Table of contents

The multiple ionization and fragmentation of N₂ in collisions with fast H⁺, He⁺, Bi²⁵⁺, and Bi⁵⁷⁺ ions was studied using a position- and time-sensitive multi-particle detector. By the coincident measurement of correlated fragment momenta a kinematically complete image of the fragmentation process can be obtained for each individual event. Of special interest are the "Coulomb explosion" processes N₂ → N^q+ + N^p+ where the dissociation energy and the ionization cross section as a function of the orientation of the molecular axis can be derived.

We have newly developed a double coincidence time-of-flight (TOF) instrument for fragment ion spectroscopy of molecular targets in low energy collisions of multiply charged ions. The double electron capture processes in collisions of ³He²⁺ ions with molecular targets of N₂, O₂ and CO at 1 keV/amu energy have been revealed by means of the three particle coincidence detection method and their branching ratios have been determined.

Charge changing cross sections of Kr^q+ in collisions with Ne and CO for q = 7–9 in addition to N₂ and O₂ for q = 8 and electron capture cross sections of ³He²⁺ in collisions with N₂, O₂ and CO have been measured at low energies, ranging from 0.006 · q to 11.9 · q eV/amu and from 0.333 to 1333 eV/amu, respectively. The measured cross sections, except for single-charge changing cross sections of Kr^q+, are strongly dependent on collision energy. Multiple-charge changing cross sections of Kr^q+ typically show a minimum in their collision energy dependence. The collision energy characteristics are explained well by taking the induced-dipole interaction potential into consideration.

Swift (6.7 MeV/u) highly charged ion (Xe⁴⁴⁺)-induced CO molecule fragmentation is investigated by a coincident determination of the time of flight of the two fragments. A determination of the kinetic energy release (KER) distributions is realized for 18 dissociation pathways up to CO⁹⁺. The results are analysed in terms of the Coulomb explosion model. The evolution of the CO fragmentation results from the perturbative regime to the strong interaction regime is also compared to ion-atom behavior. The case of the dissociative singly ionized (CO⁺* → C⁺ + O) channel is studied by Recoil Ion Momentum Spectroscopy. This technique is found to widely improve the KER resolution. High-lying components, above the double ionization threshold, have to be invoked to analyse the momentum spectrum.

We present a detailed study of the two collisional systems: He²⁺ (11 keV/u) and O⁷⁺ (4 keV/u) on CO molecule. We measured in coincidence the full momentum vector of the two fragments of the dissociated molecule and the projectile charge state. It is thus straightforward to access the dependence of the capture process on the angle between the beam and the internuclear axis of the molecule. The KER (Kinetic Energy Release) distribution for each fragmentation channel is obtained for each outgoing projectile charge state associated to single capture, stabilized and autoionizing double capture, ... . It has to be noted that, contrarily to ion-atom collisions, Auger decay of the target plays an important role: double capture by He²⁺ on CO may lead to four-fold ionization of the target.

We present calculations of charge transfer cross sections for C^q+ + H₂ collisions with q = 2, 3, 4 in the energy range 50 eV/amu < E < 2.5 keV/amu. We employ ab initio electronic wavefunctions of the triatomic systems, the eikonal method to treat the ion-diatom relative motion, and the sudden approximation for vibration and rotation of the diatom.

Single and double capture cross sections in collisions between Li⁺ and N⁵⁺ ions with H₂ molecules are calculated in the range of impact velocities v = 0.05–0.5 a.u., using the Franck-Condon approximation and a molecular close coupling expansion. Given the complex characteristics of these systems, application of ab-initio methods becomes prohibitive, and approximate techniques have been implemented.

A three-center Coulombic over-barrier model is proposed for the process of multiple electron capture from homonuclear diatomic molecules in collisions with slow highly charged ions. The present model, based on the over-barrier model for atomic targets given by Niehaus, takes further into account the two-center nature of molecular targets by introducing a three-center Coulomb potential for respective electrons. Cross sections of multiple electron capture are calculated for A^q+ + N₂ (q = 6–10) collisions. The orientation effect of molecular axis relative to the collision plane is discussed.

We present molecular correlation diagrams for the study of H⁺ + Na_n collisions (n = 8, 20, 40, 92) at low impact velocities. We infer from these diagrams that, for a given velocity, electron capture cross sections increase monotonically with cluster size, whereas the energy deposited in the cluster remains practically constant.

C₆₀-molecules are multi-ionised by Xe²⁵⁺ projectiles at collision energies of 280 keV. The produced ions and fragments are analysed in coincidence with the charge state of the outgoing projectile ion. Intact molecular ions are formed in charge states at least up to q = 10; they are stable on a time scale of several µs. From the time-of-flight spectra, average fragment sizes and average kinetic energies are determined as a function of the number of electrons stabilised by the projectile. The kinetic energies of the atomic fragments are used to estimate the charge state of the parent ion C₆₀^q+. In the case of 11 stabilised electrons (Xe²⁵⁺ → Xe¹⁴⁺), about 50 electrons are removed from the fullerene cage, which is in agreement with recent experimental results (Bernard et al, this volume).

The asymmetric decay of multiply charged fullerene ions C₆₀^q+ in charge states q = 5–8 has been studied by analysing the correlation between different fragments. Binary processes as well as sequential emission of several neutral or charged small particles is observed. Particularly, we observe a delayed emission of C₂⁺ and C₄⁺ fragments on the µs-time scale, which is competing with evaporation processes. With increasing charge q the characteristic lifetime of the fullerene ions decreases slightly, and the emission of C₂⁺ becomes more important with respect to the C₂ evaporation.

Singly charged gold cluster anions in the size range n = 16–30 have been captured, stored and size selected in a Penning trap. After application of an electron beam doubly charged gold cluster anions have been observed for 20 ≤ n ≤ 30. To our knowledge this is the first observation of metal cluster dianions. The threshold appearance size is in good agreement with a simple charged sphere model. The application of argon gas pulses simultaneously with the electron beam is found to increase the production rate by an order of magnitude.

We present experimental results for angular differential cross sections, dσ/dθ, and mean translational energy gain, ΔE_mean, as functions of the number of electrons, s, stabilized on the projectile in 26.4 keV Ar⁸⁺ + C₆₀ → Ar^(8−s)+ + Cr₆₀^r+ + (r − s)e⁻ collisions. The most probable scattering angle increases linearly with s for s = 1–7, while processes leading to neutralization (s = 8) scatter more strongly than expected from this trend. The ΔE_mean-values are positive and increase with s between s = 1 and s = 2, are virtually constant up to s = 6, and finally decrease to negative values (energy loss) for s = 7. The experimental results are compared with Monte Carlo trajectory simulations for impact parameters ranging from small to large values.

Angular differential cross sections are presented for collisions between 26.4 keV Ar⁸⁺ ions and C₆₀ molecules. Using a classical overbarrier simulation the deflection function is calculated for large impact parameters. To compare with experimental data the theoretical cross section values are convoluted with an instrumental function. Both the theoretical and experimental cross sections are absolute in magnitude and position. The experimental cross section for retaining one electron on the projectile is qualitatively well described by the theoretical cross sections for initially capturing one to four electrons from the target.

In a recent study on O^q+ (1 ≤ q ≤ 7) collisions with C₆₀, we presented fragmentation patterns dominated by electron capture processes. We now investigate the remaining important interaction mechanisms: electronic excitation due to ion-electron interactions and vibrational excitation due to ion-nucleus interactions as well as the possible electronic-vibrational coupling. Using He^q+ (q = 1, 2) projectiles we observed a decrease of evaporation peaks (ascribed to vibrational excitation) with increasing collision energy (2 keV * q ≤ E_kin ≤ 100 keV * q) and an increase of small fragments C_n⁺ as well as parent ions C₆₀^q+ (attributed to electronic excitation). The additional potential energy carried by He²⁺ (with respect to He⁺) was found to cause an extra fragmentation and ionization respectively in the low and high projectile energy regimes.

Decay channels of multicharged fullerenes C₆₀^r+ (r = 3–6), produced in collisions of slow Ar⁸⁺ ions on C₆₀ targets have been studied. By using multi-coincidence technique between outgoing projectiles, recoil fragment ions and ejected electrons, we have observed evaporation of C₂ neutral fragments in low charge states (r = 3, 4), asymmetrical fission by the loss of one monocharged fragment for r = 3 to 6 and asymmetrical fission by the loss of more than one monocharged fragments for high charge states (r = 5, 6).

Interaction of slow highly charged ions (HCI) with a thin microcapillary foil was studied for the first time through measurements of visible light emitted from the ions downstream of the foil. The microcapillary was employed so that ions which capture electrons in high Rydberg states may not violently collide with the surface in spite of the image acceleration. The wavelength range measured was from 300 nm to 600 nm. Some strong lines were found attributable to Δn = 1 transitions resulting from a single electron capture into states with n_i ~ Q and with a large angular momentum.

We obtain within the orthogonalized plane wave approximation surface-plasmon mediated neutralization rates for hydrogenic ions and for several metallic surfaces. The effects of the electronic density and of the work function of the surfaces are analyzed. The collective neutralization rates increase with density while the slope of the exponential curve for the rates increases when the work function decreases. For charged (Z) hydrogenic ions we found that these collective transitions are allowed only for states with n = Z.

