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The physics of highly charged ions, 'HCI': dominates the nature of the universe—where most of the matter is in a highly ionized stage—and determines the conditions in all terrestrial plasmas. Consequently, this topic has gained increasing interest within the last years and is still a rapidly developing field. The advances in this discipline have considerable impact in many areas ranging from applied topics—from solid and surface modifications via cluster physics to plasma problems—to fundamental aspects—from collisional interactions via spectroscopy and structure investigations to quantum electrodynamics. Since the first conference on the physics of highly charged ions initiated in Stockholm (Sweden), 1982, tremendous progress has been made in technical developments and experimental techniques as well as in theoretical understanding and methods applied in this field. The progress is mirrored by the presentations given at this biannual conference series on the physics of highly charged ions (Oxford, Great Britain, 1984; Groningen, The Netherlands, 1986; Grenoble, France, 1988; Giessen, Germany, 1990; Manhattan, USA (KS), 1992; Vienna, Austria, 1994; Omiya, Japan, 1996) and is documented in the corresponding proceedings each comprising both the invited presentations and the contributions.

The latest conference in this series, the 9th HCI, was held in Bensheim (Germany), September 14–18, 1998, and jointly organized by the heavy ion research center, GSI Darmstadt, and the Max-Planck-Institut für Kernphysik, Heidelberg. 280 scientists from 26 countries of all continents discussed the present status and the progress in the physics of highly charged ions. The invited oral presentations included 8 reviews, 8 progress reports and 16 selected topics as well as 4 reports on local research activities. Additionally, from the 256 submitted contributions 226 were posted and discussed actively during three poster sessions devoted to applied aspects, to collisional processes and to ionic structure. The scientific program, framed by astrophysical plasma aspects—the cosmic X-ray emission and the 'setting of a cosmic clock by highly charged ions'—addressed also laboratory plasmas, from the production of highly charged ions in laser generated plasmas to the interaction of those ions with plasmas. The basic atomic and electronic collision processes, from reaction microscopy to recombination, were a natural focus of the program. The transition from single collision processes via interactions with clusters to those with surfaces and finally with bulk material was another central topic. Structure and spectroscopy, especially of exotic systems like so-called hollow atoms or very heavy few-electron systems and their theoretical description, gave further fundamental aspects to the conference. Naturally, in this context particular emphasis was given to the research with highly charged ions in traps and in storage rings. A special highlight of the program was the public evening lecture on the potentialities in cancer therapy with heavy ions, an application recently introduced at GSI Darmstadt within a successful testing phase. The other local reports concerning highly charged ions in storage rings were given during working visits to the heavy ion storage ring facilities TSR at Heidelberg and ESR at Darmstadt. The full scale of the scientific program is well reflected by the present proceedings comprising both the invited and the contributed presentations.

It is a pleasure to acknowledge the involvement of all the participants, in particular of the speakers and the authors of the papers in these proceedings who contributed to the high scientific standard of the conference and made it a true success. Special thanks are given to all persons involved in the organization, in the local and international committees; S Luettges, the conference secretary who competently handled all the organizational load of HCI-98, has to be mentioned here by name in particular. All the editors are deeply indebted to the Physica Scripta team for the excellent collaboration in preparing these proceedings. And last but not least the support of the following organizations is highly appreciated: IUPAP (International Union of Pure and Applied Physics), DFG (Deutsche Forschungsgemeinschaft—The German Research Foundation), Heraeus Foundation, MPG (Max-Planck-Gesellschaft), and GSI Darmstadt (Gesellschaft für Schwerionenforschung—The German Heavy Ion Research Center). Without this support it would not have been possible to organize this conference. The conference has been recognized by IUPAP and EPS (The European Physical Society). The next, i.e. the "X International Conference on the Physics of Highly Charged Ions" will be organized by Alex Hamza and his colleagues from Lawrence Livermore National Laboratory (Livermore, CA) in Berkeley, USA (CA), in the year 2000. The organizers hope to meet there again all the colleagues working in this field and we are looking forward to the forthcoming advances in the physics of highly charged ions.

In recent years, there have been dramatic improvements in our instrumental capabilities for making x-ray spectroscopic measurements of astrophysical plasmas, and even more significant advances will occur in the next few years. It is now possible to detect and study emission line complexes from nearly all cosmically abundant elements (C through Fe) over a very wide range of physical conditions. The data have provided a wealth of new information regarding cosmic plasmas, but have also raised a number of challenges to our basic understanding of relevant atomic processes involving multiply charged ions. I review some of these recent results and provide an overview of where this field is headed in the future.

