Table of contents

A slow multicharged ion (MCI) which approaches a solid surface catches electrons from the latter into highly excited states and transiently becomes a neutral "hollow atom" (HA). Rapid autoionisation of this HA will be balanced by further ongoing electron capture until close surface contact where shielding sets in, and inner-shell vacancy recombination at and below the surface produces further slow electrons and characteristic fast electrons and soft X-rays. Until its complete neutralization the projectile continues to be attracted toward the surface by its decreasing image charge which causes a minimum impact velocity. Many aspect of this HA scenario have been studied by a number of groups for different ion species and - charge states at metal-, semiconductor- and insulator surfaces with a number of experimental methods. The most detailed comparison with available theory is possible if electron emission and projectile energy losses and -charge changes are studied in mutual coincidence for scattering along well defined trajectories at smooth mono-crystalline target surfaces. For insulator surfaces so called "potential sputtering" (PS) and strong secondary ion emission can be induced by the multi-electron capturing MCI. Our current understanding of the above-surface part of MCI-surface interactions is satisfactory and can also explain how "free" hollow atoms and -ions are produced by funneling MCI through thin capillaries or colliding them on clusters or fullerenes. Because of the approximately additive influence of potential- and kinetic ion energies on the resulting electron emission, for this part a straighforward definition of the term "highly charged ion" (HCI) can be given. On the other hand, processes occuring at and below the surface are so far less well understaood since they depend on the potential- and kinetic ion energies in a clearly non-additive way. This is particularly true for PS and excitation of target plasmons due to the potential MCI energy. PS and related phenomena are also of practical interest for new surface-analytical and nano-technological applications.

Nanoscale dots created on the surface of a highly oriented pyrolitic graphite sample by individual highly charged xenon projectile ions were observed using an in-situ scanning tunneling microscope. This is the first time that such features have been created and imaged without exposure to air. The dots vary in size with the charge state of the ion, up to a diameter of 6.6 nm for Xe⁴⁴⁺. The results are discussed in terms of the energy density deposited in the near surface region.

We present a theoretical study of the energy loss of highly charged ions undergoing distant collisions at grazing incidence angles with the surface of microcapillary targets. The interaction of highly charged ions with the internal surface of the microcapillary is treated within the framework of linear response theory. The specular reflection model and the time-dependent density functional approach are used to calculate the linear response functions. As projectiles we consider 2.1 keV/amu Kr³⁶⁺ ions and as target a Ni microcapillary. For the fraction of the ion beam that does not suffer any charge exchange, the energy loss is strongly correlated with the scattering angle. The distance-dependent stopping power and thereby the surface dielectric properties of the capillary material can be probed in detail.

Using a time-of-flight technique we have investigated the flux intensity variations for 5 keV Ar^q+ ions (q + 2,7,9,11) backscattered from Au(110) by 120° in quasi-binary collisions. A strong dependence on target orientation is found for the neutral flux, while for the charged components only a weak dependence is seen. An analysis of the observed dependences based on trajectory simulations clearly shows site-specific neutralization differences between the various possible binary collisions and a role of reionization processes in formation of multiply charged scattered ions.

Chlorine ions with charges up to 11 are scattered with energies ranging from 50 to 600 keV from a clean and flat Al(111) surface under a grazing angle of incidence. We have investigated energy loss and charge fractions of scattered projectiles. We find a weak dependence of the energy loss on the projectile charge, whereas the higher charge fractions of the scattered beam show pronounced effects on the charge of incident ions.

We have measured the secondary electron emission statistics (ES) for 5 keV N^q+ (q = 1–4) ions incident at 10° on polycrystalline aluminium, in coincidence with specularly reflected N⁰. In this arrangement the kinetic contribution to secondary electron emission is minimised. The experimental data shows that the coincident electron yield, γ, increases linearly with incident ion charge state. The kinetic emission contribution has also been determined from this data. The ES due to 2 and 4 keV He²⁺ impact on polycrystalline aluminium in coincidence with specularly reflected He⁺ and He⁰ have also been determined. The process He²⁺ → He⁰ yields a larger γ value than the process He²⁺ → He⁺.

Significant multiple electron capture has been found in collisions of 46 MeV/u Pb⁸¹⁺ ions with thin carbon foils. An increase in the many-electron capture probability has been observed for the thinnest targets, which is contrary to the normally assumed process of subsequent single-electron capture in the bulk. This shows the significant contribution of the surface for multiple electron capture. Further evidence is found for a strongly reduced capture probability from the exit surface as compared to the entrance surface. Absolute yields for electron capture and projectile ionization are presented and discussed within a Monte-Carlo simulation.

We investigated X-ray emission from highly charged slow Pb^q+ ions (q = 50–58, E = 8.5 qkeV) hitting metal surfaces. We confirmed occurrence of internal dielectronic excitation (IDE) i.e. excitation of the closed M-shell core by relaxation of "hollow-atom" electrons with a probability of unity. Calculation of singly and doubly excited states showed that population of singly excited states of n > 9 are most likely causing the core excitation. Further, we demonstrate a method for measuring the mean depth where X-ray emission occurs. The emission of the X-rays from ions with initial M vacancies occurs inside the bulk.

The interaction of multiply charged ions with fullerenes is governed by electron capture as well as nuclear and electronic stopping. The fullerene excitation due to electronic stopping leads mainly to direct ionization and multifragmentation. Our experiments show that the according patterns in the fragmentation spectra can serve as fingerprints for the electronic stopping. A quantitative analysis of the cross sections for ionization and multifragmentation reveals the characteristic linearity in the projectile velocity v as well as an oscillatory dependence on the projectile atomic number Z. The π-electron dominated electronic structure of fullerenes has some similarities to that of biomolecular species. Qualitatively similar dynamics are therefore expected to occur in HCI collisions with biomolecules. First experimental results indeed indicate a strong dependence of the fragmentation pattern on projectile charge state and velocity.

First results for the investigation of electron capture processes in collisions between multicharged ions and molecule targets using electron spectroscopy in coincidence with charged fragments, are presented. It is shown that a much more detailed investigation of the capture reaction can be achieved using molecular instead of heavy atomic targets provided that an analysis of the target dissociation is made. The collisional systems ¹⁸O⁸⁺ + Ar, CO₂ and C₆₀ have been studied at 80 keV. Non coincident electron spectra as well as first results of double or triple coincidence experiments are discussed. Kinetic energy distributions of the C_n⁺ fragments (n = 1 to 8) produced in multiple capture processes from C₆₀ target are given. A detailed investigation of the double capture process with CO₂ molecule allows the measurement of kinetic energy release distributions (KERD) which characterize the dissociation of CO₂²⁺ molecular ions; our results are found to be very similar to those measured in double photoionisation experiments.

We have succeeded in observing resonant coherent excitaion (RCE) of 1s electrons to the n = 3 states in 390 MeV/u hydrogen-like Ar¹⁷⁺ ions planar channeled in a silicon crystal through measurements of the charge-state distribution of ions transmitting the crystal. Furthermore, we directly confirmed RCE to the n = 3 states by observing the enhancement of the de-excitation X-rays, i.e., K_β X-rays under the resonance condition. The resonance profiles of the charge-state distribution as functions of the incident angle to the crystal, which uniquely relates with the transition energy, have a characteristic structure consisting of several peaks. Compared with the profile of RCE to the n = 2 states, the present profiles show a large peak shift from the j = 1/2 and 3/2 levels in vacuum, and the profiles are much wider than those expected from the Stark-split level structure of the n = 3 manifolds due to the position- (distance from the channel center in the planar channel) dependent strong static field in the crystal.

Electron emission from multi-electronic target atoms by 3.6 MeV/uAu⁵³⁺ ion impact is explored in kinematically complete experiments. DDCS as a function of the longitudinal and transverse component of the emitted electron momentum are in very good agreement with calculations using the CDW-EIS model. A study of the electron emission from specific sub-states shows that the initial state momentum distribution is mapped onto low lying continuum states with characteristic features which depend on the initial state wave- function.

We report the results of recent experiments performed at the CS (Catania) and GANIL (Caen) cyclotron accelerators, with 45 A MeV 19⁺ and 28⁺⁵⁸Ni and 95 A MeV 18⁺³⁶Ar pulsed beams, respectively. Electron velocity spectra were measured in a large angular range with different target foils (C, Al, Ni, Ag, Au). Besides electrons with the beam velocity (convoy electrons, CE) and two times the beam velocity (binary encounter electrons, BE), we observe also a high velocity tail in the case of the Au target only, which can be explained by a "Fermi-shuttle" mechanism. We also give absolute BE and CE production cross-sections. As a preliminary result of the recent GANIL experiment, we observe the lack of the CE component at the most forward angles in the case of the carbon target. For the other targets the CE yield increases as a function of the target atomic number.

Two recent experiments studying charge transfer between slow multiply charged ions and a dense CW Rydberg target are described, one studying the total charge transfer cross section and the other the details of the energy transfer. Together, these experiments provide a detailed picture of many important aspects of the charge transfer process, and confirm theoretical predictions obtained with Classical Trajectory Monte Carlo methods.

State selective electron capture by state prepared ground state ions of C²⁺, N²⁺ and O²⁺ in collisions with atomic hydrogen has been studied in the energy range 1.5–6 keV using the technique of double translational energy spectroscopy. In the case of C²⁺/H system it is clear from this comparison that whilst experiment and theory agree that the C²⁺(2s2p²) ²D state is the dominant product state there are considerable differences for the other C²⁺ product states. For the N²⁺/H system the experimental measurements confirm the predicted dominance of N⁺(2s2p²) ³D^o formation but show that significant yields of other product states are formed. The only significant O⁺ product state resulting from the one-electron capture process by ground state O²⁺³P with H at 6 keV is O⁺ (2s2p⁴) ⁴P confirming theoretical calculations.

