Effect of electrons of the multiply charged ion core on single-electron capture

The population of various electronic states of particles that arise during the capture of a single electron in hydrogen and helium atoms, as well as hydrogen molecules, by Ar³⁺ and Ne³⁺ ions with an energy of several kiloelectronvolts was studied by collision spectroscopy, viz., precision analysis of kinetic energy variation for ions formed as a result of interaction between ions and atoms. It is shown that single-electron capture in many cases is a multielectron process accompanied by the rearrangement of a multiply charged ion core. It is found that the triply charged Ne³⁺ ions formed as a result of ionization of Ne atoms by electron impacts are formed mainly in metastable states. The population of excited states of particles during their multiple ionization should be taken into account in determining the characteristics of various particles by the appearance potential method. Collision spectroscopy can be used for analyzing the metastable ion impurities in ionic beams.     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

Lifetime measurements of metastable states in CRYRING

Collinear ion beam laser spectroscopy has been utilized in the ion storage ring CRYRING in order to measure lifetimes of metastable states in singly charged ions. The laser light has been used for selective probing of the population decay of individual fine structure or hyperfine structure states. With the use of a mechanical shutter, time resolved studies could be performed with millisecond resolution. In another experiment, the cw laser light was used to optically pump stored ions from the ground state into a specific metastable state. With most of the stored ions in the metastable state, direct observation of the decay of the forbidden transition from the metastable state to the ground state could be observed passively. Measurements of metastable lifetimes in Ca⁺, Sr⁺, Xe⁺ and Eu⁺ will be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

High yields from the Stockholm electron beam ion source CRYSIS

CRYSIS is an electron beam ion source (EBIS) with a superconducting solenoid. Highly charged ions are delivered to the acceleration and storage ring CRYRING [ K. Abrahamsson et al., Nucl. Instr. Methods B 79 (1993) 269.], SMILETRAP [C. Carlberg et al., Phys. Scr. T 59 (1995) 196.] and to low energy atomic and surface physics experiments. Stable electron beam currents up to 700 mA are obtained, in order to enhance the ion yield out of the EBIS. Measurements of the total charge per pulse at different working conditions and electron beam current density measurements were done. At electron beam currents of 600 mA yields up to 2.5 × 10¹⁰ charges per pulse could be measured. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spatial and angular distribution of the heavy ion charge under channeling in crystals

A kinetic theory of passage of multiply charged heavy ions through crystals is developed that allows for diffusion in the transverse momentum space and ion-crystal charge exchange. The theory provides an adequate explanation for the observed angular distributions of heavy ions passing through oriented crystals, makes it possible to calculate the partial angular distributions of different charge states, and treats the discovered effects of “cooling” and “heating” of channeled ion beams in physical terms. The angular and spatial distribution of channeled ions with different energies is calculated. Whether a channeled beam of multiply charged heavy ions will be cooled or heated is related to the dependence of the electron capture and loss probabilities on the impact parameter when the ions interact with atomic chains. This interaction governs the run of the angular and spatial distribution of the channeled ion charge.     

Theory of the polarization of highly charged ions in storage rings: Production, preservation, observation and application to the search for a violation of the fundamental symmetries

Theoretical concepts for the production, preservation and control of polarized highly charged ion beams in storage rings are investigated. It is argued that hydrogen-like ions can be polarized efficiently by optical pumping of the Zeeman sublevels of ground state hyperfine levels and that the maximum achievable nuclear polarization exceeds 90%. In order to study the preservation of the polarization during the ion motion through the magnetic system of the ring, the concept of the instantaneous quantization axis is introduced. It is suggested that the employment of “Siberian snakes” may help to preserve the ion beam polarization in the ring. The control of the beam polarization can be achieved by different methods: by measuring the Stokes parameters for the emitted photons or by observing the angular dependence of the transition rates for polarized ions. The important motivation for the production of polarized ion beams is the possibility to observe parity nonconservation effects in the hyperfine-quenched transitions in helium-like highly charged ions, where these effects can reach an unprecedented high value for atomic physics. The possible observation of parity nonconservation effects connected with the nuclear anapole moment is also discussed. A method for the observation of the electric dipole moment of an electron in a storage ring with a polarized highly charged ion beam is proposed. This method allows, in principle, to improve the existing boundaries for the electric dipole moment of an electron. However, the requirements of the corresponding experiment are very stringent.     