We analyze two recently discovered structures in the autoionization spectra of highly charged hydrogen-like and metastable helium-like ions (q ≤ 9) colliding at energies of a few eV with metal surfaces. As opposed to higher beam energies, the projectiles spend more time on the incoming path and bounce back shortly above the first bulk layer. Thus, the ions only transiently experience the efficient screening of the bulk electron gas which has severe implication for the interaction dynamics. We demonstrate that the experimentally observed shifts in peak shapes and intensities are closely linked to these effects.

On the basis of a refined classical overbarrier model, we simulate the evolution of level occupations of slow highly charged ions (HCIs) reflected by metal surfaces. Our calculation includes the full trajectory of the projectile and incorporates atomic structure calculations which provide accurate asymptotic projectile energy levels. We scrutinize effects related to proposed interaction mechanisms, such as peeling off atomic Rydberg levels and small-distance side feeding from the target valence band to inner projectile shells. The simultaneous evaluation of projectile energy gains, final charge state distributions, and the emission of projectile Auger electrons agrees well with experiments.

Using the multistep-capture-and-loss (MSCL) model a theoretical analysis for the K satellite spectra emitted from Ar hollow atoms formed in a solid observed by Briand et al is carried out. It is found from the simulation of the spectra with the model that the feature of the spectra is strongly dependent on the magnitude of the rate of electron capture from a solid by a projectile ion.

We report on studies of low energy multicharged ion-surface interactions carried out at the ORNL MIRF (Multicharged Ion Research Facility) with recently developed apparatus that implements a UHV floating scattering chamber into which beams can be decelerated to interaction energies as low as a few eV per charge. We present initial results, utilizing a TOF system with a biasable drift tube for charge state analysis, of measurements of backscattered projectile energy loss and charge state distributions of multicharged oxygen, and argon ions incident on Au(110), with energies down to a few 100 eV/q.

Slow (v ≈ 0.06 a.u.) highly charged Ar^q+ (6 ≤ q ≤ 13) are scattered from a prepared Au(111) surface. At large angle scattering geometry with an incident angle ψ = 25° and a scattered angle θ = 75° a double peak structure is observed in the energy spectrum. The two peaks correspond to two well defined scattering trajectories, where the ions undergo single or double collisions at one or two Au atoms at the surface, respectively. Previously it was reported that the yield of multiply charged scattered ions (MCSI) increases by nearly three orders of magnitude for single collisions when the Ar ions carry initial L-shell vacancies. This strong shell-effect is partly suppressed for ions experiencing double collisions, which indicates L-shell filling due to two collisions and a longer interaction time.

The energy loss and the mean number of secondary electrons are measured as a function of the projectile scattering angle for 10 keV O^q+ ions (q = 2–7) colliding on LiF target at grazing angle. These data are interpreted in terms of trajectory dependant properties, which are essentially governed by the neutralisation dynamics on the way in of the collision. The relative role of potential and kinetic emission is evaluated and discussed.

We have measured the electron emission statistics for Xe⁺ and Xe¹⁰⁺ ions incident on clean polycrystalline copper at grazing angles of 10 and 23. The probability of no electron emission has been measured. The experimental data for 2–10 keV Xe⁺ ions is well described by binomial distributions. For Xe⁺ and Xe¹⁰⁺ the total electron yield has been determined. The potential and kinetic emission components have been deduced.

We present accurate total electron yields from clean gold for impact of singly and multiply charged ions (C, N, O, Ne) for impact velocities from below 10⁵ up to 10⁶ m/s. Special interest has been devoted to a possible projectile-charge dependence of the respective kinetic electron emission.

The secondary electron emission yields from clean copper surfaces induced at normal incidence of low energy (2.4–125 eV/amu) rare gas ions have been measured in an ultra high vacuum chamber (~10^-11 Torr). It has been found that the yields increase with the ionic charge of the incident ions over the whole impact energy range investigated. It was also found that the kinetic emission was independent of the incident ion charge. As a further result, the potential emissions measured in the present study are smaller than those previously measured as well as those expected by some scaling laws.

The "Coulomb explosion" model has been frequently invoked to explain that HCI induced sputtering- and secondary ion emission yields for certain (insulating) target surfaces dramatically increase with projectile charge state. Based on available experimental results and new theoretical estimates we will present evidence that "Coulomb explosion" can be ruled out as a dominant mechanism for potential sputtering of insulating surfaces, whereas all experimental features are found to be consistent with a model involving defect-mediated desorption.

The desorption of negative secondary ions from vacuum evaporated layers of decacyclene and phenylalanine deposited on surface upon the impact of HCI is investigated by time-of-flight SIMS. The relative yield of small and large sputtered species is measured for different charge states. A special attention is brought to the desorption of the monomer anion.

We report on energy losses of KLL Auger electrons emitted during the interaction of highly charged ions with Si(100) and Al(111) surfaces when observed under grazing angle with respect to the surface. Describing the interaction of the outgoing electrons with the solid we derive estimates for the emission depths at the time of the KLM- and KLL- Auger decay of various H-like ions.

The transmission of highly charged ions (HCI) through microcapillaries is investigated. The interaction of an HCI with the internal surface of the capillary is described in terms of a classical image potential. For Ne^q+ projectiles transmitted through a Ni capillary the impact parameter distribution and angular distribution of ions leaving the microcapillary is determined. We consider both a narrowly focussed and a dispersive incident beam relative to the acceptance angle of the capillary. Our results are compared with recent experiments.

Interactions of relativistic clothed heavy ions with solid targets have been studied for 390 MeV/u Ar¹⁷⁺ ions traversing carbon foils of various thicknesses through measurements of the convoy electrons. The electron energy spectra at 0° were obtained by a magnetic analyzer combined with a Si surface barrier detector with a depletion layer of 5 mm. The peak widths of the convoy electrons in the laboratory frame have been found to decrease as the target thickness is increased from 25 to about 500 µg/cm², and broaden for thicker targets due to multiple scattering.

A relativistic quantum mechanical transport theory is formulated which describes the production of the binary encounter electrons and allows for energy loss and angular deflection during their subsequent penetration of the target. The theory is applied to 13.6 MeV/amu and 93 MeV/amu Ar¹⁸⁺ + C collisions and compared with experimental data.

A method of coupled integral equations for partial (charge selected) energy distributions of fast ions passing through matter suggested and successfully used earlier to treat evolution of the shape of angular non-resolved spectra as a function of the foil thickness is extended to calculate unified partial energy and angular distributions of ions passing through thin foils accompanied by multiple charge exchange processes between their different fractions.

We measured the energy loss of charge-frozen clothed ions (i.e., partially stripped ions which maintain their charge states throughout the passage) in a Si solid employing a totally depleted silicon detector with 4.1 µm thickness as a target, to measure the energy deposition of 390 MeV/u Ar¹⁷⁺ directly. Under the present conditions, the probability of the electron loss is not so large, and that of the electron capture is negligibly small. Accordingly, the energy loss of charge-frozen Ar¹⁷⁺ ions in a rather thick target can be measured. Charge states of ions transmitted through the target were observed by a 2D-position sensitive detector in coincidence with the energy deposition. We have obtained an effective charge of Ar¹⁷⁺ ions for the stopping power, as Z_eff = 17.36 (±0.03).

Characteristics of relativistic ion motion through a crystal near one of the crystal axes are discussed. The main features of such motion in a crystal are compared for protons and for highly charged ions. The possibility of bending of relativistic ion beams due to their multiple scattering on atomic strings in a bent crystal is discussed.

A crossed-beams technique has been used to measure total cross sections for charge transfer in ³He²⁺ + X^q+ collisions (X^q+ = N⁴⁺, O⁵⁺) at barycentric energies between 8 keV and 200 keV. The cross section for charge transfer in He²⁺ + N⁴⁺ collisions is calculated within the molecular model and shows good agreement with the experimental data.

Single electron detachment processes from negative hydrogen ions under collisions with MeV/u highly charged ions have been investigated using the so-called crossed-beams technique. The preliminary results of the single-electron detachment cross sections obtained is found to be in crude agreement with some empirical and theoretical estimations.

The neutralization of negative hydrogen ions in collisions with fast (including relativistic velocities) highly charged projectiles is considered by using a simple approach resulting in analytical cross sections for the range of parameters where the Born approximation is invalid. A formula has been derived for the cross section of H⁻ neutralization.

Electron excitation, transfer and ionization in collisions between alpha particles and lithium-like ions are investigated by means of the Basis Generator Method (BGM), a specific coupled-channel approach to the time-dependent Schrödinger equation (TDSE). The gist of the BGM is the representation of the electronic wavefunction within a dynamically adapted finite model space including the physically accessible part of the electronic continuum. Theoretical results for total capture are in good agreement with recent experimental data.

A crossed-beams energy-loss spectrometer has been used to investigate angular distributions for electron scattering from Ar²⁺ and Ar³⁺ ions, at a collision energy of 16 eV. Results are compared with the predictions of a partial waves calculation based on a semi-empirical potential, and with the classical Rutherford formula.

Using a crossed beam method elastically scattered electrons from Ar⁷⁺ were observed at an electron energy of 100 eV. The scattered electrons are analyzed simultaneously both in energy and angular dispersion by using a toroidal electron analyzer with a two-dimensional position sensitive electron detector.

Bremsstrahlung energy losses of electrons scattered by atomic ions, including a pure Coulombic case, are obtained for low relativistic electron scattering energies. Analytical calculations are compared with those calculated numerically in the relativistic partial wave approximation.