Spectral analysis of tokamak plasmas penetrated by medium to high energy (5 keV/amu – 120 keV/amu) neutral deuterium and helium beams is explained. Attention is limited to the diagnostic exploitation of absolute measured spectral intensities. Such measurements reflect the population distributions of the excited emitting species in the beams and in the plasma and the influences upon them. For the neutral beam atom emitters, impact of plasma impurity ions is the primary influence in the medium/high energy regime, but the effective emission at tokamak densities represents the collective effect of many re-distributive processes. Also, the method of analysis is governed by the relaxation lengths of populations in the beam compared with density and temperature scale lengths in the plasma and the spatial resolution expected of spectroscopy measurements. For impurity ion emitters in the plasma, emission follows the primary charge transfer from the neutral beam donor. The distribution of capture is influenced by the various donor states and the consequential effective emission again influenced by re-distributive processes in the plasma. The current state of such charge exchange and beam emission spectroscopic diagnostics is examined from the point of view of the quality of analysis achieved, the factors limiting precision and how enhanced precision might be obtained in the future.

The radiation from tungsten ions in the extreme ultraviolet spectral region was investigated using a 2-m grazing-incidence spectrometer in conjunction with the Berlin electron beam ion trap. Operating EBIT at beam energies between 900 eV and 1.7 keV In-like W²⁵⁺ to Sr-like W³⁶⁺ ions could selectively be excited, and a bright emission band of about 2 Å width was measured which shifts from 50 to 54 Å when raising the charge state. The band of partially unresolved lines originates from 4l–4l' transitions of ions having an open 4d subshell. Atomic structure calculations with the HULLAC code package show that the narrowing and shift of the line band emission can be interpreted in the framework of the unresolved transition array using mixed configurations. The theoretical spectrum analysis applies a collisional-radiative model to account for the low electron density of EBIT and reproduces the variations of the observed emission pattern.

The study of Highly Charged Ion-induced diatomic (CO) and triatomic (CO₂) molecules fragmentation by the coincident measurement of the fragment momenta is presented. It is shown that the experimental results together with ab initio calculations including a large number of potential energy curves evidence the limitation of the Coulomb Explosion model to reproduce the dynamics of the fragmentation. The geometry modification of the CO₂ molecule during the breakup is explored and the concerted or sequential nature of the dissociation is discussed for two fragmentation channels. Finally, the fragmentation of the H₂ molecules following collisions with slow multicharged ions is analysed for various projectiles and energies. From the energy spectra, the effect of the projectile on the H⁺ fragments is evidenced. This three-body effect is discussed in connection with the results of two model calculations including or not the role of the projectile.

Fragmentation of D₂ by slow Xe²⁶⁺ ions is studied using multi-hit, position- sensitive detection of both D⁺ fragments. By detecting both ion fragments, kinematically complete pictures of the breakup dynamics following double electron removal are obtained. These data are compared to 5-body CTMC calculations. For slow collisions, two types of three-body effects are identified. The largest is attributed to interactions between the undissociated molecule and the dipole field of the projectile and is compatible with a two-step fragmentation picture. A smaller effect, attributed to quadrupole-like and higher multipole interactions between the dissociating fragments and the projectile field is also observed. This "true" 3-body effect, extracts internal energy from the exploding molecule and is incompatible with a two-step picture.

High precision mass measurements are performed at the SMILETRAP facility by measuring the cyclotron frequencies of ions trapped in the electric and magnetic fields of a Penning trap. Since the resolving power increases linearly with the ion charge state, the SMILETRAP experiment is connected to an Electron Beam Ion Source (EBIS). Using this technique we have shown that we are able to measure routinely the mass of any stable element within 1 ppb accuracy. Examples of some of the recent mass measurements and their possible applications for various fields of atomic and nuclear physics are given.

A technique for producing cold ensembles of trapped highly charged ions is described. The ions, trapped in an electron beam ion trap, can undergo a drastic contraction during the pulsed mode of evaporative cooling, if a truncated Boltzmann distribution is assumed. The underlying theory and the experimental results are presented.

Two years ago, a small electrostatic storage ring ELISA (ELectrostatic Ion Storage ring, Aarhus) was put into operation. The design of this small 7 m circumference ring was based on electrostatic deflection plates and quadrupoles. This is in contrast to the larger ion storage rings, which are based on magnetic focusing and deflection. The result is a small, relatively inexpensive, storage ring being able to store ions of any mass and any charge at low energy (< 22 keV). The average residual-gas pressure is around 10^-11 mbar resulting in storage times of several tens of seconds for singly charged ions. The maximum number of singly charged ions that can be stored is a few 10⁷. Several experiments have already been performed in ELISA. These include lifetime studies of metastable ions and studies of fullerenes and metal-cluster ions. Lasers are also used for excitation of the circulating ions. Heating/cooling of the ring is possible. Cooling of the ring leads to significantly lower pressures, and correspondingly longer lifetimes. A change of the temperature of the vacuum chambers surrounding the ion beam also leads to a change of the spectrum of the black-body radiation, which has a significant influence on weakly bound negative ions. At the time of writing, at least two other electrostatic storage rings are being built, and more are planned. In the following, the electrostatic storage ring ELISA will be described, and results from some of the initial experiments demonstrating the performance will be shown. The relative merits of such a ring, as opposed to the larger magnetic rings and the smaller ion traps will be discussed. The potential for highly charged ions will be briefly mentioned.

In the first measurements at the Freiburg electron beam ion trap FreEBIT, highly charged ions of Xe, Ba, W, Kr and Ar have been observed (with charge states up to 63+) by means of X-ray spectroscopy. Experiments with an optical imaging spectrograph have been also carried out. Forbidden transitions in the visible range have been measured, from Kr²²⁺ and Xe³²⁺ stored in the trap. The resolution achieved in the optical range is γ/δγ ≈ 4000.

We have performed studies of keV X-ray production from (Ar)_n, (Kr)_n and (Xe)_n rare gas clusters (with n between 10⁴ and 10⁶ atoms/cluster) submitted to intense (10¹⁸ W/cm²) infrared (790 nm) laser pulses. We have determined the photon energies and the absolute photon emission yields as a function of several physical parameters governing the interaction: size and atomic number of the clusters, peak intensity of the laser. Up to 10⁶ 3 keV photons per pulse at a moderate (10¹⁵/cm³) atomic density have been observed. High resolution spectroscopy studies in the case of (Ar)n clusters have also been performed, giving unambiguous evidence of highly charged (up to helium-like) ions with K vacancies production. The results obtained indicate that X-rays are emitted before cluster explosion on a subpicosecond time scale, and shed some light on the mechanisms involved in the first stage of the production of the nanoplasma induced from each cluster.

We have observed the variation in ion signal as a function of intensity within a focused laser spot. Using an aperture detector, the ion signals from narrow bands of the laser focus have been observed. By moving the laser focus along the direction of propagation, regions of different intensities are exposed to the detector. This has allowed detailed measurements to be made of ion signals as a function of laser intensity.

The magnetic dipole transitions between fine structure levels in the ground term of Ti-like ions, (3d⁴) ⁵D₂ –⁵D₃, were investigated by observation of visible and near-UV light for several elements with atomic numbers from 51 to 78. The wavelengths are compared with theoretical values we recently calculated. The differences between the present calculations and measurements are less than 0.6%. The anomalous wavelength stability predicted by Feldman, Indelicato and Sugar [J. Opt. Soc. Am. B 8, 3 (1991)] was observed. We attribute this anomalous wavelength stability to the transition from LS to JJ coupling and the asymptotic behavior of the transition energies in the intermediate coupling regime.

Z-dependence of atomic properties for n = 3 excited states of highly charged neonlike ions was investigated theoretically for Z = 50–56. Configuration interaction effects for relativistic atomic states of three electric-dipole lines: 3D (3d_5/2 → 2p_3/2), 3E (3d_3/2 → 2p_3/2) and 3F (3s_1/2 → 2p_1/2), were examined in terms of jK-coupling. A present collisional-radiative model consistently explained intensity ratios for 3F/3D observed with the Tokyo-EBIT, and predicted a vanishing intensity for the 3E line near Z = 52. The vanishing intensity for the 3E line was attributed to the Cooper-minimum type mechanism.

High-resolution X-ray spectra obtained by the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission put new demands on atomic data including line positions, excitation cross sections, and radiative rates of cosmically-abundant highly-charged ions. To address this need, we are performing measurements of the line emission from ions of cosmically abundant elements. The data are obtained at the LLNL Electron Beam Ion Trap and focus on cross sections for electron-impact excitation, dielectronic recombination, and resonance excitation as well as atomic structure measurements. We find that ratios of the electron-impact excitation cross sections of singlet and triplet levels are systematically different from the calculated values in the case of many highly charged ions. This, for example, has a profound impact on inferring optical depths from solar and stellar atmospheres. Moreover, new line identifications are presented that resolve some long-standing puzzles in the interpretation of solar data, and the importance of resonance contributions to the spectral emission is assessed.

We have measured the number statistics of emitted electrons for multiply charged Ar-ion impact on a clean LiF(001) surface under grazing incidence conditions. For projectiles reflected from the surface we have also recorded the scattering distribution on a two dimensional detector in coincidence with the number of emitted electrons. In this way it was possible to distinguish electron emission for three different types of trajectories: non-penetrating projectiles specularly reflected from the topmost surface plane, projectiles entering the target and being reflected from subsurface layers and projectiles penetrating deeply into the target. The results support existing concepts on the charge state dependence of potential- and kinetic electron emission from poly- and monocrystalline LiF gained in normal incidence experiments.

The kinetic energy release for the reaction channels C₆₀⁵⁺ → C_60-n⁵⁺ + C_n⁺ has been measured from the time of flight profiles of C_n⁺ for 2< n < 6. The related intercharge exchange distance correlates sensitively with the ionization potential of the C_n clusters and is comparable to the resonant charge transfer distance obtained with the classical barrier model. Highly charged C₆₀⁵⁺ was prepared by electron capture from a low energy Ar⁸⁺ beam and a multi- coincidence technique was used to select each reaction channel.