The amount of ¹⁸⁷Os in meteorites resulting from beta decay of the long-lived ¹⁸⁷Re nuclide (half-life 42 billion years in the neutral charge state) has been used as a measure of the time span of nucleosynthesis in our Milky Way galaxy. During the galactic evolution, however, the rhenium atoms, generated in the r-process, can be "astrated" several times into just originating stars, where they are stripped of most or all of their electrons. An experiment conducted at the ion storage ring ESR at Darmstadt established for bare¹⁸⁷Re ions a half-life of only 33 years, that is by more than nine orders of magnitude shorter than the corresponding half-life in the neutral atomic charge state. Based on this amazing result, the half-life for any other charge state of ¹⁸⁷Re can be calculated and, accordingly, the effective lifetime of ¹⁸⁷Re during nucleosynthesis can be figured out in the framework of well-founded evolution models of our galaxy. That allows a re-calibration of the rhenium "aeon clock" from which, tentatively, a lower limit of 12 billion years for the age of our galaxy was derived being, a fortiori, also a lower limit for the age of the universe. In connection with recent results for the Hubble constant this limit provides narrow constraints for actual cosmological models.

The interaction of an intense laser pulse of sub-picosecond duration with an atomic cluster larger than a few hundred atoms can be extremely energetic. The high local density within the cluster together with a dynamic dielectric resonance in the expanding cluster microplasma greatly enhance coupling of the laser to both ion and electron kinetic energies. In contrast to the few tens of eV temperatures typically produced in laser irradiation of monatomic gases, cluster targets can produce electron energy distributions in the few keV range, mean ion temperatures of 10–50 keV and peak ion kinetic energies up to 1 MeV. In addition, charge states up to 40⁺ can be produced with quite modest laser intensities (≈ 10¹⁶ W cm^-2). Cluster targets are also surprisingly efficient (>90%) at absorbing intense laser light, and thus provide a new route to producing very high energy density, highly ionized laboratory plasmas of interest to a broad range of disciplines. Here we review recent experimental results and outline some areas of current research in this new field.

High current discharges generating a fully ionized hydrogen plasma and a high energy Nd:glass laser system (100 J, 15 ns) to produce a high density carbon plasma were used to study the interaction processes of heavy ions with ionized matter. The energy loss and charge state distribution of the projectile ions have been measured. Fully ionized plasmas show an enhanced stopping power due to the effective stripping of the ion. We report experimental findings that high density carbon plasmas, not fully ionized, also show an enhanced stopping power due to charge state effects and due to the efficient energy transfer to free electrons.

The application of recently developed spectroscopic instruments in laser produced plasmas with simultaneous high spectral and spatial resolution combined with high luminosity discovered new types of x-ray spectra. These new types are characterised by the disappearance of the resonance lines and the strong emission of dielectronic satellite spectra. Several types of transitions of highly charged ions are discovered which are unknown from usual sources employed in atomic physics. New theoretical models are developed and successfully applied for the interpretation and for plasma diagnostics.

In this review we will start out with an effort to describe the present understanding of the dominant mechanisms for electron transfer in collisions between slow highly charged ions and atoms. We then follow up with an account of the most prominent phenomena in highly charged ion-surface collisions and emphasize the similarities and differences between the collision scenarios for atomic and surface targets. Finally, we will discuss recent advances in the much less mature field of highly charged ion-cluster (mainly C₆₀) collisions.

In low energy highly charged ion-C₆₀ collisions, a great number of weakly bound electrons can be transferred to the projectile ion, creating excited highly charged C₆₀ and free Rydberg hollow atoms by multi-electron captures. By coincidence measurement of the number of stabilized electrons on the projectile, the multiplicity of ejected electrons and the time of flight of charged C₆₀ or fragments, we have studied collisionnal mechanisms for Ar⁸⁺, O⁸⁺ and Xe³⁰⁺–C₆₀ systems. Different electron multiplicity distributions and scattered projectile kinetic energies have been observed for peripheral and frontal collisions. The number of electrons ejected during the collision depends not only on the initial charge state but also on the atomic structure of the incoming projectile ion. In the case of Xe³⁰⁺–C₆₀, up to 80 active electrons have been measured even in peripheral collisions.

An overview is given about the electron emission when slow highly-charged ions interact with metal surfaces. Various processes are considered starting with the formation of the hollow atom above the surface until its final decay a few layers below the surface. In this context selected experimental data of the last few years are presented and theoretical results like from the classical over-the-barrier model and transport calculations are discussed. The instant of the K-Auger transition of the hollow atom is a main aspect of the present paper. It is shown that this process takes place almost only below the surface.