The alignment of the 2p⁵3s ³P₁ level of neon-like germanium ion excited by impact with an unpolarized electron beam has been theoretically studied. For high electron-beam densities, 10¹⁹–10²¹ cm^-3, we have calculated the relative populations of the 2p⁵3s ³P₁M_J = 0 and |M_J| = 1 magnetic sublevels allowing for direct collisional excitation from the 2p⁶¹S₀ ground level as well as collisional de-excitations from the 2p⁵3p ¹S₀, ¹D₂ levels. The collision strengths for the transitions between magnetic sublevels were obtained in intermediate coupling using the University-College-London distorted-wave code. The calculations were carried out for impact electron energies from 155 to 750 Ry. It is shown that the collisional de-excitation effects result in a strong decrease of the degree of alignment and can even lead to a change of sign. The reported results might have useful applications in the modeling and diagnostics of hot and dense plasmas in which beams of energetic electrons are generated.

Spectral emissions from C³⁺ are of importance in solar, planetary and astrophysical plasmas. In these plasmas the population of excited states is often dominated by electron impact excitation. Accurate cross sections for this process are required to determine parameters such as electron and ion densities. The 2s ²S–2p ²P transition in C³⁺ is of particular interest as it allows direct comparisons among the results of several different experimental and theoretical groups. Employing the electron energy-loss technique we have measured cross sections to several eV above threshold, a region where there is still significant disagreement among previous results.

Absolute total cross sections for excitation of the Be-like ions C²⁺, N³⁺ and O⁴⁺ at electron energies close to the thresholds have been measured. For all three ions, cross sections for the resonance transition 2s²¹S → 2s2p ¹P were determined. Theoretical cross sections calculated using close-coupling approximations are in reasonably good agreement with the measured cross sections. Some data were also obtained for transitions from the metastable 2s2p ³P states of C²⁺ and O⁴⁺.

Absolute cross sections σ_q,q+n for the multiple ionization of C^q+, N^q+ and O^q+ ions were measured for the reactions: e + A^q+ → A^(q+n)+ + (n + 1)e. The measurements were performed for electron energies up to 6.5 keV using the animated crossed-beams method at the Giessen electron-ion crossed-beams set-up. K-shell ionization with subsequent autoionization or even double-autoionization strongly contributes to the cross sections.

Electron impact ionization cross sections σ_q,q+n for gallium ions have been measured for the reaction e + Ga^q+ → Ga^(q+n)+ + (n + 1)e for single (n = 1, q = 1, 3, 8) and triple (n = 3, q = 3, 4) ionization. The measurements have been performed using the crossed-beams technique in an energy range from the respective ionization threshold up to 1 keV, partly up to 6 keV.

The cross sections for single ionization of ions in the charge state q = 1 and 3 show contributions below the ground state threshold caused by ionization of ions in excited, long-lived states in the parent ion beam. The cross sections are compared with the semiempirical Lotz formula.

The cross section for Ga⁴⁺ triple ionization shows contributions from inner-shell processes. Semiempirical formulae are in good agreement with the experimental data only at energies below the cross section maximum.

Experiments in which the time evolution of the charge distribution of Argon ions in the Tokyo EBIT was measured have been carried out. These measurements involved extracting the ions from the trap after a controlled confinement time and determining the charge distribution of these ions. By repeating the procedure at various confinement times the onsets for ions of particular charge states were determined at a series of electron beam energies. This sort of data should permit electron impact ionisation cross-sections to be determined as well as providing interesting information relating to EBIT physics.

Absolute cross-section measurements are reported for electron impact ionization of Ni¹²⁺ and Ni¹⁴⁺ in an energy range from threshold to about 5.5 keV. For single ionization, data show that the first observed threshold corresponds to the metastable state [Ne]3s²3p⁴¹S₀ which is located 12 eV above the ground state. Excitation-autoionization is seen to play a dominant role for this reaction in the whole energy range. Double ionization is seen to be dominated by L-shell ionization followed by total autoionization. For Ni¹²⁺, triple ionization is found to be two orders of magnitude lower than double ionization and is attributed to direct ionization only.

Recently we proposed a new semiempirical analytical parametrization for the shape of ionization cross sections resulting from collisions of ions with various projectiles [1]. It was shown that the incorporation of the correct threshold behaviour as given by the Wannier theory is the crucial point to obtain good agreement with experimental results from threshold to high energies and leads to a unified view of collision processes in very different systems. While the shape of the cross sections as given by this parametrization already compares favourably with the experiment, a simple extension of the formula leads to an even more accurate description for electron-impact ionization of ions [2]. Using recent results from an extended Wannier theory for multiple ionization [3,4], tests of similar parametrizations for multiple ionization processes become feasible.

Radiative and dielectronic recombination of F⁶⁺ have been studied at the heavy ion storage ring TSR in Heidelberg. For a detailed investigation of rate enhancement effects at very low electron-ion center-of-mass energies experimental parameters such as the magnetic guiding field, the electron density and the adiabatic expansion factor of the electron beam have been varied systematically. Whereas measurements at different electron densities show no influence on the enhancement and while a variation of the expansion factor evokes the predicted behaviour, we see an increase of the enhancement with increasing axial magnetic field between 20 mT and 70 mT.

In the frame of quantum defect theory, the dielectronic recombination cross sections of Ca¹⁹⁺ ion are calculated by a simple relativistic configuration interaction method. Based on this calculation, the resonant transfer and excitation cross sections of Ca¹⁹⁺ with H₂ and He targets are estimated in impulse approximation. The theoretical results for Ca¹⁹⁺ + H₂ system are compared with the measurements available. The effect of target electron momentum distribution on the resonant transfer and excitation cross sections is discussed.

We have performed x-ray spectroscopic measurements of the dielectronic recombination (DR) resonance strengths for the KLn (n = 2, ..., 5) series of He-, Li-, and Be-like krypton ions. The ions were produced with an electron beam ion trap, and the strengths were obtained from a fit procedure that compares the experimental excitation function for DR to theory. The results agree well with the predictions. By looking at the KLL resonance, the time evolution of different krypton charge states was measured with this technique and compared with a model of the trap inventory.

The enhancement of dielectronic recombination of multiply charged ions by external electric fields has been studied under controlled conditions. The heavy ion storage rings CRYRING at Stockholm University and the heavy ion storage ring TSR at the Max-Planck-Institut in Heidelberg were used to store beams of Si¹¹⁺ (ion energy 280 MeV) and Cl¹⁴⁺ (ion energies 250 MeV and 110 MeV), respectively. Recombination in the electron cooler has been measured over energy ranges covering all resonances due to 2s → 2p core exitation. External electric fields up to 183 V/cm at CRYRING and up to 379 V/cm at the TSR have been applied. In the TSR experiment the influence of different magnetic guiding fields as well as the influence of different electron densities in the electron cooler on the recombination rate was investigated. A significant rate enhancement was found for 1s²2pnl Rydberg resonances with n > 25 for Si¹¹⁺ ions and n > 20 for Cl¹⁴⁺ ions.

The influence of plasma effects on three-body recombination of bare ions with electrons in cold plasma in the electron cooler is discussed in context of recombination "enhancement" observed in storage ring experiments. We show that for high-Z bare ions and low electron temperatures and densities the cooler plasma becomes "nonideal", leading to the enhancement of the three-body recombination rates. This effect is described in terms of the Debye screening length within the "rigid shift" approximation. We demonstrate, that in cold (T ≈ 1–10 K) anisotropic plasma the screening effect substantially enhances the three-body recombination rates for very high n-states. The relaxation of high Rydberg states below the field ionization cut-off, set in storage ring experiments, is discussed. The calculations are confronted with the experimental results obtained in storage ring experiments.

Using an electron beam ion trap, we have measured the relative cross sections for Fe XXIV line emission at electron energies between 0.7 and 3.0 keV. Good agreements with distorted wave and R-matrix calculations are found at energies above 1.5 keV. At lower energies, the contributions of resonant excitation are observed and agree with R-matrix calculations. Below the excitation thresholds, the intensities of dielectronic recombination satellites for capture into n ≥ 5 levels are measured.

Using the Lawrence-Livermore electron beam ion trap (LLNL-EBIT), we produce a quasi-Maxwellian plasma by sweeping the energy of the nearly monoenergetic beam so the time spent an any energy is proportional to the Maxwell-Boltzmann probability at that energy. To verify the accuracy of the quasi-Maxwellian, we measure line emission due to dielectronic recombination (DR) and electron impact excitation (EIE) of Mg¹⁰⁺ and Ne⁸⁺, for a range of simulated temperatures. The ratio of DR to EIE lines in heliumlike ions is a well understood temperature diagnostic. The spectroscopically inferred temperatures are in excellent agreement with the simulated temperatures.

In search for interference between radiative and dielectronic recombination (RR and DR) absolute recombination rate coefficients for Ar-like Sc³⁺ and Ti⁴⁺ ions have been measured at the Heidelberg heavy ion storage ring TSR. Whereas the Sc³⁺ experiment suffers from statistics too poor for a line shape analysis, the Ti⁴⁺ recombination spectrum clearly does not exhibit asymmetric lineshapes due to quantum mechanical interferences as recently predicted theoretically for Sc³⁺ ions. The broad Ti³⁺(3s²3p⁵3d²²F) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero center-of-mass (c. m.) energy, such that its width is larger than its distance to threshold. At zero c. m. energy an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed.