Recombination of electrons with highly charged heavy ions at very low energies

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesE _cm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U²⁸⁺ ions nearE _cm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onE _cm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.     

Atomic collisions involving C60 and collective excitation

Here we review and discuss some of our recent investigations on collective excitation in a free C₆₀ molecule and its influence on the atomic collisions. In particular, emphasis has been given for collisions with fast highly charged ions. It is demonstrated, from the charge-state-dependence studies of recoil-ion spectra, that the plasmon excitation plays a dominant role in the single and double ionization process. The observed linear charge-state-dependence is in contrast to the expected behavior predicted by ion-atom collisions models. This behavior was observed for different projectiles and at different energies. The time-of-flight recoil-ion mass spectroscopy experiments involve 1–5 MeV/u C, O, F and Si ion beams with different charge states, ranging between 4⁺ and 14⁺. In addition, the influence of the collective excitation on the electron capture process was also investigated. The wake-field induced Stark-mixing and splitting of sub-levels of projectile-ions following electron capture from C60 carries signature of the collective plasmon excitation. For the electron capture studies X-ray spectroscopic technique was used for collisions with bare and dressed S and Cl ion beams. The results on the TOF data on fullerene target obtained in last few years will be summarized.     

Peculiarities in the cross sections of electron capture by phosphorus ions

The effect of decreasing the cross sections of electron capture by phosphorus (P⁵⁺) ions penetrating through gaseous media has been revealed experimentally. This effect is a violation of the known uniform dependence between the electron-capture cross section and the ion charge. Such an anomaly was not observed in measurements performed with argon ions under the same conditions. A possible reason of decreasing the cross sections of electron capture by P⁵⁺ ions may be autoionization of excited P⁴⁺ ions. The latter are formed during electron capture into excited states by metastable particles of a beam of P⁵⁺ ions.     

Formation,Dynamics, and Characterization of Nanostructures by Ion Beam Irradiation

Ion beam irradiation is a potential tool for phase formation and material modification as a non-equilibrium technique. Localized rise in temperature and ultra fast (～10^?12 s) dissipations of impinging energy make it an attractive tool for metastable phase formation. As a matter of fact, a major component of materials science is dominated by ion beam methods, either for synthesis of materials or for its characterization. The synthesis of nanostructures, and their modification by ion beam technique will be discussed in this review article. Formation of nanostructures using ion beam technique will be discussed first. Depending on species (e.g., mass and charge state) and energy range, there are various modes for an energetic ion to dissipate its energy. The role of the electron will also be covered in this article as a basic principle of its interaction with matter, which is same as for an ion. By using a simple reactive ion beam or electron induced deposition, a secondary phase can be nucleated by ion beam mixing techniques, either by using inert gas irradiation or reactive gas implantation on any desired substrate. Nucleation of secondary phase can also be executed by electron irradiation and direct implantation of either negative or positive ions. Post implantation annealing processes are required for the complete growth of clusters formed in most of these ion irradiation techniques. Implantation processes being inherently a non-equilibrium technique, defects always have a role to play in phase formation, amorphization, and beyond (blister formation). When implanted with large energy, even electrons, one of the lightest charged particles, also manifest these properties. Electronic and nuclear energy losses of the impinging charged particle play a crucial role in material modification. Doping a nanocluster, however, is still a controversial topic. Some light will be shed on this topic with a discussion of focused ion beam.     

HITRAP: A Facility for Experiments with Trapped Highly Charged Ions

HITRAP is a planned ion trap facility for capturing and cooling of highly charged ions produced at GSI in the heavy-ion complex of the UNILAC-SIS accelerators and the ESR storage ring. In this facility heavy highly charged ions up to uranium will be available as bare nuclei, hydrogen-like ions or few-electron systems at low temperatures. The trap for receiving and studying these ions is designed for operation at extremely high vacuum by cooling to cryogenic temperatures. The stored highly charged ions can be investigated in the trap itself or can be extracted from the trap at energies up to about 10 keV/q. The proposed physics experiments are collision studies with highly charged ions at well-defined low energies (eV/u), high-accuracy measurements to determine the g-factor of the electron bound in a hydrogen-like heavy ion and the atomic binding energies of few-electron systems, laser spectroscopy of HFS transitions and X-ray spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The contribution ofK-electron capture for the production of highly charged Ne recoil ions by 156 MeV bromine impact

