles with a gaseous target were used to populate exclusively the first excited state, 1s2s, of He-like tin, which provided a clean two-photon spectrum. The measured two-photon spectral distribution was compared with fully relativistic calculations. The obtained results show very good agreement with the calculations for He-like tin.

Whereas our understanding of total cross sections for ionization and capture processes in ion‐atom collisions is widely viewed as having arrived at a state of adequate maturity, the same cannot be said at all about the dynamics of collisions, multi‐electron processes or the electron continua (in target and projectile) which are at the origin of total cross sections. We depict how these processes can be studied favourably in storage ring environments. We present examples of resonant and non‐resonant electron transfer processes, radiative and non‐radiative. This is elucidated via the relation of the electron nucleus bremsstrahlung at the high energy tip of the bremsstrahlung spectrum to the radiative electron capture cusp (RECC) and a new approach to determining molecular orbital binding energies in superheavy quasi‐molecules in resonant KK charge transfer.

The ratio R of helium double-to-single ionization cross sections was measured for ${\mathrm{Ne}}^{10+}$ and ${\mathrm{Ni}}^{28+}$ projectiles at velocities of 0.39\ensuremath{\le}${\mathit{v}}_{\mathit{P}}$/c\ensuremath{\le}0.93 intending to explore the high-velocity limit of R for highly charged ion impact. For ${\mathrm{Ne}}^{10+}$ projectiles R was observed to become independent of ${\mathit{v}}_{\mathit{P}}$ for ${\mathit{v}}_{\mathit{P}}$/c\ensuremath{\gtrsim}0.73 and an asymptotic experimental value R=(2.57\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ has been established. This is in good agreement with experimental results for proton, antiproton, electron, and positron impact and in excellent accordance with the theoretical prediction of (2.59\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$.

By applying the deceleration technique for bare uranium ions at the ESR storage ring we studied the Radiative Electron Capture process in collisions with low-Z target atoms. This allowed us to extend the current information about the time-reversed photoionization process of highly-charged heavy ions to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The angular differential data obtained prove theoretical predictions that at high-Z higher-order multipole contributions and magnetic corrections play an important role even at energies close to the threshold.

A kinematically complete experiment for double ionization of He by 3.6 Mev/u Au53+ impact has been performed using a high-resolution integrated multi-electron recoil-ion momentum spectrometer. Nine out of twelve momentum components of the outgoing fragments were measured. The electrons sum-momentum is found to be in the forward direction and the recoil ion almost compensates the electrons being ejected into the backward hemisphere. The momentum change of the projectile is found to be small compared to the momenta of the ejected reaction products, which reveals similarity with photoionization. The strongest interaction among the collision partners is found to exist between the recoil ion and the electrons.

The spectral distribution of the $1s2s\text{ }^{1}S_{0}\ensuremath{\rightarrow}1{s}^{2}\text{ }^{1}S_{0}$ two-photon decay of He-like tin was measured using a novel approach at the gas-jet target of the ESR storage ring. Relativistic collisions of Li-like projectiles with low-density gaseous matter have been exploited to selectively populate the desired $1s2s$ state. Compared to conventional techniques, this approach results in a substantial gain in statistical and systematic accuracy, which allowed us to achieve for the first time a sensitivity to relativistic effects on the two-photon decay spectral shape as well as to discriminate the measured spectrum for Sn from theoretical shapes for different elements along the He-isoelectronic sequence.

Absolute cross sections for single and double ionization of helium have been measured for highly charged (24

International audience; We have measured the two-photon 2E1 transition from the singlet 1s2s 1S0 to the 1s21S0 ground state of helium-like tin. The experiment was performed by using initially Li-like tin ions in collision with a nitrogen gas target at the ESR storage ring at the energy of 300 MeV/u. The two-photon emission arises from the decay of the exclusively formed 1s2s states following the ionization of one K-shell electron from the Li-like ions. The obtained spectral shape of the 1s2s1S0 - 1s21S0 (2E1) transition was compared with the one calculated within a relativistic approach. The spectral shape is displayed and its sensitivity with regard to details in the theoretical description of the 2E1 decay is discussed.