Recombination of Au²⁵⁺-ions has been investigated in a single-pass merged-beams experiment at the UNILAC of GSI in Darmstadt. Very low energies in the electron-ion center-of-mass frame were particularly addressed. At E_rel = 0 eV we found a recombination rate exceeding the expectations by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

Dielectronic recombination and radiative recombination have been studied for several lithium-like ions (Au⁷⁶⁺, Pb⁷⁹⁺, Bi⁸⁰⁺ and U⁸⁹⁺) as well as for hydrogen-like Bi⁸²⁺ and completely stripped Bi⁸³⁺. Experiments were performed at the electron cooler of the storage ring ESR of the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. In the dielectronic recombination spectra analyzed so far for the lithium-like ions, 2s_1/2 → 2p_1/2 excitations with capture of the free electron into Rydberg states with n ≥ 20 and 2s_1/2 → 2p_3/2 excitations with capture into n = 5 (uranium) and n = 6 (gold, lead, bismuth) are observed. These measurements provide a stringent test of atomic structure and recombination theory for high-Z heavy-ions. In particular, the extrapolation of the 2p_1/2nl_j sequence (n ≥ 20) to the series limit renders possible to extract the 2s_1/2 → 2p_1/2 Lamb-shift in lithium-like high-Z ions at an expected accuracy level which can test QED theory.

Dielectronic recombination cross sections for Li-like Au ions are calculated and compared with experimental results. Good agreement is obtained for the capture to n = 6 and n = 20 levels lying in the range of electron energies of 8–60 eV.

The influence of quantum electrodynamics (QED) to the process of radiative electron capture (REC) is investigated in the region of highly relativistic projectile energies by modifying the wave functions according to the Uehling potential. Exact relativistic calculations are employed for the radiative recombination. The deviation from non-QED calculations is found to amount to up to 2% in U⁹²⁺ for a projectile energy of 10 GeV/u and forward emission angles of the emitted photon.

The nonrelativistic dipole approximation (Stobbe theory) for the atomic photoelectric effect and its inverse reaction, radiative recombination, is contrasted with an exact relativistic formulation taking into account relativistic electron motion and all multipole orders for the interaction with the photon. A comparison is performed for total cross sections as a function of energy and nuclear charge as well as for angular distributions. It is found that even for small velocities of the continuum electron, discrepancies between both approaches persist for high nuclear charges.

A Monte Carlo simulation program of x-ray spectra produced in collisions of fast ions with a gas-jet target of a storage ring is presented. The calculated spectra are compared with experimental data obtained at the ESR storage ring of GSI in Darmstadt.

We have measured complete doubly differential cross sections for electron emission coincident with the charge state of the outgoing recoil ion for strongly perturbing collisions of heavy ions F⁹⁺, I²³⁺ with He, Ne and Ar.

We measured energy spectra of electrons emitted from sputter-cleaned amorphous carbon targets bombarded with swift ions of constant velocity (9 MeV/u, v_P = 19v₀) with a new UHV set-up at the heavy ion accelerator GANIL. The charge of the projectile q_p varied from 6 to 45. At large electron energy, a q_p² scaling law is observed. For energies below the 1s ionisation threshold, strong reduction effects with increasing q_p occur. A carbon Auger hypersatellite line due to double ionisation of the K shell appears for high projectile charge only. From these Auger spectra, the ratio of double and single ionisation was deduced as a function of q_p and found to increase from a threshold up to a saturation.

We have investigated the energy and angular distributions of the low energy electron emission in a pure three-body ion-atom collision involving atomic hydrogen as target. The double differential ionization cross sections have been measured for C⁶⁺ + H (v = 6–10 a.u.). The CDW-EIS calculations provide an excellent agreement with the data except some discrepancies in the backward angles. These observations clearly show that the two center mechanism plays a major role in emission of low energy electrons.

In collisions of fast and highly charged ions with neutral atoms the ionization of the target atom is one of the most probable reactions. Therefore the energy and angular distributions of the emitted electrons have been investigated for more than three decades. But up to now there are no reliable experimental techniques for measuring electrons with energies below E_e ≤ 100 meV. Over the last years we have developed a new kind of electron momentum analyser which overcomes the restrictions of the conventional electron spectrometers in the low energy regime. In combination of this novel electron spectrometer with a high resolution recoil ion momentum spectrometer we have performed a kinematically complete experiment investigating the single ionization of Helium by 3.6 MeV/u Au⁵³⁺ ions. The longitudinal momentum distribution of the low energy electrons is strongly asymmetric in the forward-backward direction due to the interaction with the long reaching coulomb potential of the outgoing highly charged projectile ion. For the ultra-low energy electrons the momentum distribution shows a sharp maximum at p_e = 0.

Double differential cross sections in the 20–550 eV energy range and in the full angular range of 0°–180° for electron emission were measured by the impact of 150 keV/u C⁺ ions on He and Ne atoms. An unexpected, broad structure around 300 eV electron energy has been observed at backward emission angles relative to the beam direction. Our CTMC calculations support the hypothesis that the new structure is due to double scattering of the target electrons on the screened fields of the projectile and the target. According to the present impact-parameter Born calculations, the average degree of ionization is about 50% for C⁺+ Ne collisions, i.e., a multiple ionized system is created in the collision.

Double differential emission cross sections d²σ/dE_edΩ_e (DDCS) of electron emission in collisions of highly charged and partially stripped ions with gas targets deviate dramatically from the predictions of simple theories like the First Born and Binary Encounter (BE) Approximation (C Kelbch et al 1992 Z. Phys. D 22 713 and P Richard et al 1990 J. Phys. B 23 L213). In our experiment the electron emission cross sections were investigated in collisions of partially stripped Bismuth ions with Helium gas targets at projectile energies in the range 1.4 ≤ E_P ≤ 6.0 MeV/u. DDCS of electron emission have been measured as a function of the electron emission energy E_e and the observation angle ϑ_e. The data show systematic enhancement of the BE electron emission for all projectile energies. For the lowest projectile energy the predicted diffraction structures in the Binary Encounter peak were observed at angles between 35° and 45°.

The impact parameter version of the four body-symmetric Eikonal approximation (SE2) is introduced to study double excitation of dielectronic atoms by heavy ions at high impact energies. The total cross sections are calculated for double excitation to the 2p² (¹D) state and the sum of the 2p² (¹D) and 2s2p(¹P) states of helium atoms by impact of protons, C^q+ (q = 4–6) and F^q+ (q = 7–9) ions. Theoretical results are compared with measurements and other existing calculations.

Hydrogen ionization in soft collisions with relativistic charged projectiles is considered. Analytical expressions have been obtained for differential cross sections of hydrogen ionization accompanied by the emission of slow electrons. An asymmetry in the angular distribution of soft electrons is found which can be interpreted as a result of two effects: i) an absorption of virtual quantum momenta by the atomic system, ii) so-called "post collision interaction".

A complete momentum-space map of the double ionisation events in the collision of 100 MeV/u C⁶⁺ ions with helium has been obtained. From this map the angular distribution of two ejected electrons is generated as a function of the momentum transferred by the projectile to the atom. Analysis of the angular distribution of the fragments in the plane transverse to the projectile axis (the azimuthal plane) shows a separation of events into two domains, depending on the momentum transferred by the projectile to the target. For momentum transfers smaller than 1.2 a.u., both electrons are distributed independent of the azimuthal angle. For momentum transfers larger than 1.2 a.u., the electron with larger energy is distinctly emitted along the direction of momentum transfer, and the one with smaller energy is distributed isotropically. The value of 1.2 a.u. is approximately equal to the mean value of the momenta of the bound electrons in the helium atom and demarcates indirect, soft collisions and direct, hard collisions respectively. For soft collisions, the electron angular distribution shows certain similarities with the angular distributions in photo-double ionization.

Single and double ionization of helium and neon by 1 GeV/u U⁹²⁺ ion impact was explored in a kinematically complete experiment using multi-electron recoil-ion momentum spectroscopy. Cross sections, the emission characteristics of ions and electrons as well as the momentum balance are quantitatively discussed in terms of photoionization of the atom in the field of virtual photons generated by the projectile (Weizsäcker-Williams 1934). Electron-electron correlation after double ionization of neon as well as a first result from the recoil-ion momentum spectrometer at the ESR of GSI are presented.

We have used the energy and angular distribution of the low energy electron emission to derive the doubly differential final-state longitudinal momentum distributions of the recoil-ions in ion-atom ionization. The complementary nature of the electron spectroscopy and the recoil-ion momentum spectroscopy has been investigated using a formulation based on three body kinematics. The separation of the three-body and binary collision branches of recoil-ion distributions is a novel feature of the present technique.

State-selective single-electron capture processes for collisions of Ne^q+ (q = 3 and 4) recoil ions with N₂ and O₂ targets have been studied experimentally by means of translational energy-gain spectroscopy technique. The energy-gain spectra were measured on a differential energy-gain spectrometer at impact energies of 1.25 qeV/u and scattering angles between 0° and 8°. For the charge state q = 3, the spectra show that the dominant reaction channels are due to capture into n = 3 states. Weaker processes are also observed due to electron capture accompanied by excitation of the target products and metastable states of Ne³⁺ ions. For q = 4, the capture occurs mainly into 3d and 3d', respectively, for the targets N₂ and O₂.