Electron multicapture processes at large impact parameters have been studied for collisions between 150 keV Xe³⁰⁺ (v ≈ 0.2 a.u.) and C₆₀. By detecting the number of ejected electrons in coincidence with an outgoing Xe²⁸⁺ ion and a C₆₀^r+ ion (3 ≤ r ≤ 6) we have obtained electron spectra corresponding to the ejection of (r - 2) electrons in the continuum. Experiments have been performed for several extraction fields and the electron collection probability has been extracted from each spectrum. A charge trajectory simulation allows an estimation of the kinetic energy distribution of the ejected electrons. We find that most of the detected electrons have energies less than 100 eV.

Magnetic dipole transitions between the fine structure levels (3d⁴) ⁵D₃–⁵D₂ of titanium-like silver and tin have been observed as emission lines from the Oxford electron beam ion trap (EBIT). The precision of the measurement system is validated by observation and comparison of well known lines in Kr II and He I with the database values, justifying uncertainties of 4–12 ppm.

A technique is described whereby measurements of ions extracted from an electron beam ion trap can be used to deduce their temperature dynamics. The measured temperature dynamics shows the expected trend as a function of charge and also gives evidence for Landau-Spitzer heating, ionization heating and evaporative cooling.

Charge exchange followed by radiative stabilization are the main processes responsible for the recent observations of X-ray emission from comets in their approach to the Sun. A new apparatus was constructed at JPL to measure, in collisions of HCIs with atoms and molecules, (a) absolute cross sections for single and multiple charge exchange, and (b) normalized X-ray emission cross sections. The ions are produced by the JPL HCI Facility and passed through a neutral-gas target cell. The product charge states are analyzed by a retarding potential difference technique. Results are made absolute by measuring target pressure, and incident and product ion currents. X-rays emitted from the product ions are detected with a Ge solid-state detector having a resolution of approximately 100 eV. X-ray astronomy has taken major steps forward with the recent launch of the high-resolution satellites Chandra and Newton. The cross sections reported herein are essential for the development of the solar wind comet interaction models inspired by these observations.

Absolute cross sections for the electron-impact multiple ionization and fragmentation of negatively charged fullerene ions C_n^–(n = 60, 70, 84) → C_n-m^q+ (q = 1, 2, 3 and m = 0, 2, 4) have been measured for electron energies up to 1 keV. A scaling law has been observed for the magnitude of the ionization cross sections. Furthermore, the multiple ionization of a fullerene anion is found to be a sequential process. A novel ionization mechanism is proposed which can be expected to be valid also for other negatively charged molecular or cluster ions.

Scanning tunneling microscopy (STM) with atomic scale resolution has been applied to study surface defects in highly oriented pyrolytic graphite (HOPG) which have been produced by impact of 150 eV singly and multiply charged Ar ions (charge state up to 9+). The most prominent surface defects are protrusions. Their area density is in good agreement with the applied ion dose, implying that about every single ion impact causes one protrusion. A (√3 × √3) R 30° surface reconstruction, as characteristic for interstitial defects in HOPG, is observed in the vicinity of most defects. As the most remarkable result we find that the measured size of the hillocks (mean diameter and height) increases with projectile charge state.

Molecular dynamics simulations of the cooling of highly-charged ions captured into a Penning ion trap (RETRAP) along with Be⁺ ions are described. Experimentally, the Be⁺ ions were directly cooled continuously using a laser, and collisional coupling to these laser-cooled ions cooled the highly-charged ions. The simulation is described as a tool for experimental interpretation and refinement. Under conditions closely related to the experimental parameters, the highly charged ions were found to form ordered structures, and to exhibit characteristics of a magnetized non-neutral plasma.

Orientation dependence is investigated for multiple electron transfer from homonuclear diatomic molecules in slow collisions with highly charged ions. Angular distributions of the molecular axis with respect to the incident beam are calculated for r-electron transfer (1 ≤ r ≤ 10) from nitrogen molecules with the three-center Coulombic over-barrier model developed by the present authors. The quadrupole anisotropy parameter β₂ obtained indicates universal variation with the incident charge q; β₂ ≃ – 1 at the threshold (i.e. q = r), taking the minimum of β₂ ≃ – 0.1 at q ≃ r, and converging to β₂ ≃ – 0.06 for q ≫ r. The present model generally derives oblate distributions (β₂ < 0), in striking contrast to the statistical energy-deposition model, which gives prolate distributions (β₂ > 0) for multiple ionization by fast-ion impact.

Charge changing collisions between slow protons (v ∼ 0.2 a.u.) and C₆₀ fullerenes have been studied with the technique of coincident time-of flight mass spectrometry. Despite the low collision energies, the fullerene target is multi-ionised and formed in charge states 1 to 3, the outgoing scattered protons are neutralised or negatively charged. The relative importance of different processes (single and double capture, transfer ionisation and target excitation) as a function of the scattering angle, as well as corresponding angular distributions are discussed

Sodium clusters Na_n in the size range 20 < n < 600 have been ionized in single collisions with multiply charged ions at velocities between 0.4 and 0.9 a.u.. Depending on the initial charge state z of the projectile (1 < z < 28), stable cluster ions are produced in charge states r ranging from 1 to ∼ 11. As the stability of multiply charged clusters depends on their internal energy, we have analyzed the appearance sizes in order to determine the cluster temperature and hence the energy which has been transferred to the target during the ionizing collision. This energy depends on whether penetrating collisions are required to create a given charge state or not. In the case of highly charged projectiles (distant collisions) the transferred energy can be as low as 1–2 eV for five-fold ionization

A new, very high magnetic field superconducting ECR ion source, VENUS, is under development at the LBNL 88-Inch Cyclotron. It will boost the maximum energies and intensities for heavy ions from the cyclotron particularly for ions with mass greater than 60. It will also serve as R&D ion source for the proposed Rare Isotope Accelerator (RIA) project in the US, which requires up to 10 pµA of U³⁰⁺. The superconducting magnet structure consists of three solenoids and six racetrack coils with iron poles forming the sextupole. The coils are designed to generate a 4 T axial mirror field at injection and 3 T at extraction and a radial sextupole field of 2.4 T at the plasma chamber wall. Test results of the magnet coils, which exceeded design requirements with minimum training, are presented. The magnet assembly with its cryostat will be enclosed by an iron shield and therefore must be designed to withstand any possible forces between coils and iron, which can be as high as 3.4 · 10⁵ N (35,000 kgf). The low energy beam transport line (LEBT) and mass analyzing system of the ion source is designed to transport a proton- equivalent current of 25 mA at 20 kV extraction voltage. The design of the ion source and LEBT will be discussed.

Autoionization spectra of doubly and triply excited Li atoms and doubly excited states of Li⁺ have been recorded. The excited Li atoms and Li⁺ ions were created by impacting 30 KeV Li³⁺ ions on C₆₀, Ar and Xe target. The spectra reveal previously unresolved structures which are tentatively assigned.

An experimental study has been made on multifragmentation of free C₆₀ molecules using highly charged neon ions at an incident energy of 30 MeV. Production cross sections for fragment ions were obtained systematically at projectile charge states q = 2 ∼ 9 using time-of-flight coincidence method. Total cross sections for all product ions are found to increase approximately as q^1.2. On the other hand, cross sections for individual ions exhibit significantly different q-dependence for different ions. As an overall feature, most product ions begin to increase slowly at low q values and then sharply as q² for q ≥ 7. Production of C₁⁺ ions is found to be enhanced exhibiting a q^2.7 dependence for q ≥ 7.

High-resolution data are presented for initial kinetic energy distributions (KEDs) of fragment ions from C₆₀ bombarded by 1.5-MeV Li⁺ and 6.0-MeV Si³⁺ ions. The KEDs were obtained by means of a peak profile analysis of time-of-flight spectra measured in coincidence with secondary electrons emitted simultaneously. It is found that the values of KE decrease monotonically with increasing cluster size; e.g., mean values are about 12 eV (C₁⁺) and 1 eV (C₁₁⁺) for Si³⁺ impacts. Also, we found that the KEDs for small ions C_n⁺ (n > 1) showed single-peak distribution patterns. The most important result obtained in this work is the double peak distribution pattern observed for C₁⁺ ions. This newly observed structure suggests clearly that two different production mechanisms must be taken into account for the production of C₁⁺ ions in C₆₀-multifragmentation process.

We report on measurements of absolute scattered projectile charge fractions for Ar^q+ ions (q = 2–13) with incident energies in the range 2.4–35 keV, that have been 120° backscattered from Au(110) in quasi-binary collisions. Use of a time-of-flight technique that incorporates a biased drift region permitted full separation of all scattered charged states, including neutrals. At fixed projectile energy, the scattered ion fractions are relatively independent of incident charge state until the projectile L-shell is opened, at which point large enhancements are observed, amounting for 2+ scattered ions to more than a factor of twenty. For fixed projectile charge state, the scattered neutral fractions all decrease monotonically with increasing energy, with the higher (q > 1) charge fractions showing a corresponding increase, while the q = 1 fractions initially rise to an extremum and then fall off with yet higher incident energies. Comparisons are made with the recent experimental results of Pešic et al.

The production of hollow atoms through the collisions of fast electrons with a solid is studied. These electrons are produced by high-intensity short-pulse laser irradiation on a solid. The inner-shell ionization and excitation processes by the fast electron impact are investigated. It is found that ionization processes give more significant contribution to the production of hollow atoms.

Usually in an electron beam ion source (EBIS) the electrons are radially confined by a strong magnetic field, resulting in a constant electron density. In a XEBIST (X = crossover, T = trap) without confining magnetic field the current density is determined by beam spreading due to the beam space charge and optimized by proper matching with a lens system. By continuous production of ions and the increase of their charge states the space charge neutralization of the electron beam increases until a stationary state is reached Therefore, the effective current density in the trap will increase. Caused by the magnetic self field of the electron beam a further compression can be observed. According to this relativistic pinching effect even higher current densities may be expected at electron energies of some 100 keV than with magnetic confining field. In order to demonstrate this behavior we have set up an XEBIST to operate up to 65 keV and observed in first experiments at 22 keV an increase of the average current density from 10 to 150 Acm^-2 at 40 ms neutralization time. Highly charged ions like Ar¹⁶⁺ could be produced in this way.