We present a new series of experiments on the interaction of highly charged ions with thin films. The main goal concerns the investigation of the interplay between the projectile and the surface electronic structure. The dynamics of the hollow atom is analyzed through high-resolution Auger electron spectroscopy. LiF and C₆₀ thin films have been investigated; the results from LiF permit definite conclusion on the respective role of the band gap and the binding energy in bulk LiF during the projectile neutralization and relaxation. Results from C₆₀ raise new questions on the actual nature of the processes responsible for the fast filling of the projectile L-shell.

Using a recently developed multi detector, a large variety of multi-coincidence studies for ion-surface interaction becomes possible. In particular this includes simultaneous measurements of projectile energy losses and angular scattering patterns in combination with statistics, angular and energy distributions of secondary electrons emitted. Results are presented for grazing scattering of protons and O^q+ (q = 2,3,5,6; E = 10 keV) ions from LiF single crystal which demonstrate the potential of the technique for investigating ion-surface interaction and deliver interesting new information on projectile energy losses and secondary electron emission in correlation with scattering angle. As a future development a possible combination of this technique with the voltage labeling method for studying backscattered projectiles will be discussed.

Electronic sputtering in the interaction of slow (v < 10⁶ m/s), highly charged ions (e.g., Au⁶⁹⁺) with solid surfaces increases secondary ion yields by over two orders of magnitude compared to sputtering with singly charged ions. We discuss advantages of highly charged ions for analysis of semiconductors and biomolecular solids in a time-of-flight secondary ion mass spectrometry scheme.

Measurements of electron multiplicity spectra in coincidence with reflected projectiles for well defined angles of incidence (typically ψ ≤ 5°) allow for the first time to unambiguously separate contributions from potential and kinetic electron emission in HCI-surface interactions. From such measurements we could demonstrate that potential electron emission is solely dependent on the vertical component of the impact velocity. Molecular dynamics calculations performed in order to simulate the interaction of projectiles with ideal as well as rough surfaces help to understand characteristic differences in the number of emitted electrons for surface channeled projectiles and those undergoing scattering in collisions with surface imperfections.

Relativistic channeling experiments of highly-charged ions provide valuable information of a variety of atomic states and processes in the unique environment of a crystal. A periodic crystal potential forces an ion trajectory away from an atomic string/plane and depresses electron loss/capture probabilities. It leads to smaller energy loss, trajectory-dependent Stark level mixing due to the crystal field and resonant coherent excitation (atomic level excitation by the periodic crystal field) of highly-charged heavy ions. Recent progress at HIMAC is reported with an emphasis of resonant coherent excitation observed in charge exchange and energy loss of the highly-charged ions.

Complete experimental studies of the production and transport of core and Rydberg nl states have been performed for Ar¹⁷⁺ at high velocity (v = 23 a.u.) on different carbon targets; a range of target thicknesses from single collision condition to equilibrium has been investigated. Considering the collision system chosen, two models, based on a collisional approach—a rate-equation model and a classical Monte Carlo simulation—can be applied and compared. Our calculations show that they both represent well the experimental Rydberg l state populations but fail to describe the core states (n = 2,3,4). The observed l-mixing for these states is much larger and faster than predicted. This discrepancy could possibly be explained by the Stark mixing induced by the projectile polarization of the medium, and neglected, thus far, in these two models.

The Electron Scattering Model, ESM, for ion-atom collisions refers to the scattering of a quasi-free (loosely-bound) target electron in the field of a highly charged projectile ion. Many atomic processes have been successfully described by the ESM, which relates a differential scattering cross section for an ion-atom collision process to the cross section for the corresponding pure electron-ion collision process. The following processes have been studied in ion-atom collisions: resonant and non-resonant electron-ion elastic scattering, resonant and non-resonant inelastic electron-ion scattering, and dielectronic recombination. Recently, features have been observed in electron double differential cross sections from ion-atom collisions that have been attributed to "super elastic" electron-ion scattering, and intra-atomic double electron scattering in the case of molecular targets. Also, evidence for triply-excited states formed by resonance excitation has been observed. A survey of the results of these studies and the status of this field of research will be presented.

We report on calculations of differential electron spectra for double ionization of helium by fast highly charged ion impact based on a first order Born (shake-off) model. These spectra are shown to be very sensitive to the electron–electron interaction. The potential of recoil ion momentum spectroscopy for investigating the correlated dynamics of electrons in multiple ionization of atoms by fast heavy ion impact is discussed.

For the first time, state-selective electron capture in ion-ion collisions has been investigated experimentally. An experimental method which is mainly based on the specific features arising in ion–ion collisions due to the crossed-beams kinematics has been newly developed. We are able to determine the electronic states populated in the collision process as well as angular differential cross sections for the respective reaction channels.