State-selective one-electron capture involving pure beams of ³P ground state and ¹D metastable O²⁺ ions has been investigated by Double Translational Energy Spectroscopy. Measurements have been made in which the relative importance of the excited product channels are established without the ambiguity inherent in earlier Translational Energy Spectroscopy measurements.

The Kr⁸⁺-Li(2s) collision has been studied by photon spectroscopy in the near UV and visible wavelength range (200–600 nm). A particular attention has been paid to the polarisation degree of each observed line in order to determine the behaviour of m-sublevel distribution versus energy collision.

A high resolution cold target recoil-ion momentum spectroscopy (COLTRIMS), capable of measuring simultaneously the longitudinal and the transverse recoil momenta, has been used for the study of state-selective single- and double-electron capture by C⁵⁺ ions from He at laboratory impact energies between 0.5 and 50 keV/u. The longitudinal momentum spectra for single-electron capture indicate that the dominant reaction channel observed is due to capture into the n = 3 state of the product C⁴⁺ ions. This reaction channel contributes mainly to the total cross section for single-electron capture, in agreement with the close-coupling calculations. At a collision energy of 35 keV/u, the spectrum for the autoionizing double-electron capture illustrates that capture into the asymmetrical C³⁺(2l, n'l') states (n' = 3 to ∞) dominates, while capture into the symmetrical C³⁺(2l, 2l') and C³⁺(3l, 3l') states show less contributions. True double-electron capture occurs predominantly into the asymmetrical C³⁺(2l, n'l') states with n' = 3 to ∞.

We have measured the degree of polarization of the emission line corresponding to the 1s²2s6h–1s²2s7i transition of O⁴⁺ ions produced by the double electron transfer in the collisions of O⁶⁺ ions with He, Ne, Ar, Kr, Xe, H₂, N₂ and O₂. For all targets, the polarization degrees for this line have positive values in the collision energy range of 60–120 keV. This result indicates that the excited states produced in the double electron capture reactions are strongly aligned to the M_L = 0 magnetic substate as in the case of most single electron capture process. However, the values for He and H₂, both of which are two electron system, are slightly larger than those for others which have more electrons. It is possible that a certain multiple electron process has an influence on the alignment of this excited state.

Using high resolution electron spectroscopy, positions and lifetimes of many Be-like singlet states of the 1s²3lnl' Rydberg series (n = 3 to 5) of oxygen and neon have been measured for the first time. This was achieved by a fitting procedure which takes into account an accurate definition of the post-collisional electron lineshapes. These states are produced after a double electron capture by multicharged ions has occurred in O⁶⁺(1s²) + He, H₂ and Ne⁸⁺(1s²) + He collisions at about 4 keV/amu collision energy.

Electron Capture and transfer ionization by Ar^(8–12)+ in collisions with argon atoms were studied by using the position sensitive and time-of-flight techniques. Some of the measured projectile charge changing cross sections (σ_q,q−k) and phenomenological cross sections (σ^r_q,q−k) are reported in this paper. The measured cross sections are compared with the predictions of the molecular Coulombic overbarrier model by considering the autoionization decay and electron evaporation of multiply excited states in projectiles and recoil ions respectively.

In order to study the charge transfer processes between highly charged ions and atoms or molecules in the collisional energy range of below 100eV/q, an experimental setup for the state selective differential cross section measurement of multi-electron capture processes was prepared. The apparatus is set at the 30° beam line of the RIKEN 14.5 GHz Caprice ECR ion source. Highly charged ions extracted from the ion source at 2keV/q into the beam line, are decelerated down to 10–100 eV/q and then energy selected by a double hemispherical analyzer. A typical current of 300 pA was achieved for the energy selected 40 eV/q Ar⁶⁺ beam with an energy width of 0.3 eV/q. The energy distribution of scattered ions for 36.6 eV/q (7.3 eV/u) Ar⁸⁺-N₂ collisions at scattering angles from 0° to 10° has been measured.

The heavy-ion storage ring ESR provides a unique possibility to decelerate highly charged ions up to bare uranium to energies which are far below the energy required for the efficient production of high charge states. This technique allows for a detailed study of atomic collision dynamics and, in particular, for an (n,l,j)-sensitive investigation of electron capture processes by means of x-ray spectroscopy. The results of a first state selective capture investigation performed for decelerated bare uranium ion are discussed and their relevance for Lamb shift investigations on high-Z hydrogenlike ions is emphasized.

Single K-shell excitation and double-K-shell-vacancy production in atomic Li by fast N⁷⁺ (10.6 MeV/u) and Ar¹⁸⁺ (95 MeV/u) projectiles have been investigated. Auger electron emission, occurring in the energy range 50–100 eV and resulting from the deexcitation of singly- or doubly-K-shell excited states, respectively, was measured for various emission angles in the range 20°–160°. For incident Ar¹⁸⁺, high-resolution spectra were also obtained, thereby enabling the study of alignment effects for the specific Li configurations produced in the excitation process. In the case of double-K-shell-vacancy production, the two K vacancies are produced mainly by K-shell ionization plus K-shell excitation. Electron-electron effects are important in producing these latter double-K-shell-vacancy events, as well as three-electron transitions.

In this paper, we present our study on the K shell electron loss of hydrogen-like O and F ions by C atom in the impact energy range 20 MeV to 100 MeV. The PWBA calculations for electron loss neglecting screening by the target electrons are in good agreement with the measured cross-sections at high impact energy. Large deviations are seen in the measured loss cross-sections at intermediate impact energy and this is attributed to capture ionization (CI) which is important in nearly symmetric collision systems.

Mean ionization probabilities of the outer L- and M-shells in the projectiles (p_L and p_M, respectively), associated with the K-vacancy production in the collisions Fe, Co, Ni, and Cu + Au at 0.1–1.5 MeV/u bombarding energy, are reported. They have been obtained by using the energy and yield shift method for the K x-ray lines. An interesting behavior of p_L at lower (0.1–0.5 MeV/u) energies, which suggests non-equilibrium L-shell vacancies, has been observed.

Experimental data of inner-shell ionization cross sections and outer-shell ionization probabilities per electron, obtained for the collision system. Cu + Bi in the energy range of 0.1–1.625 MeV/u, are in good agreement with the predictions of the statistical model of Mittleman and Wilets. The found dependence of the diffusion constant determined for different final vacancy states on the excitation (ionization) energy is in fair agreement with the model prediction.

We have measured subshell resolved L-K electron transfer cross sections from the L subshell of the Yb target to the vacant K-shell of the Si projectile at energies varying between 2–4.5 MeV/A. In addition, the L subshell ionization cross sections are also obtained for Yb. It is observed that contrary to expectations, the L₂ subshell electron transfer as well as ionization cross sections are substantially lower than the corresponding cross sections for the L₁ subshell.

In contrast to the first Born approximation predicted quadratic enhancement, saturation has been observed in the excitation cross-sections of 120 MeV S¹⁴⁺ ions in interaction with gas targets. The measured cross-sections agree with the theoretical calculations in the framework of the Schwinger variational principle. The symmetric eikonal CDW approximation, however, underestimates the cross-sections with the heavier target atoms.

Calculations of projectile K-shell electron excitation cross sections for He-like ions during ion-atom collisions have been performed in the distortion approximation by the use of Herman-Skillman wave functions. The calculated results are compared with the experimental data for several targets. The excitation cross sections deviate from the first-Born approximation and show the saturation effect as a function of target atomic number. This effect can be explained as the distortion of the projectile electronic states by the target nucleus.

The electron loss of multiply-charged dressed ions by heavy neutral atoms can have a significant contribution from collisions with small impact parameters. This can render one of the two competing mechanisms which govern the electron loss, i.e. the screening, highly non-perturbative. The other mechanism (antiscreening) is due to electron-electron interactions and its contribution can be treated perturbatively. The dependence of the total electron loss cross sections on the target atomic number, Z₂, presents a strong saturation as the value of Z₂ increases. Calculations based on the Plane Wave Born Approximation present such a behavior for the antiscreening but not for the screening, since this saturation is related to a non-perturbative regime. In this work we compare data for the total electron loss cross sections of C³⁺ and O⁵⁺ ions by H, He, Ne, Ar, Kr and Xe targets, with energies ranging from 1.0 to 3.5 MeV, with calculations for the screening contribution based on the free-collision model. This comparison shows that, for highly-charged ions, the electron capture and direct target ionization channels play a major role in the description of experimental electron loss data.

The L₂₃–MM spectrum of argon emitted after 1s photoionization represents a superposition of several vacancy satellite spectra. They are separated by means of electron-ion coincidence spectroscopy. Excellent agreement of the gross spectral structure is found with a calculation in relativistic single configuration average approximation. With the aid of data from the literature we identify sequentially emitted L₂₃[L₂₃]–MM[L₂₃] and L₂₃[M²]–MM[M²] lines and locate intermediate 2p⁵3p⁴ levels. Elaborate multiconfiguration Dirac-Fock calculations of transition energies and intensities have been performed for these two spectra. Their results show very good agreement with observation, thereby confirming and extending the experimental analysis. Other lines are shown to be known from the well investigated L₂₃–MM and L₂₃[M]–MM[M] spectra. Virtually all recognizable lines of the densely packed cascade spectrum are assigned.