Dielectronic recombination (DR) of very heavy Li-like ions including Pb⁷⁹⁺, Bi⁸⁰⁺, and U⁸⁹⁺ with Δn = 0 core excitations (1s²2s_1/2 → 1s²2p_1/2,3/2) has been investigated at the electron cooler of the Experimental Storage Ring (ESR) of GSI in Darmstadt, Germany. Absolute rate coefficients and resonance energies of DR via intermediate doubly excited Rydberg states 1s²2p_1/2nℓ (n ≥ 20) and doubly excited states 1s²2p_3/2nℓ_j (n = 6 for Pb⁷⁹⁺ and Bi⁸⁰⁺, and n = 5 for U⁸⁹⁺) of recombined Be-like ions have been determined. For the 1s²2p_1/2nℓ series Rydberg states up to n ≈ 50 were resolved in the measurement. In addition individual fine structure components of the Rydberg resonances can clearly be distinguished. For the first time in such a heavy system the series limit has directly been observed in DR. For these very heavy ions strong relativistic effects as well as the Breit interaction have to be taken into account in order to explain the experimental findings. The experimental data are compared with fully relativistic multi configuration Dirac Fock (DR/MCDF) calculations

To demonstrate isotope shift measurement for high-energy highly charged ion beams, we measured precisely the 1s2s ³S₁–1s2p ³P_0,1,2 transition energy of He-like ¹²C ion accelerated up to 0.9 MeV/u and ¹³C ion to 0.6 MeV/u. For the precision measurement, the uncertainty coming from the ambiguity in the absolute ion beam velocity was suppressed by means of that the resonance energy was measured by two laser beams which propagate in parallel and anti-parallel directions to the ion beam. As the results, an isotope shifts of these transitions were obtained with an accuracy of 10%.

A simple theoretical model based on gradual Auger neutralization of a highly charged ion as it approaches the surface, with consequent positive charge deposition in surface layers and their expansion due to Coulomb repulsion provides the means to make some estimates that could explain the creation of very shallow blisters and craters on surface, as well as sputtering of up to 10³ atoms in a single ion impact. Calculation of the dependence of blister size on projectile charge, based on charge evolution, gives some results fitting the experimental data. The model deals not just with the conducting properties of the solid, but with its structure as well, for instance the layered structure of mica. While the general source of energy remains the same, the particular mechanism of its realization depends largely on the composition, structure and electronic properties of the solid. The composition of the ejecta is discussed within the framework of the shock wave approach.

The vibrational distribution of H₂⁺ (X²Σ_g⁺;v) after charge transfer in H⁺+H₂ and C²⁺(2s2p;³P)+H₂ collisions is obtained using the SEIKON approach and ab initio molecular data. We also present H₂ vibrational excitation cross sections in H⁺+H₂ collisions.

Electronic sputtering of nitrogen on carbon was studied as a function of the electronic energy loss dE/dx and the projectile charge q with swift highly charged heavy ions (Z = 6–73, q = 6–54, E = 6–13 MeV/u). The fluence dependence of low energy electron yields induced by swift heavy ions allows to deduce the cross section σ_N2/C for sputtering of nitrogen from carbon. A strong correlation of this desorption cross section (or the sputtering yield) is found with the parameter dE/dx and in a lesser extend with q. The dependence on q is close to the cubic charge dependence observed for the emission of H⁺ secondary ions which are believed to be emitted from the very surface. On the other hand, the dependence on dE/dx is close to a quadratic one thus rather pointing towards a thermal evaporation-like effect.

We have determined exit charge states and energy losses of slow, highly charged gold ions after transmission of thin (7 ± 3 nm) diamond like carbon foils. Average charge states of up to 37+ were observed for Th⁸⁰⁺ at 8.2 keV/u (0.57 v_Bohr). The final charge state is populated through re-arrangement processes after the projectile has left the foil. High final charge states indicate that none of the M-shell vacancies in Th⁸⁰⁺ could be filled inside the foil. The increase of the average exit charge state as a function of M-shell vacancies is a sign of strongly enhanced X-ray fluorescent yields for hollow atom decay in vacuum.

We present a theoretical study of convoy electron emission resulting from transmission of relativistic 390 MeV/amu Ar¹⁷⁺ ions through carbon foils of various thicknesses. Our approach is based on a Langevin equation describing the random walk of the electron initially bound to the argon nucleus and later in the continuum. The calculated spectra of ejected electrons in the forward direction exhibit clear signatures of multiple scattering and are found to be in good agreement with recent experimental data.

A new version of computer codes based on balance equations for all ionic charge states and theory of electron and ion confinement in the open magnetic trap of an ECR ion source was developed and tested for the numerical simulation of ion injection into the ECR plasma. The presented numerical simulations of pulsed ion injection into the ECR source have shown some advantages of the combination of ECRIS and laser ion or neutral sources. The pulse loading of heavy elements in the ECR source could give a high current output of highly charged ions due to a narrow charge state distribution of ions in the source. The extraction current of every charge state has a pulse structure with a duration of a few milliseconds. Different charge states have different build-up times. It is possible to select the extraction of a charge state by changing the time length of the RF pulse after the time of ion or neutral beam injection. There are only few charge states at every moment of time in the source and the charge state distribution of the extracted current is very narrow in comparison with ordinary ion loading. This effect should be strongly enhanced in the afterglow mode of operation and results of simulations promise to produce ion pulses of Pb²⁷⁺ of about 1 mA in the afterglow mode (0.3 ms duration) in the SERSE (INFN-LNS, Catania) type ion source. The presented simulations are in qualitative agreement with existing experimental data of neutral flow injection from laser ablation plasma.

The effectiveness of highly charged ion production and X-ray bremsstrahlung in an electron-cyclotron resonance (ECR) ion source is determined by the energy distribution function of electrons in the source plasma. Three components of electrons are introduced to describe the electron energy distribution in ECR plasma. The new model and equations were used in a new version of computer codes for numerical simulation of highly charged ion production and bremsstrahlung emission in the afterglow mode of ECR source operation. The multicomponent consideration of electrons in the ECR plasma is applied to explain physically and to simulate numerically the effect of long bremsstrahlung (100–300 ms) in the afterglow regime that was discovered recently in the ECR source at RIKEN.

Stopping forces on swift heavy ions have been evaluated theoretically, with an emphasis on the velocity regime where screening due to electrons bound to the projectile cannot be neglected. Bohr's classical theory is an appropriate starting point, but major complications arise from higher-order Z₁ effects, shell correction and projectile excitation/ionization. These effects have now beeen accounted for, and good agreement is being achieved with experimental results over a wide velocity range without introduction of adjustable parameters.

We have developed a novel delay-line anode design based on L/2 shifted maeander-lines for the readout of open-faced and sealed MCP-based detectors. In combination with the sealed detector (image intensifier) we are able to provide position and time sensitive single photon detection from near UV to near IR.

Low-energy electrons ejected by 1–4 keV Ne⁴⁺ ion impact on an Al surface were measured. Spectral structures found near 11 eV were attributed to the decay of plasmons induced by potential energy of the projectile. The data were used to determine absolute values for electron yields from the plasmon decay, which were primarily studied as a function of the incidence angle of the Ne⁴⁺ projectile. Strong variations of the plasmon yield are observed when the angle of incidence reaches that of specular reflection indicating significant changes in the contributions to plasmon production from below and above surface.

Using the multistep-capture-and-loss (MSCL) model, stabilization processes of Kr hollow atoms formed below the surface of a solid have been analyzed through the simulation of the K X-ray spectra. It is found that the Coster–Kronig (CK) transitions lead to the increase of the intensity of only KL⁷ and KL⁸ satellite lines in the simulated spectra.

New experimental data on the transport of Kr³⁵⁺ inner-shells initially populated either by capture or by excitation processes are presented. Absolute Lyman (np → 1s) intensities, directly related to the np state populations, as well as 3l_j substate populations have been determined over a range of carbon target thickness allowing to study the transport from single collision conditions to equilibrium. Results are compared with predictions of different transport simulations which take into account multiple collisions, the strong target polarization induced by the incoming HCI (the wake field), and radiative decay. Very good agreement is found between theory and experiment for the np populations up to n = 5 where induced wake mixing becomes visible. The simulations also explain the behavior of the 3l_j populations of Kr³⁵⁺ which exhibit a strong sensitivity to the presence of radiative decay during transport and the effective value of the wake field.

We investigated the influence of the scattering geometry and incident energy on the neutralization degree of highly charged Ar^q+ (q = 7–13) ions scattered from single Au crystal. It is found that at large scattering angle the probability of detecting multiple charged ions after interaction is high. We point out that besides the interaction time, additional ionization in hard collisions is also important. The results are supported by model calculations which consider electron exchange processes along the ion trajectory.

X-ray spectroscopic measurements of highly charged xenon ions produced in the Dresden electron beam ion trap (DEBIT) are presented and compared to simulation results. The measurements are done using both a Si(Li) solid state detector and a crystal diffraction spectrometer being equipped with a SiO₂ (100) crystal and an energy sensitive CCD camera. The simulation is performed by a code that calculates the charge state evolution in an EBIT by solving the particle and energy rate equations. In addition the energies of the most intensive lines of the wavelength dispersive spectrum are compared to calculations using the GRASP atomic structure code. The calculated charge states and energies are in good agreement with the experimental results.

We report the first measurement of the kinetic energy release distributions of multiply charged oxygen ions produced in collisions of O₂ molecules with 5.9 MeV/amu Xe¹⁸⁺ and Xe⁴³⁺ ions. The slow O^q+ ions generated in the collision are detected in coincidence by a time- and position sensitive multi- particle detector. An analysis of correlated momenta of molecular fragments provides the kinetic energy release distribution for various reaction pairs O₂ → O^q1+O^q2+ with a total charge up to q = q₁ + q₂ = 11. The position of the maximum and the width of the distributions are well described by the recently suggested Average Bunch Statistical (ABS) model.