During the last five years, a powerful technique emerged which led to a new insight in HCI-Atom collision dynamics: the Recoil Ion Momentum Spectroscopy (RIMS). Its high momentum resolution makes it act as a dynamical microscope. Its 4π detection solid angle allows additional coincident analysis such as electron spectroscopy. The way towards a multifragment 4π detector for atomic collisions is open. This review will present the latest achievements of the RIMS in the field of HCI-Atom or Molecule collisions.

A five-body classical trajectory Monte Carlo model has been developed to study fragmentation of H2 and HD after double electron removal collisions by highly charged ions. The theoretical model includes all two-body Coulomb interactions in the post-collision regime. A systematic study of the energy partitioning between the nuclei has been made for C⁶⁺ impact from 1 eV/u to 1 MeV/u. Major deviation from isolated molecule Franck-Condon behavior is found for impact energies E ≤ 1 keV/u. At the lowest impact energies the target ions are very energetic with a strong contribution being from collisional transfer from the projectile. The target ion energy spectra at high impact energy is due to the Coulomb explosion of the isolated doubly ionized molecule in the known Franck-Condon transition, resulting in a distinct separation of the energy peaks for the HD isotope. At energies around 10 eV/u to 1 keV/u, or projectile charge divided by speed of q/v ≈ 30–300, both slow and fast target ions are observed which result from the competition between the momentum transfered during the collision with that from the Coulomb dissociation of the two ions.

Ionization of triatomic molecules in collision with 120 keV Ar⁸⁺ and the subsequent molecular fragmentation has been studied by means of a position sensitive time-of-flight (TOF) technique. The triple coincidence signals for these molecules are identified for the dissociation channels of (CO₂)ⁿ⁺ where n = 3–6. The dissociation of (NO₂)ⁿ⁺ where n = 3–5 has also been studied. The velocity vector of each fragment ion is determined for individual collision events and the angle between the velocity vectors is calculated. For the highly ionized molecules (n = 6), the angle is consistent with the angle of the neutral molecule, both in the case of linear and bent molecules. For the lower charge states considerable deviation from the neutral molecule bond angle has been found. These results suggest that production of higher charge states is favorable for the fulfillment of a pure Coulombic dissociation scheme.

Double translational energy spectroscopy has been successfully used to study state selective electron capture processes in atomic and molecular targets involving slow (v < 1 au) C²⁺, N²⁺ and O²⁺ primary ions in prepared ground or metastable states. The results show that previous TES and total one electron capture measurements involving primary ions beams containing unknown fractions of metastable states require cautious interpretation.

The multiple electron capture in N⁷⁺ + Ar and F⁷⁺(1s²) + Ar systems is investigated at 70 keV with a new electron-recoil ion charge coincidence experiment. The whole electron energy range has been studied. Up to six electrons are found to be captured into autoionizing states. The recoil ion charge distribution associated with the emission of electrons is similar for both systems and found to be in good agreement with the prediction of Niehaus's model roughly adapted to take into account autoionizing cascades. New findings for the coincident double and triple captures are briefly discussed. A capture of an inner L-shell electron of Ar into the K-shell of the projectile is also observed in N⁷⁺ + Ar collisions.

The magnetic trapping mode of the Livermore high-energy Electron Beam Ion Trap is exploited to study charge transfer reactions between cold (few eV/amu) highly charged ions and gases. By selectively puffing neutral gases and monitoring the x-ray emission, state-selective measurements of the charge transfer reaction channels are possible. The observed K-shell x-ray spectra show prominent emission from high-n levels decaying to the n = 1 ground level, which is enabled by electron capture into states with low orbital angular momentum. A comparison with modeling calculations, therefore, allows a determination of the range of principal and angular momentum quantum numbers involved in the reaction.

The study of multielectron processes, where two or more electrons are involved, has usually been carried out by using independent electron approximations associated with one electron processes. In our studies of the double electron capture reaction at intermediate and high collision energies by impact of bare heavy ions on He(1s²) targets, an independent electron treatment of distorted wave approximations has been employed but also four body models were used, in order to shed some light on the role played by the electronic correlation during the collision. Theoretical results using distorted wave approximations calculated in both frameworks are presented and compared with experimental data available at present and with other theoretical calculations.

The physics of elementary atomic processes in relativistic collisions between highly-charged ions and atoms or other ions is briefly discussed, and some recent theoretical and experimental results in this field are summarized. They include excitation, capture, ionization, and electron-positron pair creation. The numerical solution of the two-center Dirac equation in momentum space is shown to be a powerful nonperturbative method for describing atomic processes in relativistic collisions involving heavy and highly-charged ions. By propagating negative-energy wave packets in time the evolution of the QED vacuum around heavy ions in relativistic motion is investigated. Recent results obtained from numerical calculations using massively parallel processing on the Cray-T3E supercomputer of the National Energy Research Scientific Computer Center (NERSC) at Berkeley National Laboratory are presented.