A method is presented to measure separate cross sections for collisional deexcitation of metastable ions via radiative and non-radiative processes. The principle of the experiment is to first determine the total collisional deexcitation cross section in an attenuation-like measurement. After this the non-radiative part is determined separately in a measurement where the ionized target atom (unique to the non-radiative process) is detected. Here we discuss recently published results on deexcitation of metastable He⁺(2s) ions colliding with Ar at 1.65 keV/amu (H T Schmidt et al 1998 Phys. Rev. A 57 R4082 and 1998 Phys. Rev. A 58) as well as preliminary results for a Xe target. In both cases we find that the dominating contribution from radiative deexcitation agrees with the result of a semi-classical calculation of the 2s-2p mixing driven by the induced dipole field of the target atom modified by taking competing processes, for which we measured the cross sections, into account. Finally the extension to H-like projectile ions in higher charge states is discussed.

The technique of cold-target recoil-ion momentum spectroscopy is used to study the process of ionization in slow to intermediate-velocity collisions. Some evidence for the role of the quasi-molecular promotion in the ionization process is presented. The two collision systems He⁺–He and Ne⁺–Ne are chosen for illustration.

A four-body classical trajectory Monte Carlo method and a simple binary encounter model was applied in the study of K-shell vacancy production in N⁷⁺ and Ti collisions in the impact velocity range 12–30 a.u. The total cross sections of the single and double K-shell vacancy productions are calculated and the ratios between them are compared with experimental data. Independent electron approximations fail to reproduce the measured double/single vacancy production ratios.

We present analytical calculations of transition matrices in ion-atom ionizing collisions. The final correlated state is represented by a series expansion in terms of two-body Coulombic wave functions. We study the particular case when this series is the multivariable confluent hypergeometric function Φ₂. Transition matrices are also represented by a series using undistorted and distorted wave approaches for the initial state. We show that this series is strongly convergent and analyze the contribution of their different terms to the double differential cross sections within the undistorted approach.

The analytical properties of three body Coulomb functions are discussed through the introduction of complex momenta of the electron relative to the heavy ions. We analyze the characteristics of the energies thresholds and the behavior of the density of states when E = 0. We make use of the well known C3 wave function and the recently proposed Φ₂ state. The analytical continuation of the momenta in both models enables us to obtain bound-continuum states. We show that the Φ₂ bound state derived with this procedure is the same as in the C3 model for high impact energies, and resembles a bound state in the continuum in the intermediate energy regime.

A quantum mechanical approach to time-dependent many-electron problems, which relies on an effective single-particle picture, is applied to the collision system F⁹⁺ + Ne at 1 MeV/amu. We present an analysis of multiple-electron capture and ionization and discuss the effects of dynamic screening and autoionization processes.

The probabilities and cross sections for the 1s²¹S → 2s²¹S and 1s²¹S → 2s2p ¹P transitions induced in collisions of fast charged projectiles with helium atoms have been investigated in the framework of the operator approach to the few-body problem (V A Sidorovich, Physica Scripta, accepted for publication). Calculations have been performed for the case of the 2s²¹S excitation of helium by projectiles with Z_p varying from 3 to 9 at collision energy E_p = 1.5 MeV/u. The calculated cross sections are in satisfactory agreement with the available experimental data. The cross section dependence upon the projectile charge is established. The wavefunctions for few-electron atom, satisfying the boundary condition U = 0 (U is the interconfiguration interaction potential) in the past or in the future, and the closure relation for joint system functions are presented.

Using a classical treatment we calculate ionization and capture cross sections for stripped ions A^q+ (q = 1, ..., 8) colliding with H, H₂. For the diatomic target, we employ an independent particle model with a single center effective hamiltonian. An initial hydrogenic distribution is employed. The impact energy range is 6 keV < E < 30 MeV, and for conciseness, we only compare our data to experiment.

The electron capture reaction from hydrogenic targets by heavy bare ions is studied. From well known models, such as the Continuum Distorted Wave and the Boundary Born Series, we develop a new approximation within the distorted wave formalism. In the entry channel the initial distorted wave function is chosen as in the first order of the Boundary Born Series whereas in the exit channel, the final distorted wavefunction is taken as in the CDW model. Single differential and total cross sections are evaluated and compared with other theories as well as with experimental data.

The formation of excited states via Coulomb excitation in relativistic ion-atom collisions can be studied for high-Z ions by the observation of the radiative decay of the excited levels to the ground state. This has been demonstrated in a first study, recently performed for H- and He-like Bi ions [1]. However, in that study solid targets were used and the results suffered from low counting statistics. Here, we report an extension of these investigations performed at the gas-jet target of the ESR storage ring where the K-shell excitation has been measured for H-like Au ions in collisions with gaseous Ar atoms.

The simultaneous excitation-ionization and excitation-capture processes in collisions of 223 MeV/u He-like uranium ions with heavy target atoms are identified by measuring the x-ray (L → K) transitions in coincidence with projectiles undergoing charge exchange. The state selective cross sections for the population of the L-shell sub-levels of the projectile are determined, and in the case of excitation-ionization data compared to the detailed relativistic calculations. It is emphasized that the experimental data can serve as an important benchmark of the independent particle approximation and fully relativistic perturbative models, for the domain of relativistic ion-atom collisions.

A general expression for the cross section of the inelastic collision of relativistic highly charged ions with heavy (relativistic) atoms is obtained using the generalized eikonal approximation. In the ultrarelativistic limit the obtained formula coincides with the known exact one and has the standard nonrelativistic limit. As an application of the obtained result probabilities and cross sections of single and double K-vacancy production in U⁹²⁺ – U⁹¹⁺, U⁹⁰⁺ collisions are calculated.

The investigation of quasimolecular x-rays from superheavy collision systems with bare or H-like projectiles seems to be a promising approach to get information about the behaviour of inner shell electrons with energy eigenvalues in the vicinity of the negative continuum. We present calculations of the MO x-ray spectra for the system U⁹²⁺-Pb for varying impact energies. Furthermore we analyse the contributions due to electrons in higher lying states. The results are discussed with respect to the experimental determination of the transition energies in the superheavy quasimolecule.

Probabilities for electron-positron pair production with capture of the electron into the 1s_1/2 state in U⁹²⁺–U⁹²⁺ collision at 0.47 MeV/N are calculated with the finite-element method. Using the time reversal symmetry we obtain these probabilities by developing the U⁹¹⁺(ls_1/2) state in time on a three-dimensional finite element grid and projecting on the target states of the negative continuum.

The production of fermion pairs in ultrarelativistic heavy-ion collisions has attracted renewed attention in the recent past. This was in part inspired by the observation that at ultrarelativistic energies the underlying theory simplifies greatly. We briefly review the foundations for these simplifications and state the pair production amplitude exact in all orders perturbation theory in the ultrarelativistic limit. The resulting impact parameter dependent cross section is derived in the Weizsäcker-Williams approximation which is extended to include Coulomb corrections. The cross section shows deviations from the Born result, giving an enlarged total cross section for peripheral collisions. Accounting for the nuclear form factor we find an additional enhancement.

The experimental determination of the magnetic moment (g-factor) of the electron bound in hydrogen-like ions represents a clean test of Quantum Electrodynamics, because it is not very sensitive to nuclear structure effects. Experimental data on the g-factor of the bound electron are available only for the hydrogen atom and the ⁴He⁺-ion. In this paper we present the first result for the g-factor of hydrogen-like carbon (¹²C⁵⁺). The experimental accuracy is high enough to verify the relativistic contribution to the g-factor on the 10^-3 level.

Some of the most critical tests of the QED Lamb shifts are currently being pursued in the x-ray regime by investigating the core levels of medium Z atoms such as vanadium. The current approach to the measurement of the Lamb shift in such a system is outlined. We present major progress in understanding and reducing the systematic errors within our system. The error budget has been reduced to a level where critical tests of QED can be performed. Recent observations of Lyman α in hydrogenic vanadium at the NIST Electron-Beam Ion Trap are presented.

X-ray spectra from Ne-like Xe, Cs and Ba have been observed with the Tokyo electron beam ion trap and a flat crystal spectrometer. For several n = 3 to 2 transitions in Ne-like Ba and Cs, wavelengths have been determined with an accuracy of about 200 ppm. The experimental wavelengths are compared with previous theoretical and experimental results. X-ray transitions in Ne-like Ba have also been used to measure the spatial distribution of x-ray radiation with a spherically bent crystal. The possibility to apply such techniques to diagnostics of an EBIT is discussed.

Ground-term fine-structure levels and their magnetic-dipole (M1) transition properties of Titanium-like ions are investigated theoretically through the atomic number Z = 37–103. An anomalous stability in wavelength of J = 3–2 M1-transition reported by Feldman (1991 J. Opt. Soc. Am. B 8 3) is explained in terms of a concept of the critical atomic number beyond which the relativistic nature dominates in the corresponding fine-structure. The Tokyo-EBIT facility (1996 J. Phys. Soc. Jpn65 3186) enabled us to observe the corresponding line in the visible range for Xe⁺³² and Ba⁺³⁴ ions. The measured wavelengths are consistent with measurements by the NIST group (1995 Phys. Rev. Lett.74 1716), however there remains a considerable discrepancy from present calculations.

We present experimental data on visible lines produced by Kr (q = 11+ to 22+), Xe (q = 18+ to 35+), and Ba (q = 28+ to 36+) ions, corresponding mainly to 3s^l 3p^m 3dⁿ configurations, obtained with the LLNL electron beam ion traps. Tentative assignments for the lines are made.