In order to investigate the ion-optical parameters of the AECR-U injection line into the 88-Inch Cyclotron, an electrostatic-deflection-type emittance scanner has been designed and constructed. It allows fast on-line measurements, while tuning the ion beam through the cyclotron. Emittance measurements have been performed for various high charge state ions. First results indicate a strong mass dependence of the normalized beam emittance. For example the normalized rms emittance for protons (0.24 π · mm · mrad) is four times higher than for O6+ (0.06 π · mm · mrad) and about 8 times higher than Kr19+ (0.03 π · mm · mrad). Furthermore it was found, that the emittance values are approximately independent of the current at the medium ion beam intensities. The predominant factor on the beam emittance is shown to be the plasma stability. The emittance measurements and the results are discussed in the paper.

By rf-modulating the current of an electron beam ion source and trap (EBIS/T) the ion distribution is significantly influenced by the simultaneously modulated radial trapping potential. It has been observed that highly charged ions are preferentially accumulated near the axis, showing a substantial reduction of their emittance. Furthermore, by using the appropriate cyclotron frequency adjusted according to the shift caused by the space charge of the electron beam it is possible to remove ions with specific mass to charge ratio from the trap. Rf-heating of low charged argon ions during stepwise ionization is enhancing the yield of highly charged argon ions.

In this work we measured the time dependence of the light intensity of the 1s²2s ¹S_1/2–1s¹2p ¹P_3/2 resonance transition in Li-like Al ions after passing through a 10 µg/cm² carbon foil. The measurements were done at beam energies of 0.8 MeV/u and 2.5 MeV/u. Also, the 1s5g–1s6h transition in He-like Al ions, after passing through the same thickness carbon foil, at 2.5 MeV/u was investigated. The results show very obvious decreases of the light intensity for the Li-like transition with beam exposure time of the foil at both beam energies, while a slow increase was observed for the He-like line intensity. In this work several different foil aging effects on line intensities are discussed and a new normalization scheme is proposed to improve the reliability of beam foil spectroscopy method in lifetime measurements.

In this work, we studied hollow ions produced in collisions of Ar ions with metallic targets of atomic numbers 12 ∼ 73, at incident energies 43 ∼ 95 MeV. We observed K_α hypersatellites and satellites of Ar ions and obtained L shell electron populations with vacant and half vacant K shell. The results are very different to those reported for hollow atoms formed in slow collisions.

Single electron capture from excited H(2s) and H(2p) states by impact of protons, alpha particles and Li³⁺ is theoretically studied using the Continuum Distorted Wave-Eikonal Initial State approximation. Total cross sections are calculated for proton and alpha particles, and partial cross sections for capture to final bound states with principal quantum number n_f = 4 are determined for Li³⁺. Comparisons with other existing theoretical predictions are presented.

Electron-impact ionization cross sections σ_{q,q+ n} have been measured for the reaction e + Sm^q+ → Sm^(q+n) + (n + 1)e for single, double and triple ionization and initial charge states q = 1, 2, 3. The measurements have been performed using the dynamic crossed-beams technique in an energy range from the respective ionization threshold up to 1 keV. Furthermore, we have measured the single-ionization cross section for Sm¹²⁺ using the dynamic crossed- beams method and an energy-scan method. The cross sections are compared with semi-empirical formulae. In most cases the measured cross sections are described insufficiently. In the single-ionization cross section for Sm¹²⁺ a remarkable number of narrow structures resulting from resonant excitation with subsequent double-autoionization (REDA) are observed.

The formation of doubly-vacant, i.e., "hollow", states in 0.5–2.0 MeV/u Li-like C³⁺ ions colliding with helium has been investigated using high-resolution Auger electron spectroscopy. From the measured spectra, hollow two-electron states corresponding to 2l2l' configurations can be identified, as well as hollow three-electron states corresponding to 2l2l'2l'' configurations. The results suggest that the electron–electron interaction is important in producing the observed double-K-shell-vacancy states.

Using both the LLNL high-voltage electron beam ion trap, SuperEBIT, and its low-energy counterpart, EBIT-II, we are currently performing spectroscopic measurements with electron beam energies ranging from 150 eV to 150 keV on ions ranging from near neutral Ne to ions as highly charged as T1⁸⁰⁺. Our measurements span photon energies from visible light to hard X-rays and focus on electron-ion interaction cross sections, line identifications and QED measurements, the determination of nuclear parameters, the investigation of charge transfer reactions, and radiative transition rates. An overview of some of the new instrumentation and a subset of the current experiments is given.

Absolute double differential cross sections for the electron production at zero degree laboratory observation angle were measured for high velocity hydrogenic carbon, nitrogen, oxygen and fluorine ions on molecular hydrogen. The energies of these ions were chosen so the elastic scattering resonance 2p²¹D for each case can be clearly observed near the peak of the binary encounter electron distribution. Close coupling R-matrix calculations of elastic differential cross sections of electron impact of these ions were related to the measured ion-atom cross sections by using the elastic scattering model (ESM). Excellent agreement was found between theory and experimental data.

Radiative recombination rate coefficients obtained using the nonrelativistic dipole result of Stobbe for the RR cross section are compared to other nonrelativistic approaches. A good agreement is found with widely used formulas. The RR rate coefficients can be calculated for different beam temperatures and for arbitrary nl sublevels up to high Rydberg states. The computation technique used allows us a fast evaluation of the dipole matrix elements without any additional approximation.

In order to improve the performance of existing ECR ion sources, different magnetic field configurations and their influence on the extracted ion currents have been investigated. Furthermore the source performances were found to be strongly dependent on the microwave frequency. At optimum frequency the microwave power requirements could be significantly reduced allowing the use of a low-power solid-state microwave system.

We measured energy distributions of electrons emitted in backward direction from sputter cleaned amorphous carbon foils under swift HCI bombardment. The shape of these spectra was found to depend strongly on the projectile charge q_p (q_p = 6 ... 39) for fixed velocity v_p (v_p = 19 atomic units, 9 MeV/u). We observe (for high projectile charge only) a K²VV hyper satellite line at 310 eV due to the double ionization of the K-shell in addition to the well known KVV peak of carbon (at 257 eV). From a careful analysis of the shape of these carbon Auger spectra including background subtraction and deconvolution techniques, we clearly observe a projectile charge dependence of the shape of the primary Auger electron spectrum. This effect is strongly connected to a modification of the electronic structure of the solid target at the moment of the Auger decay. We discuss different possible interpretations of this effect, such as an increase of the mean electronic temperature or a structural modification of the material.

Optical spectra of Ne, Ar and Kr have been excited in an electron beam ion trap (EBIT). A prism spectrograph was used for broad band (3700–6000 Å) spectral surveys with moderate resolution to establish the excitation thresholds and thus the charge states of transitions in highly charged ions. A high-resolution transmission grating spectrometer for high precision measurements yielded detailed spectra. The "inverted trap" technique was employed for in situ wavelength calibrations.

Absolute cross sections for electron impact single and double ionization of Ni^q+ ions (q = 10, 11, 13 and 15) have been measured for energies from threshold to about 5 keV. Experimental results for Ni¹¹⁺, Ni¹³⁺ and Ni¹⁵⁺ are in good agreement with the calculations of Pindzola et al, obtained in the average configuration distorted-wave approximation for the 3s²3pⁿ (n = 5, 3 and 1) ground configurations, respectively. The Ni¹⁰⁺ results show a significant ionization signal below the ground state [Ne]3s²3p⁶ threshold due to ions formed in the metastable configuration [Ne]3s²3p⁵3d in the parent ion beam. The theoretical calculations including direct ionization and excitation- autoionization'from both the ground and the excited configurations underestimated the cross section. The difference is probably due solely to the 3s → nl excitations. In this isonuclear sequence, the whole results shows that ions with an odd number of electrons behave differently from ions with an even number. Our measurements show clearly that double ionization is dominated by L-shell ionization followed by autoionization.

The O IV, F V and Ne VI spectra have been investigated by collision spectroscopy. Photons emitted by boron-like ions following berrylium-like ions – Cs electron capture collisions have been recorded in the 60–600 Å wavelength range. Displaced terms have been distinguished by using highly metastable incident O⁴⁺, F⁵⁺ and Ne⁶⁺ ion beams. Previously unreported or unassigned lines have been identified. Branching ratios of 2s–2p transitions have been measured.

A multi-configuration Dirac–Fock (MCDF) method has been used to study the 2s²2p (²P_3/2 –²P_1/2) M1 transitions of Ar¹³⁺ and all the M1 transitions among the 2s2p levels of Ar¹⁴⁺ by including correlation effects to a rather large extent. Some detailed comparisons of the present theoretical results with the newest experiments and a few existing calculations are presented.

The quasi-relativistic configuration interaction method is applied to calculate wavelengths and intensities of the 1s3lnl'–1s²nl'' satellite transitions for lithium-like argon. From these satellites, a theoretical line spectrum is obtained by a convolution of the relative intensities with an experimental resolution of 0.0003 Å. Our calculated spectra show excellent agreement with a recent high-precision measurement by Smith et al. [Phys. Rev. A 54, 462 (1996)], both in the positions and the relative intensities of the observed peaks

Wavelengths of the 2s ²S_1/2–2p ²P_1/2 fine structure transition in lithium-like ions in the medium heavy Z range have been measured using beam foil excitation and grazing incidence spectroscopy. The achieved precision of 70 ppm allows for a 0.1% QED test which establishes a new benchmark. It provides sensitivity to two-photon QED contributions. Very good agreement with the newest calculations based on hydrogenic wave functions and applying a strict QED formalism is observed. All accurate published data along the isoelectronic Li-like series for Z > 20 are summarized. Their values and their accuracies are compared to the calculations and to the relative QED contributions, respectively.

In this work, theoretical triply differential cross sections for ionization of several hydrogenic atoms from excited states by fast electron impact are computed'in the case of asymmetric coplanar geometry. The final wavefunction is chosen as an approximate solution to the three-body scattering problem with correct asymptotic conditions. The initial wavefunction also satisfies correct boundary conditions. Results for Li²⁺ ions in their 2s and 2p excited states are presented. In addition, it is shown that a simple scaling law for the triply differential cross sections obtained for ionization of hydrogenic targets from their ground state is also satisfied by excited hydrogenic targets.