The present status of QED tests for heavy, highly charged ions with one or a few electrons is reviewed. The results of numerical calculations (to all orders in Za) of the Lamb shift for hydrogen-, helium- and lithium-like ions are compared with recent experimental results. Also numerical calculations of the hyperfine structure and Zeeman effect in heavy hydrogen-like ions are discussed.

Electroweak radiative corrections to the matrix element ⟨ns_1/2|Ĥ_PNC|n'p_1/2⟩ are calculated for highly charged hydrogenlike ions. These matrix elements constitute the basis for most parity nonconserving (PNC) processes in atomic physics. The operator Ĥ_PNC represents the parity nonconserving relativistic effective atomic Hamiltonian at the tree level. The deviation of these calculations from the calculations valid for the momentum transfer q² = 0 demonstrates the effect of the strong field, characterized by the momentum transfer q² = m_e² (m_e is the electron mass). This allows for a test of the Standard Model in the presence of strong fields in experiments with highly charged ions.

Higher order photon transitions such as M1, M2 and two-photon decay are conveniently studied using highly-charged few electron ions. Here we discuss two examples from recent experiments which were done using the ATLAS facility at Argonne National Laboratory. The first is a test of Relativistic Quantum Mechanics involving a precision measurement of the spectral shape of the two-photon decay of the 1s2s¹S₀ state in He-like nickel and the second is a test of the theory of damping in quantum mechanics involving observation of E1–M1 interference in the electric field quenching of metastable H-like ions.

We have measured 2s–2p fine structure transition energies in highly charged Ni, Zn, and Ag ions using beam–foil excitation and grazing incidence spectroscopy at the GSI–UNILAC accelerator. The precision of the results provides tests of two-photon QED screening corrections to the 2s–2p transition energies. The experimental values are compared with other measurements and with recent relativistic calculations along the lithium–isoelectronic sequence.

Penning and Kingdon ion traps have been used to study low-energy multiply-charged ions with charge states up to 80+ during the last few years. The ions have been captured into the traps from beams of external multiply-charged ion sources, or have been produced inside the trap. Measurements of cross sections for electron capture from neutrals to ions and studies of relative double electron capture rates have been completed. The lifetimes of metastable levels of ions, precision spectroscopy on multiply-charged ions in traps, and cooling of trapped ions using lasers, ion-ion elastic collisions, and parallel-tuned circuits, are briefly reviewed. Prospects for the future of highly-charged ions in traps are also discussed.

We have been using the Tokyo EBIT to study a wide range of electron ion interactions and to measure the energies of transitions in highly charged ions. Transition energies were investigated using visible and x-ray spectroscopy. The two electron contribution to the ground state energy of Helium-like Krypton was measured using absorption-edge spectroscopy. Dielectronic recombination processes were measured by mapping the yield of x-rays as a function of electron energy whilst the time dependence of the evolution of charge states was measured, towards the determination of ionization cross-sections. A brief review of these activities is given.

Recent experimental studies of electron-ion recombination performed at heavy-ion storage rings as well as related theoretical work is reviewed. The topics covered are high resolution spectroscopy of doubly excited states by dielectronic recombination, relativistic effects, interference between radiative and dielectronic recombination, test of QED in high-Z HCI, recombination rate enhancement at low energies, recombination in the presence of external fields and applications to astrophysical and laboratory plasmas.

A polarization recombination channel of electron on heavy ions with a complex core is under investigation. The channel is connected with a dynamic polarization of an ion's core which results in radiation of the core and an inelastic transition of electron. This channel is estimated in the frame of statistical model of the complex ion. It is shown that the contribution of polarization radiation may be comparable or exceed the standard radiation contribution. The significance of interference effects is demonstrated as well.

With the advent of the relativistic storage ring ESR, a new generation of highly precise collision experiments has become accessible even for the heaviest ion species. At the ESR, the interaction of brilliant beams of cooled high-Z ions with low-dense gaseous matter can be studied without any beam collimation guaranteeing background-free experimental conditions. Furthermore, a unique feature of the ring is its capability to decelerate even bare uranium ions to moderate energies close to about 10 MeV/u, i.e. it allows to scan the beam energies from highly relativistic encounters to almost slow collisions. Currently, the main experimental activities focus on final state resolved electron capture, photon angular distribution and alignment investigations.