Electron beam ion traps (EBIT) lifetime measurements on forbidden lines in the visible spectrum have reached a precision of only about 5%, whereas some EBIT lifetimes measured on x-ray transitions are good to 0.5% already. Scientific and technical problems encountered when trying to improve on these limits are discussed.

An experiment at the Heidelberg heavy-ion storage ring TSR has been developed for accurate atomic lifetime measurements in the 0.1 ms to 100 ms range. The measurements use optical detection in the ultraviolet range, of intercombination and forbidden transitions in a variety of ions of fundamental and astrophysical interest.

Spectra of multiply charged oxygen ions were studied in the region between 1800 and 6000 Å by the beam foil method. The beam energy was 2, 4, 6 and 9 MeV for the charge state identification. Most of the intense lines are classified as transitions between high n, l levels in O III to O VIII, where n ranges from 9 to 15. The lines due to transitions 5l-6l' in Li-like O VI were investigated in detail in the 1800–2250 Å region. Many lines around 2070 Å were attributed to the transition of the type of 1s2s5l⁴L–1s2s6l' ⁴L' and 1s2p5l⁴L–1s2p6l' ⁴L', where L = l and L' = l'.

Highly charged neon ions were studied by the beam foil method in the visible region. The lines due to transitions of 2s7l-2s8l' in berylliumlike Ne VII were found to be split into l-components by the perturbation of the nearby lying multiply excited 2p5l levels to the 2s8l [1]. In the present work the decay was measured by the beam-foil time-of-flight technique. The transitions 2s7i–2s8k (388.48 nm) and 2s7h–2s8h (386.34 nm) show a very slow decay with multiexponential features due to the heavy cascading. The remaining transitions with lower l decay faster depending upon the admixture with 2p5l states. The observed multiexponential decay of 2s7i–2s8k was well reproduced on the basis of the excitation model according to σ(n) ∝ n⁻³ and σ(l) ∝ 2l + 1.

We have measured the decay time of the 1s²2p ²P_3/2 level of Li-like Ar¹⁵⁺ using beam-foil spectroscopy. The experiment was done at a beam energy of 2.6 MeV/u and with an Al foil instead of a C foil which is usually used in beam-foil experiments. The signal was recorded by means of a CCD detector system installed on a 2.2 meter grazing incidence spectrometer. This system allowed recording of the spectral line-profile decay. The result shows different decay times for different parts of the measured line profile, thereby corroborating our previous studies of the same transition in Ar¹⁵⁺ by using a C foil and the same energy. In the latter work some difficulties were revealed in determing reliable lifetimes for excited levels in highly charged ions. We discuss the causes of the uncertainties in decay time (hence lifetime) measurements and suggest a scheme for increasing the reliability of beam-foil lifetime experiments.

Beam-foil experiments on ions of the iron group have been done at the Bochum Dynamitron tandem accelerator. For elements Mn to Ge, prompt and delayed EUV spectra have been recorded in order to disentangle the contributions from short-lived and long-lived levels, in Cl-, S-, P- and Si-like ions. A number of decays of long-lived levels have been identified with the help of Cowan Code and MCDF calculations.

Excited state lifetimes ranging in value from picoseconds to nanoseconds are reported for selected highly charged ions. The measurements utilize beam-foil excitation and photon-counting extreme ultraviolet spectroscopy with position sensitive detection. Lifetimes are reported for both allowed and forbidden transitions in several classes of highly charged ions.

A first measurement of the spectral distribution of the two-photon decay of the 1s2s ¹S₀ level in heliumlike gold is reported. This study extends our earlier work for heliumlike krypton and nickel to higher atomic numbers Z into the fully relativistic regime. The measured spectral shape of the two-photon decay differs from that observed at intermediate Z but is in agreement with the results of a recent fully relativistic calculation of Derevianko and Johnson.

Energy levels of highly charged ions as a rule cannot be classified using LS coupling due to rapid increase of relativistic effects. It is suggested, for optimal classification of energy spectra, to calculate them in LS coupling and to transform the weights of the wave functions, obtained after diagonalization of the energy matrix, to the other coupling schemes. F-like ions are considered as an example.

Wavelengths and oscillator strengths of electric dipole transitions from the 2p³3l configurations of S IX are calculated. Relativistic and correlation effects are accounted for in Hartree-Fock-Pauli approximation and in the basis of transformed radial orbitals. Fairly high accuracy of results is achieved.

We report on progress in the calculation of three-electron states making use of B-spline basis sets. In particular we discuss the advantages and disadvantages of using a Hartree-Fock basis (expanded in B-splines) compared to the use of hydrogenic basis states. Preliminary results are presented for the ²S terms in Li below the 1s2s ³S limit at 64.4 eV. The ²S terms have been studied less extensively than other core-excited states in Li. In this particular case the choice of basis has a large influence on the quality of the results.

Recent years have seen a growing number of large-scale atomic structure calculations using both nonrelativistic and relativistic theories. For investigations of multiple and highly charged ions, of course, a relativistic structure code like the widely known GRASP program is required. In a revised version of this program, namely GRASP92 (F A Parpia, C F Fischer and I P Grant 1996 Comput. Phys. Commun.94 249), systematic studies of level energies and a few other bound-state properties are now being supported. — Here, we briefly introduce a new package RATIP which extends GRASP92 towards the computation of various Relativistic Atomic Transition and Ionization Properties. A short overview of the capabilities of RATIP along with current developments will be given.

Multiconfiguration Dirac–Fock (MCDF) wave functions have been applied for many years to study excitation energies, transition probabilities, and lifetimes along various isoelectronic sequences. More recently, large-scale computations became feasible also for open-shell atoms and have been carried out for selected, multiple charged ions of the iron group. Though such large-scale studies may provide accurate data for many transitions among the low-lying levels, in some cases considerable effort is required, and sometimes accurate computations still remain unfeasible even for present-day technology. This applies particularly for ions which have more than one or two electrons outside of a closed–shell core. To demonstrate both, the capabilities of the MCDF method along with its practical limitations for accurate ab-initio studies of branching ratios and lifetimes, here we discuss the decay of the 3s3p⁶²S_1/2 level for the two Cl-like ions Fe X and Ni XII for which are markable scattering of theoretical and experimental data has been found in the literature.

A model-potential method using selecting B-spline basis functions in a configuration-interaction (CI) scheme is applied to study the bound states of the beryllium isoelectronic sequence (Z = 5–7). The energies and wave functions of the bound 1s²2snl (l ≤ 2), and 1s²2pnlL (L ≤ 2, L ≠ l) states are calculated and used to calculate the oscillator strengths. Our length results of the oscillator strengths for electric dipole transitions are accurate and can be achieved efficiently for various states.

The energy shift due to the nuclear polarization is estimated for the ground state of the hydrogen-like ²⁰⁸₈₂Pb and ²³⁸₉₂U ions. In previous studies, only the contribution from the longitudinal nuclear polarization has been considered. We further calculate the contribution from transverse polarization as well. It is found that the transverse component is comparable to, or even larger than, the longitudinal component.

We study the VP correction to the energy levels in the hydrogen-like electronic and muonic atoms. The result is obtained for the ground state of hydrogen-like atoms (muonic and electronic) analytically without any expansion over Zα.

Results of a calculation of the relativistic nuclear recoil corrections to the 1s and 2s state energies of hydrogenlike ions are presented for extended nuclei in the range Z = 10–92.

We present results of a calculation of the two-electron self-energy and vacuum-polarization corrections to the 2p_1/2–2s transition energy in Li-like uranium and bismuth. The calculation is performed to all orders in αZ.

A new method for the calculation of the first-order self-energy for tightly bound electrons is presented. The method is based on the multiple commutator expansion and the partial-wave renormalization.

An electron string is a state of a one component electron plasma, which can arise when multiply reflected electrons are accumulated in a strong solenoidal magnetic field. Based on the Electron Beam Ion Source Krion-2 in the reflex mode of operation electron strings were used for production of highly charged Argon ions. Evolution of K x-ray spectra during the confinement of Ar ions was obtained. Dielectronic recombination of Ar¹⁶⁺ ions with string electrons was detected. The possibility of K x-ray diagnostic for studies of electron strings is discussed.

An electron beam ion trap (EBIT) is under construction at the University of Freiburg. It will be operated both as an ion trap and as an ion source, and its design specifications will allow production of hydrogenic ions from even the heaviest elements. It will be possible to extract and accelerate the ions to energies up to 350 kV/q.

An all-permanent magnet ECR "multi-mode" ion source with emphasis on the production of metallic ions has been built. The main feature of this ion source is a stepped plasma chamber with a larger diameter in the resonance region in order to allow the propagation of higher microwave modes than the ground mode. Ions from metallic elements can be produced using both the insertion technique and an evaporation oven. First spectra obtained for Oxygen and Bismuth are shown.

A new concept to assist the extraction and to improve the quality of highly charged ion beams from an electron cyclotron resonance ion source (ECRIS) is the injection of an intense and well focused electron beam from the extraction side into the plasma of the ion source [1]. A first test setup and first results have been presented elsewhere [2]. In this article an improved setup and new results are presented.

The production of metal ions from volatile compounds (MIVOC) in an electron cyclotron resonance ion source (ECRIS) depends on the flow rate of material into the source plasma. In the original method, the material container (MIVOC chamber) is mounted externally. The flow rate of material depends on the conductance of the pipe connecting the container to the plasma chamber and can be regulated via a dosing valve. In the ATOMKI-ECRIS the conductance is not high enough to apply external mounting. By placing the material container into the ECR ion source itself and by applying a special technique to dose the flow rate of material, beams of more than 10 µA of highly charged Fe and Ni ions have been produced.