X-ray spectra in the range from 5.5 to 19 Å from highly charged tungsten (Z = 74) ions were recorded in the tokamak ASDEX Upgrade from plasma discharges with electron temperatures up to 5 keV. Lines emitted from W³⁹⁺ to W⁵⁰⁺ were identified by comparison with ab initio calculations using the fully relativistic parametric potential code RELAC. In the short wavelength region the measured spectra could be simulated to a high degree of coincidence by using ion density distributions from impurity transport calculations and line intensities calculated by a collisional-radiative model, whereas in the long wavelength region not all of the stronger predicted lines are found. The measured absolute intensities from several spectral lines are used to calculate the tungsten density present in the tokamak plasma. The extracted tungsten concentrations are in the range of 10^-5–10^-4 indicating that a sensitive W impurity diagnostic can be based on the presented spectral lines.

First-order perturbation theory is a widely used method when studying the excitation and ionization of high-Z, hydrogen-like ions. In many applications, this theory has been found helpful explaining the ionization of projectile electrons in collisions with different target materials. It has recently been questioned by several experiments, though. Here, we comment on the validity of perturbation theory at intermediate and high energies and show that an improved screening model, which is based on the (many-electron) density distribution in the target atoms, helps understand most of the experimental findings for light and medium-Z elements up to energies of several (ten) GeV/u

Multiconfiguration Dirac–Fock wave functions have been applied to study the level structure and transition probabilities of high-Z nickel-like ions in the range 74 ≤ Z ≤ 84. Several level crossings are found which strongly influence the decay dynamics of the excited ions and explain why some of these lines cannot (so easily) be observed for all elements along the isoelectronic sequence. For two selected ions, the role of configuration interactions and relaxation is explored by large-scale expansions.

We calculate cross sections for electron–positron pair production in collisions of heavy ions using a coupled-channel approach. We use an expansion of the wave functions in terms of atomic wave functions centered around the target as well as the projectile

An experimental program to study low temperature dielectronic recombination (DR) in the iron L-shell ions Fe¹⁷⁺ to Fe²³⁺ is currently in progress at the TSR heavy-ion storage ring in Heidelberg, Germany. L-shell iron ions play an important role in determining the line emission and thermal and ionization structures of cosmic photoionized plasmas. Successful modelling of these plasmas requires accurate ionization and recombination rates, particularly for low temperature DR which is the dominant recombination mechanism for most iron ions in photoionized plasmas. The dependence of DR on the detailed atomic structure makes it challenging to determine the needed DR rate coefficients theoretically, especially in many-electron systems. Our systematic survey of Li- to F-like Fe-ions is designed to produce high-resolution DR spectra which can be used to benchmark theory for ions of increasing complexity and to provide absolute low-temperature DR rates required in astrophysics.

We describe polarization spectroscopy measurements of N⁴⁺(1s²3p ²P_3/2) following single electron capture by N⁵⁺(1s²) from He and H₂. The degree of polarization P for the 3s²S_1/2–3p ²P_3/2 transition has been measured as a function of projectile velocity ranging between 0.38 and 0.54 a.u. In both of the collision systems, P has a nearly constant value in this velocity range, namely 0.2 for He and approximately zero for H₂. The strong target dependence observed in the degree of polarization is discussed.

Transitions between high spin core-excited states of highly charged Mg-like Ions (Ti, Fe and Ni) have been investigated using beam-foil spectroscopy. The identification of the 2p⁵3s3p²⁵D₄–2p⁵3s3p3d ⁵F₅ transition has been aided by calculations and iso-electronic methods.

The determination of atomic masses from highly ionized atoms using Penning Traps requires precise values for electronic binding energies. In the present work, binding energies of several ions (from several elements) are calculated in the framework of two relativistic many-body methods: Relativistic Many-Body Perturbation Theory (RMBPT) and Multi-Configuration Dirac– Fock (MCDF). The ions studied in this work are: Cl (He and Li-like), Se (F and Ne-like), Cs (He, Be, Ne, Al, Cl, Ar, K, Kr, Xe-like and neutral Cs), Hg, Pb and U (Br and Kr-like). Some of them are presented in this paper. Cesium has been treated in more details, allowing for a systematic comparison between MCDF and RMBPT methods. The Cs ions binding energies allow for the determination of atomic Cs mass, which can be used in a QED-independent fine structure constant determination.

At MeV energies and beyond the inner-shell vacancy production cross section associated with the photoelectric and Compton effect decrease with increasing photon energy. However, when the photon energy exceeds twice the rest energy of the electron, ionization of a bound electron may be catalyzed by the creation of an electron-positron pair. Distinctly different from all other known mechanisms for inner-shell vacancy production by photons, we show that the cross section for this "vacuum-assisted photoionization" increases with increasing photon energy and then saturates. As a main result, we predict that vacuum-assisted photoionization will dominate the other known photoionization mechanisms in the highly relativistic energy regime.

Charge changing cross sections of C^q+ in collisions with He and H₂ for q = 2–5 have been systematically measured at low energies, ranging from 1–1800 eV/q. The measured cross sections for He target for the most part have stronger dependence on the collision energy than those for H₂ target.

Electron capture processes by protons from highly charged hydrogenic ions in dense plasmas including dynamic screening effects are investigated using the semiclassical version of the Bohr–Lindhard model with the straight-line path. The dynamic interaction potential in dense plasmas is obtained by considering the longitudinal component of the plasma dielectric function. The scaled semiclassical electron capture probability is obtained as a function of the impact parameter, Debye length, and projectile velocity. The dynamic plasma screening effect on the scaled semiclassical electron capture probability is found to be more significant for low projectile velocities. When the projectile velocity is smaller than the electron thermal velocity, the dynamic screening effect is weaker than the static screening effect. However, when the projectile velocity is greater than the electron thermal velocity, the dynamic plasma screening effect leads to the static plasma screening effect.

Angular distribution of C²⁺ ions produced in C⁴⁺(1s²) + He → C²⁺(1s²2s²) + He²⁺ at collision energy of 270 and 470 eV are presented. Measurements have been carried out with an experimental setup for the state selective differential cross section measurements of electron capture processes which utilizes the 14.5 GHz Caprice Electron Cyclotron Resonance (ECR) ion source at RIKEN. It is found that results of calculations based on the potential, which was obtained from the previous experimental results at 1520 eV, do not reproduce the present experimental results at 270 and 470 eV.

The fragmentation of molecular ions produced in collisions of Kr⁸⁺ ion with N₂ has been studied at low energies below 200 eV/amu by a double-coincidence TOF technique. All processes taking place in single-, double-, and triple-charge changing collisions were identified and their branching ratios were measured. The distributions of kinetic energy released in fragmentation of molecular ions were also determined. New phenomena of molecular ion peak splitting and a first experimental evidence for the oriented fragmentation occurring have been observed.

Charge transfer in collisions of Si⁴⁺ ions with He atoms below 100 keV/u is studied by using a molecular orbital representation within both the semiclassical and quantal representations. Single transfer reaction

Si⁴⁺ + He → Si³⁺ + He⁺

has been studied by a number of theoretical investigations. In addition to the reaction (1), the first semiclassical MOCC calculations are performed for the double transfer channel

Si⁴⁺ + He → Si²⁺ + He²⁺

Nine molecular states that connect both with single and double electron transfer processes are considered in the present model. Electronic states and corresponding couplings are determined by the multireference single- and double- excitation configuration interaction method. The present cross sections tie well with the earlier calculations of Stancil et al., Phys. Rev. A 55, 1064 (1997) at lower energies, but show a rather different magnitude from those of Bacchus-Montabonel and Ceyzeriat, Phys. Rev. A 58, 1162 (1998). The present rate constant is found to be significantly different from the experimental finding of Fang and Kwong, Phys. Rev. A 59, 342 (1996) at 4,600 K, and hence does not support the experiment.

We have calculated doubly differential cross sections (DDCS) for electron emission in 3.6 MeV/amu Au⁵³⁺ collisions with neon and argon atoms in the continuum distorted wave with eikonal initial state approximation. DDCS for a given final charge state of the recoil ion are extracted from the differential ionization probabilities by use of a binomial analysis. Our results are in very good agreement with experimental data for net- and single-electron emission, when the calculations are based on the optimized potential method to describe the target atom. By contrast, artificial structures in the low-energy continuum are found if a Hartree–Fock–Slater model potential is used for argon targets.

We have measured the electron beam radius with the Tokyo-EBIT using a Thomson scattering method. Dependences of the beam radius were investigated on beam energy, beam current and magnetic field at the center region of the trap. The variations of the measured beam radius against the beam current and the magnetic field were similar to those in the Herrmann's prediction. However, it is seemed that the beam radius was dependent on the beam energy, which is different from the theory.

The formation of projectile autoionizing 2l2l' states has been investigated for 0.5–1.75 MeV/u (v = 4.5–8.4 a.u.) F⁸⁺ projectiles colliding with neon. Double transfer to 2l2l' configurations is found to occur with much higher probability than transfer excitation. Most of the double capture events are attributed to the transfer of two K-shell electrons, or a K-shell electron plus an L-shell electron. A large contribution from the 1s(2s2p ³P) ⁴P state is observed. Apparent anomalies in the observed relative intensities of the 1s(2s2p ³P) ²P_– and the 1s(2s2p ¹P) ²P₊ states are ascribed to quantum mechanical interference effects.

We measured DR resonances in Lithium like Kr³³⁺ in the energy range 3–14 eV. The absolute energy scale was determined within 30 meV and the relative energy scale (difference between states) to within 10 meV. Our results are in good agreement with calculations obtained by relativistic many-body perturbation theory. The contributions from QED effects in this system are around 1.5 eV.