The recoil ion production cross sections in 2MeV/amu Br ⁿ⁺+Ne⁰→Br ^n′++Ne ^q+ were measured using a projectile ion — recoil ion coincidence technique where the final charge states of both collisions partners were detected simultaneously. Multiple ionization was found to be the dominant process for the production of low charge state recoil ions whereas the production of highly charged recoil ions is accompanied by electron capture from the Nek-shell. The derived ratio of single to double Ne-k electron capture probabilities indicates deviations from a binomial statistics distribution.     

Probing a cooled beam of decelerated highly-charged heavy ions extracted out of the ESR by a channeling experiment

A cooled beam of decelerated highly-charged heavy ions is slowly extracted out of the cooler and storage ring ESR, by combining the deceleration technique and the charge exchange extraction mode. The quality of the external ion beam is tested by a channeling experiment. Bare Au⁷⁹⁺ ions are injected into the ESR at an energy of 360 MeV/u, decelerated to 53 MeV/u, and finally cooled strongly in the electron cooler. By breeding of neighboring charge state ions via radiative recombination in the electron cooler H-like ions are produced. The H-like ion fraction is extracted out of the storage ring. This extracted Au⁷⁸⁺ ion beam is probed by a channeling experiment measuring the extinction rate of the projectile Kα X-ray yield around the [110] axis of a thin silicon crystal. This revised version was published online in August 2006 with corrections to the Cover Date.     

Beam dynamics in an ultra-low energy storage rings (review of existing facilities and feasibility studies for future experiments)

Storage rings operating at ultra-low energies and in particular electrostatic storage rings have proven to be invaluable tools for atomic and molecular physics. Due to the mass independence of the electrostatic rigidity, these machines are able to store a wide range of different particles, from light ions to heavy singly charged bio-molecules. However, earlier measurements showed strong limitations on beam intensity, fast decay of ion current, reduced life time etc. The nature of these effects was not fully understood. Also a large variety of experiments in future generation ultra-low energy storage and decelerator facilities including in-ring collision studies with a reaction microscope require a comprehensive investigation of the physical processes involved into the operation of such rings. In this paper, we present review of non-linear and long term beam dynamics studies on example of the ELISA, AD Recycler, TSR and USR rings using the computer codes BETACOOL, OPERA-3D and MAD-X. The results from simulations were benchmarked against experimental data of beam losses in the ELISA storage ring. We showed that decay of beam intensity in ultra-low energy rings is mainly caused by ion losses on ring aperture due to multiple scattering on residual gas. Beam is lost on ring aperture due to small ring acceptance. Rate of beam losses increases at high intensities because of the intra-beam scattering effect adds to vacuum losses. Detailed investigations into the ion kinetics under consideration of the effects from electron cooling and multiple scattering of the beam on a supersonic gas jet target have been carried out as well. The life time, equilibrium momentum spread and equilibrium lateral spread during collisions with this internal gas jet target were estimated. In addition, the results from experiments at the TSR ring, where low intensity beam of CF⁺ ions at 93 keV/u has been shrunk to extremely small dimensions have been reproduced. Based on these simulations, conditions for stable ring operation with extremely low emittance beam are presented. Finally, results from studies into the interaction of ions with a gas jet target at 3–30 keV energy range are summarized.     

Field enhanced dielectronic recombination of Si11+ ions

The enhancement of dielectronic recombination by applied electric fields has been observed and measured for the first time in a wide range of controlled and measurable fields using multiply charged ions. The heavy ion storage ring, CRYRING, at Stockholm University was used to store a beam of Si¹¹⁺ and collide it with a cold electron target. The observation of a substantial monotonic increase of the rate coefficient for the group of higher Rydberg states is in puzzling disagreement with theoretical calculations of electric field enhanced dielectronic recombination. This revised version was published online in July 2006 with corrections to the Cover Date.     