Theories for electron transfer to the continuum have encountered considerable difficulties to take into account the intrinsic many‐electron processes in the capture channel. This may partially be attributed to large momentum transfers involved and thus collision systems are mostly not in the realm of first order perturbation theories. For this reason we have studied collision systems where simultaneously distinct competing electron transfer processes are found to be active, like radiative (RECC) and non‐radiative electron capture to continuum (ECC) in the relativistic domain where one or two even active electrons are involved; here another, though distinct, transfer process, the projectile electron loss to continuum (ELC), permits additionally to study the dynamics of ionization very close to threshold. We have studied these electron transfer processes simultaneously in forward electron emission in two systems of different projectile Compton profile, U88++N2 and Sn47++N2 collisions using the forward elect...

The authors have measured the azimuthal ( phi p) and polar angle ( theta p) scattering distribution of projectiles in coincidence with recoil ions in charge states from 3+ to 8+ for the collision 0.53 MeV u-1 F8++Ne to F6++NeqR++(qR-2) e-. The coincident scattered particle distributions as a function of the azimuthal and polar angles phi p, theta p are obtained for different degrees of ionisation, ranging from one to six electrons ionised. Transverse projectile and recoil momenta have been experimentally determined. For a high degree of ionisation the authors observe strong deviations from two-body nuclear kinematics, i.e. for a given collision the resultant transverse momentum of all electrons emitted into the continuum increases with the number of ejected electrons and has a direction which is mostly not coplanar with the scattering plane.

We report the first observation of correlated x rays emitted by a heavy highly charged ion such as hydrogenlike uranium in the process of radiative recombination. The angular correlations between these x rays for the first time revealed a coherence of the populated state in this ion. The results also indicate a strong contribution of the spin-orbit interaction in radiative recombination.

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.

Radiative recombination transitions into the ground state of cooled bare and hydrogenlike uranium ions were measured at the storage ring ESR. By comparing the corresponding x-ray centroid energies, this technique allows for a direct measurement of the electron-electron contribution to the ionization potential in the heaviest He-like ions. For the two-electron contribution to the ionization potential of He-like uranium we obtain a value of $2248\ifmmode\pm\else\textpm\fi{}9\text{ }\text{ }\mathrm{eV}$. This represents the most accurate determination of two-electron effects in the domain of high-$Z$ He-like ions, and the accuracy reaches already the size of the specific two-electron radiative QED corrections.

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.

The ESR storage ring offers unique conditions for precision investigations in the realm of atomic structure and collisions research at highest nuclear charges. For atomic structure studies, a very recent experiment will be discussed aiming on a precise determination of the electron-electron QED contribution to the groundstate ionization potential in He-like uranium. For the regime of collision dynamics, the close interplay between dynamics and atomic structure is outlined. Here, for the case of the 2p3/2 → 1s1/2 transition in H-like uranium, a strong interference between the El and M2 transition amplitudes has been identified which gives rise to a remarkable modification of the angular distribution of emitted photons.

Absolute cross sections for single and multiple projectile loss from MeV/u low‐charge‐state argon and xenon ions are presented. The impact energies were 0.74 and 1.4 MeV/u and the incoming charge states were 1,2 and 3. Using the present and data from the literature, it was shown that the multiple loss cross sections scale as (ΣBE)−n, where ΣBE is the sum of the binding energies required to remove a certain number of electrons. The power n was found to be velocity dependent, varying roughly from 2.5 to 1 for velocities ranging from approximately 0.2 to 20 (units of 108 cm/s).

Observed enhancement of K-shell radiative recombination (RR) of bare uranium ions with cooling electrons is interpreted in terms of of distant transverse collisions in magnetized electron-cooler plasma described within the semiclassical geometrical model (SGM), which was recently proposed. The Monte Carlo simulations based on the proposed approach explains the enhancement measured in RR of U^{92+} ions with cooling electrons for the K-shell.