Balmer spectra produced in collisions between He-like uranium projectiles and gaseous targets (N₂, Ar, Kr, and Xe) were measured at the jet-target area of the ESR storage ring. The X-ray radiation were recorded in coincidence with the down charged projectiles having captured one electron from the target atom. By means of spectra simulation, all the significant Balmer transition lines were identified. Further, the j-differential cross sections for electron capture into projectile subshells were deduced from the well resolved spectra. For the heavier targets, where NRC contributes considerably, systematic deviations between experiment and the continuum distorted wave approximation are observed

We present ab initio calculations of cross sections for electron capture by C²⁺ ions in their ground (¹S) and metastablee (³P) states, from H and H₂. We explain the difference between C²⁺–H₂ collisions, where the content of metastable ions is critical, and C²⁺–H collisions, where it is less important. We also present preliminary results for electron capture by O²⁺ in ground (³P) and metastable (¹D, ¹S) states from H.

Angular distributions of elastic scattering electrons from Ar⁷⁺ and Ar⁸⁺ have been measured at a collision energy of 100 eV and in the angular range between 34°–85° simultaneously by using a toroidal analyzer. Relative differential cross sections for the e + Ar⁷⁺ system have been obtained after calibration for the detection efficiency by using the results of the e + Ar⁸⁺ system, where the differential cross sections are assumed to be the Rutherford cross sections. The measured cross sections for the e + Ar⁷⁺ system are compared with both Rutherford cross sections and R-matrix calculations.

A crossed-beams energy-loss spectrometer has been used to investigate angular distributions for electron scattering from Ar²⁺ and Xe²⁺ ions, at a collision energy of 16 eV. For Ar²⁺ the measurements are compared with the predictions of a partial waves calculation based on a semi-empirical potential, where it is shown that the interference term governs the position of the observed minimum in the angular distribution.

We present cross sections for electron capture in C⁴⁺, N⁵⁺ + H₂ collisions in the energy range 50 eV/amu ≤ E ≤ 6 keV/amu, calculated using a model potential approximation to treat the interaction of the active electron with the cores. To avoid the difficulties of the standard independent particle model at low velocity, we have applied a modified technique where the independent particle model is used to evaluate the matrix elements of the two-electron Hamiltonian.

Absolute rates for photorecombination (PR) of lithium-like Sc¹⁸⁺ ions have been measured at the Heidelberg heavy-ion Test Storage Ring (TSR). The investigated electron energy range covers all DR resonances related to 2s → 2p_1/2 and 2s → 2p_3/2 Δn = 0 core excitations. Resonances associated with 1s² 2p_3/2 10ℓ_j intermediate states are found at energies between 0 and 0.3 eV. They are measured with an energy resolution reaching down to about 18 meV for the lowest-energy resonances. A comparison between the experimental results and calculations based on relativistic many-body perturbation theory (RMTB) yields a good agreement even though deviations at very low energy could not be explained yet.

Calculated total cross sections for electron-impact ionization of C³⁺ using a unified theoretical approach are compared with high-resolution high- precision measurements clearly demonstrating interference of resonant multi-electron reaction channels with direct ionization and inner-shell excitation. In particular, the so-called READI process, i.e. K-shell excitation with simultaneous capture of the incident electron (resonant-excitation) and a subsequent three-electron interaction resulting in simultaneous ejection of two electrons (auto-double-ionization) has been studied in unprecedented detail. The calculation is in very good agreement with the experimental cross section.

The first experimental results of a high-resolution study of Th and U Lγ-X-rays induced in collisions with 230 MeV and 360 MeV oxygen ions, undertaken to investigate the dynamics of the multiple ionization in M-, N- and O-shells and the structure of M- and N-shell X-ray satellites, are reported. The measurements were performed using the transmission curved- crystal spectrometer installed in DuMond geometry. An instrumental resolution of about 10 eV was achieved for Lγ (L-N,O) X-ray transitions, allowing clear separation of M-shell X-ray satellites for Th and U. Moreover, the N-shell satellite structure in the Lγ₆ (L2-O4) X-ray transition was observed for 360-MeV O ion impact on a thorium target. This, to our knowledge, is the first observation of this kind. Other observed Lγ X-ray diagram lines are shifted and widened, with respect to the photoinduced X-ray spectra excited by an X-ray tube, due to the unresolved N-shell and, probable the O-shell, satellite structure.

We report measurements of photon emission cross sections following electron capture of C²⁺, N³⁺, N⁴⁺ and O³⁺ ions (obtained from an ECR ion source) in collision with ground state lithium atoms at keV energies. We examine the sources of error and obtain best estimates of the relative values of the cross sections having accuracies in the range from 15% to 50%. The accuracy of the absolute cross sections may be lower for capture by N³⁺ and N⁴⁺ ions because of the uncertainty in the fraction of metastable ions. We discuss this problem.

Making use of linear response method and density functional theory, we have studied the role of dynamical electron correlation in photo-absorption processes. By varying nuclear charge, we have made systematic studies of so-called 4d giant resonances appearing in Xe-like iso-electronic sequences. By changing nucleus from Xe to Cs⁺ and Ba⁺⁺ the 4d-ionization threshold moved to the higher energy side but the dynamic correlation effect does not change so much. Particular attention was paid to the analysis of Ba⁺⁺ photo-absorption data. We could identify the observed excitation peaks and find the role of dynamic correlation in the photo-absorption processes

Within a series of systematic measurements of the dielectronic recombination (DR) of lithiumlike ions in the presence of external fields we have determined the absolute recombination rates of Ti¹⁹⁺ and Ni²⁵⁺ ions. In both cases the electron energy range covers all DR resonances attached to 2s → 2p_1/2 and 2s → 2p_3/2 Δn = 0 core excitations. The experimental resolution permits the study of the field effects for the two 2p_jnℓ series of Rydberg DR resonances separately. For both the Ti¹⁹⁺ and the Ni²⁵⁺ ions the electric field induced enhancement is by factors of up to 1.8 stronger for the 2p_3/2 series than for the 2p_1/2 series. Moreover, we find that the magnitude of the field induced DR rate enhancement becomes smaller as the nuclear charge increases. The dependence of the DR rate coefficient on magnetic fields in an E × B field configuration, that was only recently established by theory and experiment, is also found for Ti¹⁹⁺, whereas for Ni²⁵⁺ a change of the magnetic field over the range 42–80 mT did not significantly influence the DR rate enhancement induced by the electric field.

Within a series of measurements of the dielectronic recombination (DR) of lithium-like ions we have determined the enhancement of the recombination rate in the presence of crossed electric and magnetic fields for Ne⁷⁺ and O⁵⁺ ions. In both cases the electron energy range covers all DR resonances attached to 2s → 2p_1/2 and 2s → 2p_3/2 Δn = 0 core excitations. For increasing field the enhancement factor first increases linearly with the electric field and then saturates. In order to investigate the field effect on high-n Rydberg states the ion energy in the O⁵⁺-experiment was changed from 9.4 MeV/u to 5 MeV/u and 3.26 MeV/u. With the variation of the ion energy the field ionization of Rydberg states in the analyzing magnet is influenced. This enabled us to study the field enhancement for a narrow bandwidth of n-states.

Electron-ion recombination of completely stripped Bi⁸³⁺ was investigated at the Experimental Storage Ring (ESR) of the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany. It was the first experiment of this kind with a bare ion heavier than argon. Absolute recombination rate coefficients have been measured for relative energies between ions and electrons from 0 up to about 125 eV. In the energy range from 15 meV to 125 eV good agreement is found between the experimental result and theory for radiative recombination (RR). However, below 15 meV the experimental rate increasingly exceeds the RR calculation and at E_rel = 0 eV it is a factor of 5.2 above the expected value. A variation of the electron density by a factor of 3 had no significant influence on the recombination rate which is in agreement with other experiments. An additional variation of the magnetic guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted in periodic oscillations of the rate which are accompanied by considerable changes of the transverse electron temperature.

Using an electron beam merged with an energetic ion beam in a storage ring, non-resonant recombination between multiply charged ions and free electrons is studied by measuring directly the rate of recombination products leaving the interaction zone. The relative electron-ion energy was varied from meV energies up to ∼ 50 eV. Above ∼ 0.5 eV the measured rates are well represented by the calculated RR into final states up to the experimental Rydberg-ion detection limit. Between ∼ 0.01 and 0.5 eV the calculated RR rates are 10% experimental data. As the relative energy is decreased below 0.01 eV, the observed recombination rates rise strongly (to ≲ 2.2 times the predicted RR rate), reproducing the low-energy recombination enhancement found in other recent experiments.

Dielectronic recombination of Pb⁵³⁺ has been studied and a resonance is detected only ∼ 10^-4 eV above the ground state. The possibility to determine the Pb⁵³⁺ (4p_1/2) – Pb⁵³⁺ (4s_1/2) energy splitting with a similar accuracy from the determination of the resonance position is discussed. Such a precision can only be achieved by calculations which treat quantum electrodynamics (QED) in a many-body environment at a level which can still not be reached. A fully relativistic many-body calculation of the splitting is described and the uncertainties are discussed.

Electron capture and excitation processes in collisions of B^q+ (q = 1–5) ions with He atoms have been studied theoretically by using a semiclassical close-coupling method with eight to seventeen channels at collision energies between 30 eV/amu and 20 keV/amu. In addition, for the (B³+He) system quantum mechanical calculations have been carried out at collision energies below 10 eV/amu. Ab initio calculations and generalized valence bond calculations with a model potential have been performed to determine the potential energy curves and radial and rotational couplings including electron translation factors. The cross sections thus obtained are compared with those reported earlier and short discussion is also given.

Explosive fragmentation of highly charged CS₂^q+ ions formed by low energy collision between Ar^6,8+ and an effusive molecular beam of CS₂ was studied using triple coincidence measurements of the fragment ions combined with a position-sensitive time-of-flight (TOF) detector technique. Detailed analysis of the 3-dimensional velocity vectors of the fragment ions resulting from multiple ionization predicts a reconstructed bond angle that is reasonably consistent with the known structure of the neutral CS₂ molecule.