Possible Sources of Electron Neutrinos with a Modulated Monochromatic Component

A directed beam of electron neutrinos can be created by means of accumulating ions with β⁺-unstable nuclei in a storage ring. Such a neutrino beam may contain a significant monochromatic component, if these nuclei decay via electron capture with a sizable probability. Under specific conditions— in particular, in the case of employing hydrogen-like ions—this monochromatic component can be modulated. Requirements for β⁺-unstable nuclei of such hydrogen-like ions are discussed, and β⁺- decaying nuclides most appropriate for creating intense sources of electron neutrinos with amonochromatic component admitting modulation are indicated.     

First state selective electron capture measurements with trapped highly charged ions

The first state selective electron capture cross section measurements at eV energies are reported for collisions between C⁴⁺ ions and H₂ molecules. The cross sections are measured in a crossed beam experiment by means of Photon Emission Spectroscopy. The ion beams are decelerated in an octopole ion trap where the trap is used to guide the ion beam through the collision region. The experimental results are in excellent agreement with previous measurements at higher energies. The 3-state atomic orbital calculations of Gargaud and McCarroll generally agree with our measurements although there are some discrepancies at lower energies. However, the results for C⁴⁺ are still on a relative scale. To put our measurements on an absolute scale the N⁴⁺+ H system is investigated at keV energies. These results are in good agreement with the data of previous experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dielectronic recombination at low energy range with Boron-like Ar13+at the CSRm

In order to extend the dielectronic recombination (DR) experiments from the main cooler storage ring CSRm with intermediate charge state ions to the experimental cooler storage ring CSRe with highly charged ions and even radioactive ions, a test electron-ion recombination experiment of B-like Ar¹³⁺ ions was performed with a section of the new electron beam energy detuning system, which will be installed at the CSRe. We present the measured and also the calculated DR spectra for the ∆n = 0 resonances from 0 to 3 eV. The experimental results agree very well with the previous DR experimental data from CRYRING, and the energy resolution reached 30 meV full width at half maximum. This test experiment demonstrates that the reliability and stability of the newly developed electron beam energy detuning system are satisfactory for the upcoming DR experiments at the CSRe. However, we found large discrepancies between the experimental result and the calculation in both resonance positions and the intensities of the recombination spectrum below 0.7 eV, which indicate that precise calculation of the DR spectrum of multielectron ions is still a challenge.     

Atomic structure and collision dynamics with highly charged ions

The research progresses on the investigations of atomic structure and collision dynamics with highly charged ions based on the heavy ion storage rings and electron ion beam traps in recent 20 years are reviewed. The structure part covers test of quantum electrodynamics and electron correlation in strong Coulomb field studied through dielectronic recombination spectroscopy and VUV/x-ray spectroscopy. The collision dynamics part includes charge exchange dynamics in ion-atom collisions mainly in Bohr velocity region, ion-induced fragmentation mechanisms of molecules, hydrogen-bound and van de Waals bound clusters, interference, and phase information observed in ion-atom/molecule collisions. With this achievements, two aspects of theoretical studies related to low energy and relativistic energy collisions are presented. The applications of data relevant to key atomic processes like dielectronic recombination and charge exchanges involving highly charged ions are discussed. At the end of this review, some future prospects of research related to highly charged ions are proposed.     

Review of highly charged ion production with ECR ion source and the future opportunities for HCI physics

In recent years, there is very intense worldwide research and development work on electron cyclotron resonance ion source (ECRIS). Remarkable progress represented by the third generation superconducting ECRIS has been made with regards of intense highly charged ion beam production such as >600 eμA Ar¹⁶⁺, >10 eμA Ar¹⁸⁺, and hundreds of enA He-like Kr³⁴⁺. A low energy heavy ion platform named Low Energy heavy ion Accelerator Facility (LEAF) that features a next generation 45 GHz ECRIS, a 300 kV high voltage platform, a 0.5 MeV/u radio-frequency quadrupole, and several multidisciplinary experimental terminals is under construction at the Institute of Modern Physics (IMP). This paper will report on the recent progress with ECRIS dedicated to highly charged ions and the status of LEAF at IMP that will provide new opportunities for highly charged ion physics in the near future.