Spectroscopic investigation in the 10–80 nm spectral range of the plasma generated by an Electron Cyclotron Resonance (E.C.R.) ion source has been realized. The results are based on oxygen. O V intensity line ratio measurements inside the plasma are investigated as a function of the RF power. Results are correlated with the extracted O⁴⁺ current. The electronic density and temperature have been derived from the computed results of Kato et al. Finally, first determination of ionic densities and lifetimes have been performed.

Absolute ion densities in an Electron Cyclotron Resonance (ECR) ion source plasma have been measured using high resolution x-ray spectroscopy of (n = 2) → (n = 1) emission lines from highly charged argon ions. Ion densities have been correlated to extracted currents and ion confinement times have been deduced. Confinement time evolution with charge state and plasma performances is presented and discussed.

With a powerful relativistic algorithm the three-dimensional trajectories of 10⁴ ECR-heated and confined electrons are calculated in a standard ECRIS with a deep minimum of |B| and a new ECRIS with a very flat minimum of |B|. The spatial electron (plasma) densities and electron energy densities deduced from these trajectories yield new and surprising insight in the performance of ECRIS.

The results of numerical simulations of highly charged ion production in the new Superconducting ECR ion source (SERSE) at INFN-LNS, Catania are presented. SERSE is an ECR source operating in the so-called "High B mode", with the confining magnetic field up to 2.7 T, mirror ratio up to 7, different combinations of 14 GHz and 18 GHz RF generators for the plasma heating, plasma chamber diameter of 13 cm and length of about 50 cm. The final results of the numerical simulation are used for the estimation of main plasma parameters, the interpretation of the experimental data and the prediction of SERSE parameters during the next development.

A new storage ring system, HIRFL-CSR, is now in construction in the National Laboratory of Heavy Ion Research Facility of Lanzhou, China. The new facility consists of a main ring (CSRm) and an experimental ring (CSRe). With the flexibility of the production and the investigation of highly charged ions and radioactive ion beams the new HIRFL-CSR facility will make many frontier atomic physics researches possible in the near future. The future physics researches at the HIRFL-CSR are now under consideration. In this paper an overview of the HIRFL-CSR project is given, and the main atomic physics programs to be carried at the HIRFL-CSR are presented.

The design and the realization of the first beam-line section for postaccelerated highly charged ions at the 14GHz-ECRIS-(ve)RFQ accelerator facility at the Institut für Kernphysik (IKF) has been completed. The results of emittance measurements after the (ve)RFQ (Radio Frequency Quadrupole accelerator) and the final structure of the new beam-lines for the postaccelerated ions are presented.

A supersonic gas-jet target for investigations of the detailed collision dynamics in fast ion-atom collisions by means of recoil-ion-momentum spectroscopy has been constructed for insertion into the heavy-ion storage ring CRYRING, situated at the Manne Siegbahn Laboratory, Stockholm University. The aim is to create a target with a density of 10¹² cm^-3 and a temperature of less than 10 mK (for the case of a He target) without seriously affecting the excellent vacuum conditions of CRYRING. The gas target will become a part of the storage ring, and hence fast beams of protons, highly-charged ions, and molecular ions will be available as projectiles. Here we present test results for parts of the system. These all indicate that the design criteria will be fulfilled.

Details of a new high gain zero-degree Auger projectile electron spectrograph using a hemispherical analyser and a 2-dimensional position sensitive detector (PSD) with multichannel plates and a resistive anode encoder are presented. A four-element lens mounted at the entrance of the analyser, provides a virtual slit for the incoming electrons by focusing them while at the same time decelerating them to improve their energy resolution. Electrons enter through an aperture at a position R0 which is displaced (along the energy dispersion axis) with respect to the commonly used central entrance position at ½(R₁ + R₂). The analyser has an acceptance energy range of 20% and an energy resolution of 0.9%. An ion-optics trajectory simulation indicates improved focusing properties for this off-center position thus avoiding the need for cumbersome fringing field correction schemes. Test measurements of high resolution projectile Auger spectra produced in 21.7 MeV collisions of F⁸⁺ and F⁷⁺ projectiles with H₂ and He are presented.

A thin back-illuminated CCD based photon detector system has been installed on the Rowland circle of a 2.2 meter grazing incidence spectrometer, replacing the previously used Ceratron single channel detector. For reasons to be described the CCD chip surface was not angled tangential to the spectrometer Rowland circle. The spectrometer is used for beam-foil spectroscopy experiments at the RIKEN laboratory. The performance of the CCD was tested using a beam of 183 MeV Nb ions.

The viability of using beams of molecular ions as a target for strong field fragmentation studies using intense ultra-short laser pulses is demonstrated. In this way the production mechanism for multiply charged ions in strong fields may be elucidated.

A new type of quasi-continuous spectra of femtosecond laser plasma in the vicinity of multicharged H-like and He-like ion resonance lines were observed and interpreted for the first time. It is shown that such spectra were generated by multicharged hollow ions and are caused by super high density conditions provided by a high contrast laser pulse.

Highly charged ions are produced by picosecond and femtosecond laser irradiation of solids at intensities up to 10¹⁶ W cm^-2, The highly dense and excited plasma produced emits hard x-ray radiation which is measured using solid state detectors. Analysis of K x-ray emission from aluminium indicates the presence of highly charged ions Al^q+. The dependence of x-ray emission on polarization and wavelength is also reported We present a simple and powerful technique for estimating the absolute x-ray yield from such plasmas. The analysis is based on the Bose-Einstein statistics of the photon arrival at the detector.

X-ray band spectra from laser produced highly ionized hafnium plasmas have been observed in the 4.6–5.0 Å range. The contributions from electron-impact excitation and dielectronic recombination in this region are studied in detail using a quasi-relativistic configuration interaction method. It is found that the spectral structure due to electron-impact excitations is almost the same as in dielectronic recombination. The calculated spectra of 3d–5f transitions of the Ni–Ge ionization states can explain the observed features of the band spectra in this region very well.

The influence of quantum interference effects induced by mixing of coherences and populations in the fluctuating plasma microfield on the redistribution of resonance radiation by HCI is studied in terms of the atomic density matrix. The thermal ion motion is considered within the model microfield method. Results show that the thermal ion motion (ion dynamics) induces QIEF as well, and both are essential in line centres and nearest wings of the scattered radiation.

The interaction of heavy ion beams and laser light with matter is of central importance for the inertial fusion and x-ray laser research. Recently developed techniques in x-ray spectroscopy have provided extremely unusual emission spectra near the target even in traditional experiments. It will be shown that reasonable interpretation and diagnostic can be achieved only incorporating new concepts in the dielectronic satellite line formation. Theoretical models are developed which provide good agreement with experimental results. Charge exchange processes are proposed for the formation of hollow atoms.

X-ray emission of a laser produced plasma (LPP) was investigated in the spectral range from 2.7 nm to 4.5 nm. X-ray spectra with spectral, spatial and time resolution of the LPP were measured with a multilayer mirror (MLM) monochromator at distances 5–15 mm from the target. It was found that in the investigated wavelength range two lines dominate: the 1s²–1s2p line in He-like carbon (λ = 4.02 nm) and the 1s–2p line in H-like carbon (λ = 3.37 nm). The intensities of the lines were measured as a function of the energy of a Nd: glass laser, which produced the laser plasma. The intensity of the line at 4.02 nm was measured as a function of the distance from the target.

A close coupling (CC expansion) is used for investigation of the role of free-free transitions in the ionization problem. The results are obtained for ionization of hydrogen-like ions by linearly polarized monochromatic fields. The problem of stabilization is discussed jointly with the basic distinction between three-dimensional and one-dimensional models.

In order to study the sputtering mechanism of light atoms by slow highly charged ions, H⁺ ions sputtered by Xe⁷⁺ from an untreated C target were measured with a two-dimensional (2D) position sensitive detector. The kinetic energies of Xe⁷⁺ at the moment of surface impact were 700 eV to 4200 eV incident at 45 degrees with respect to the target normal. We found that (1) the 2D position distribution of sputtered H⁺ ions, which reflects the initial angular and energy distributions, has a single peak with its position shifting toward upstream side of the incident ions, (2) this shift increases with decreasing incident energy, and (3) the estimated width of the energy of the sputtered H⁺ in the direction parallel to the target is much wider than that of sputtered heavy ions.

Proton sputtering from an uncleaned surface of a CuO mesh has been experimentally studied for very slow highly charged ions of 500 eV Ne^q+ (q = 4–8), Ar^q+ (q = 4–13), Kr^q+ (q = 5–17) and Xe^q+ (q = 7–24). It is found that (i) for q < = 10, the sputtering yield of protons showed a power-law dependence (~q^γ) with an exponent of γ = 5±1 for all ions measured, and (ii) for q < 10, the yield saturated and deviated from the power law. Such a characteristic feature has been reproduced well by a model based on the classical over-barrier model.

To study the effects of electronic excitation by swift heavy ions on defect production, we have irradiated high-T_c superconductors and metals by various ions in the energy range of 90 MeV to 3.84 GeV at low temperatures, and measured the electrical resistivity as a function of ion fluence. We have found that the irradiation effects depend not only on the electronic stopping power but also on the ion-velocity.