In this work, the Independent Event Model (IEVM) is employed to analyze the total electron loss and the loss-ionization processes in intermediate-velocity collisions between multiply-charged projectile ions and neutral targets, for collision systems with three and four electrons in processes where up to three active electrons are involved. The IEVM was applied to the analysis of the single loss of Li⁺ and Li²⁺ ions by He atoms with the simultaneous single and double ionization of the target. All the possible ionization mechanisms for both collision partners in each exit channel were considered. The probabilities were calculated using either a straight-line version of the semi-classical approximation or the coupled-channel method. The anti-screening contribution was also taken into account. The calculations describe well the experimental energy dependence and the high-velocity absolute values for the cross sections for single electron loss accompanied or not by the ionization of the target.

By means of a crossed-beams technique the total cross sections for charge transfer and ionization in He²⁺ + B²⁺ collisions have been measured at barycentric energies from 10 to 256 keV and from 40 to 240'keV, respectively. Moreover, the angular-differential cross sections for charge transfer at CM energies of 10.18 and 15.3 keV have been determined. The total and differential cross sections are calculated with the Basis Generator Method (BGM) and are in good agreement with the experimental data.

To provide predictions of the Lamb shift of highly charged ions on the level of accuracy of about 10^-6 has been achieved after exact results for the contributions of all two-photon self-energy diagrams have been performed. We report on the present status of our Lamb-shift calculations including all QED-corrections of first- and second-order in the finestructure constant α and all relevant nuclear effects. An excellent agreement between the most recent experimental data for Lamb shift of the 1s-ground state in hydrogenlike uranium can be stated. This can serve as a sensitive test of QED in the strongest electric fields accessible in nature. In a second part of this article we report about an all-order numerical evaluation of the one-photon selfenergy at low nuclear charge (Z = 1, ..., 5). A sensitive comparison of our numerical approach with analytical approach to the one-photon selfenergy confirms the consistency of these two different approaches (numerical and analytic) to very high precision.

Processes associated with double electron capture into bare U⁹²⁺-ions have been observed under single collision conditions. Regions of very low cross sections, close to mbarn, have been explored successfully. In particular, an attempt to register photons with twice the energy of single K-REC photons has been performed. Moreover, K-REC spectra associated with double charge exchange have been analysed in terms of their angular distribution. As a result, evidence for correlated electron capture has been found

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

To establish the dynamical aspects of shape resonance, we investigate the electron capture processes N⁵⁺ + H → N⁴⁺(4s, 4p) + H⁺ by using a wave packet propagation method that can treat non-adiabatic collision processes. It is found that the incident wave packet is trapped into quasi-bound potentials when the collision energy coincides with that of the rovibrational state of the quasi-molecule. The wave packet component trapped in the N⁵⁺ + H effective potential transfers to electron capture channels rather than returns to the initial channel by tunneling again.

The charge transfer cross sections of Li⁺ (1s²) ions in collisions with B⁵⁺, C⁶⁺, N⁵⁺ (1s²), and O⁶⁺ (1s²) ions, and of Li²⁺ (1s) ions with Be⁺ (1s²2s) ions are calculated in the collision energy range of 0.02–32 keV/amu by using a semi-classical close-coupling method with molecular-state expansion.

We present results from a new high-precision high-resolution experiment and detailed unified R-matrix calculations for electron-impact ionization of Li-like O⁵⁺. In particular, interference of direct ionization with a two-step mechanism is observed, which involves dielectronic capture and subsequent simultaneous ejection of two electrons.

Atomic lifetime measurements in the millisecond range have provided transition rates of intercombination and forbidden transitions in a number of ions. However, at the often claimed high level of precision, discrepancies appear between the results from different devices (Kingdon-type electrostatic ion trap, radiofrequency ion trap, electron beam ion trap and heavy-ion storage ring) measuring the same or comparable atomic systems. This calls for an assessment of the quality of typical measurements.

Recently, a new method of analysing electron correlations based on intensity interferometry has been applied to double ionization of He and Ne by fast ion impact [1]. The data reveal sensitively correlation effects while they appear to be very insensitive to the collision dynamics. In order to analyse the role of the initial state electron correlation a statistically defined correlation function based on intensity interferometry was calculated for the ground state of He. In a comparative study of model wave functions we demonstrate that correlation can be considered from a statistical point of view which offers a new tool to study correlation effects in many-particle systems.

We demonstrate that the transverse interaction, which has been ignored in the previous studies, is essential to the nuclear polarization (NP) effect on an energy level in a heavy ion. The NP correction is calculated for the hydrogen-like ²³⁸₉₂U ion with the lowest electric quadrupole excitation and the electric dipole giant resonance. The transverse interaction is found to remarkably reduce the NP effect for the latter nuclear excitation. It is concluded that, in general, the effect of the transverse interaction is important, unless the nucleus has a dominant excitation mode at an energy as low as ℏc⟨r⟩, where ⟨r⟩ is the electronic mean radius.

Spectra of highly charged Ar, Kr, Xe and Fe ions, produced in an Electron Beam Ion Trap (EBIT), have been recorded in a broad X-ray energy band (0.2 keV to 10 keV) with a microcalorimeter detector. The first analysis of the Kr spectra has been completed and most of the spectral lines have been identified as transitions of B- to Al-like Kr. Line intensity ratios of Fe XVII have been measured and compared with theoretical models.

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

We have detected characteristic M X-ray emission of highly charged heavy ions (Ta^q+, q = 39–48) from single collisions with molecular gas(N₂) and rare gas atoms (He) at low energies (8 keVq). The number of electrons transferred at this low energy were determined by a coincidence with the recoil-ion time-of-flight. Characteristic M radiation is seen with high intensity for charge states where initially the M-shell was closed. This radiation is explained by a theory which assumes formation of bound doubly excited states in resonance with the singly excited capture state. A clear dependence of the spectral shape on the ion charge states is observed.

We report a combined experimental and theoretical study of an intermediate velocity collision system. Our aim is to investigate the possibility of emitting electrons which are accelerated by the Fermi-shuttle mechanism due to double and triple scattering effects. Electron emission in the 20–2000 eV energy range and in the full angular range of 0°–180° were measured by the impact of 150 keV/u C⁺ ions on Ne atoms. Classical trajectory Monte-Carlo (CTMC) calculations, show structures belonging to both the projectile-target (P-T) and the target-projectile (T-P) types of double scattering, and signatures for the triple P-T-P scattering. However, T-P and P-T-P shoulders together, calculated by CTMC at forward angles, form a structureless monotonic spectrum. This finding is in agreement with experiment.

To study collision processes of slow and ultra-slow highly charged ions (HCI) with atoms, molecules, and surfaces, we have been constructing a slow highly charged ion facility at RIKEN. A 14.5 GHz Caprice type ECR ion source (ECRIS) is used as the primary ion source of the facility. At present, the energy of the ion beam from the ECRIS ranges from 10 eV/q to 20 keV/q, where q is the charge state of the ions. In order to further reduce the energy (< 1 eV/q), we are developing two different schemes, which is one of the most important feature of the facility. The two schemes are (1) a positron cooling of HCIs and (2) a multiple ionization of neutral atoms or singly-charged ions with a femtosecond (fs) Tera-Watt (TW) laser. This paper presents an overview of the present facility and the schemes to produce ultra-slow HCI beams, which are expected to open new research fields.

The 10th International Conference on the Physics of Highly Charged Ions, the "HCI-2000", took place at the Clark Kerr Campus in Berkeley, California, USA, from July 30 through August 3, 2000. The conference, which was attended by over 170 participants from 19 countries, was organized by the American Vacuum Society (AVS) and the Lawrence Livermore National Laboratory. The "HCI-2000" conference covered various fields in physics where highly charged ions play an important role. In particular HCI production, structure and interaction with matter are examples of important impact areas. This was reflected through the major topics of the conference:

HCI-Surfaces, HCI-Cluster, HCI-Collisions, Recombination, Plasma Spectroscopy, HCI-Fragmentation, New Traps, Laser Production of HCI, and HCI Spectroscopy

These topics were covered through an intense program including 2 plenary lectures, 11 invited lecturers, 25 oral contributions, and 200 posters. A welcome address was presented by W. Goldstein (Associate Director of Physics, Lawrence Livermore National Laboratory), and an informal evening was spent during the dinner cruise on the S.F. bay. The presentations showed an impressive array of instrumental advances consisting of ion sources, accelerators, storage rings and ion traps as well as sophisticated detector technology. Their application was demonstrated through excellent contributions from advanced studies of interaction phenomena of highly charged ions with matter. The fundamental physics based on the reported new data allowed speculation on possible applications of highly charged ions in other fields of science and technology.

This is the trend of the HCI conferences, which have incorporated and emphasized different areas of physics. Instrumental aspects like ion source and detector development were discussed in view of possible application in nano-science. Here the understanding of the interaction dynamics of HCIs with complex systems is essential. Various contributions in this area revealed a need for theoretical interpretation of new interaction phenomena.

The 10th HCI conference became a success due to the efforts of the participants and contributors, which we gratefully acknowledge. The organizing committee, the international advisory board, and the organizers wish to express their gratitude to the conference sponsors: the Northern California Chapter of the American Vacuum Society, the Physics and Chemistry & Materials Science Directorates at Lawrence Livermore National Laboratory and the Division of Chemical Sciences at the USDOE Office of Science. And in particular we wish to express our thanks to the conference manager, Ms. Della Miller, conference registrar, Ms. Heather Korff from the AVS and conference secretary, Ms. Donna Vercelli from LLNL. We wish also to thank all the referees for their efforts in refereeing the many contributions published in this topical issue. The next, the 11th International Conference on the Physics of Highly Charged Ions will be organized by Dominique Vernhet and her colleagues at the Groupe de Physique des Solides équipe des interactions ions matière to be held in Caen, France in 2002. We hope to meet again with all of our colleagues working in the HCI field and look forward to further exciting advances in the physics of highly charged ions.

List of members of the Local Organizing Committee and the International Organizing Committee for the 10th International Conference on the Physics of Highly Charged Ions.

A list of participants at the 10th International Conference on the Physics of Highly Charged Ions and their email addresses.