Multiple Electron Capture Research Articles

Multiple electron capture processes between highly charged ions and molecules

We have determined the absolute cross sections for one electron capture and total electron capture between slow, highly charged ions and multi-electron molecule targets (H2, N2, CO, CO2, CH4) under single collision conditions. It is found that the cross sections increase as the charge of the projectile ions increases and as the ionization potential of the target molecules decreases, as previously observed in rare gas targets. Furthermore, we have found that a scaling law previously proposed for atoms, which is based on the extended classical over barrier model, can also reproduce our data for molecular targets.

Experimental evidence for dielectronic excitation producing Ne K vacancies in 35-keV N7++Ne collisions.

The method of Auger electron spectroscopy was utilized to measure cross sections for the production of K-Auger electrons in N7+ + Ne and N6+ + Ne collisions at an impact energy of 35 keV. In addition to the K-Auger electrons from the nitrogen projectile following multiple electron capture, K-Auger electrons of the Ne target are observed for the system N7+ + Ne. This observation is attributed to dielectronic excitation, produced by electron-electron interaction, where a K- and an L-shell electron of Ne are transferred into the K shell of N7+.

Electron capture and dissociation of the molecule in slow collisions

Single and multiple electron capture processes are studied in slow (8 - 16 keV) collisions between and molecular nitrogen. The contributions of dissociative and non-dissociative channels are determined as well as the kinetic energies of the fragments of the (q = 1 - 6) ions. From the kinetic energies of the fragments dissociation energies of the molecular ion are deduced. Under the assumption of isotropic dissociation the following results are obtained: at most 7% of single-electron capture leads to dissociation of into and at least 90% of the double-electron capture results in fragmentation into . If more than two electrons are captured, all -ions dissociate into . For even values of q we observe r = s, otherwise ; i.e. the maximum difference between r and s is 1. A charge distribution closest to the most symmetric one is preferred. The measured dissociation energies are smaller than those expected from the model of Coulomb explosion. Therefore, potential curves for the and molecular ions have been calculated, and dissociation energies deduced from these potential curves have been compared to the measured ones. In both cases dissociation from the lowest molecular state of the multicharged molecule seems most probable.

Method for production of a 3He beam with high polarization and high intensity.

We describe a concept in the production of a highly polarized {sup 3}He{sup +} beam using multiple-electron capture and stripping collisions between a fast incident {sup 3}He{sup 2+} ion and a polarized alkali-metal atom at a strong magnetic field of 1--2 T. The yield and polarization of the outgoing {sup 3}He{sup +} ion are calculated by solving the rate equations. The yield shows a characteristic pattern as a function of the alkali-metal-atom vapor thickness as a result of the multiple-electron capture and stripping collisions. The polarization shows a gradual decrease at low vapor thicknesses and a steep increase up to unity at about 2{times}10{sup 15} atoms/cm{sup 2}. By using 30-keV/amu {sup 3}He{sup +} as a primary beam, a fully polarized {sup 3}He{sup +} beam of about 0.5 mA may be possible. It is also suggested that not only polarized {sup 3}He ions but a polarized fast atomic {sup 3}He beam can be produced by the present method, where the expected polarization and beam intensity are {similar_to}0.8 and {similar_to}6{times}10{sup 15} atoms/sec, respectively.

Direct and indirect stabilisation mechanisms in multiple electron capture

Direct and indirect stabilisation mechanisms in multiple electron capture

Dissociative electron capture in [formula omitted] collisions

Single and multiple electron capture processes are studied in slow (200–400 eV/amu) collisions between Ar 8+ and molecular nitrogen. The amount of dissociative charge transfer reactions is determined as well as kinetic energies of the ionic fragments of N 2 q+ ( q = 1–6) ions. Dissociation energies are obtained from the kinetic energies of the molecular fragments. The following results are obtained: 7% of single electron capture leads to dissociation of N 2 + into N + N + and nearly 90% of the double electron capture results in fragmentation into N + + N +. If more than two electrons are captured, all N 2 q+ ions dissociate into N r+ + N s+ . For even values of q we observed r = s, otherwise r = s + 1; i.e. the maximum difference between r and s is 1. A near symmetric charge distribution among the resulting atomic ions is preferred.

Study of multiple capture processes by recoil ion momentum spectroscopy for 150 keV Ne 10+Ar collisions

We report preliminary investigations of multiple electron capture in the collision of 150 keV Ne 10+ ions with Ar targets. These processes can be described by the following reaction: Ne 10+ + Ar → Ne (10− s)+ + Ar r+ + ( r- s)e −. The Ne 10+ projectile removes r electrons from the Ar target among which s electrons are stabilized on the projectile and ( r- s) are ejected into the continuum. Distributions corresponding to the production of up to septuple-charged recoils were obtained in coincidence with the charge state (10 − s) of the scattered projectiles. Stabilization rates and average scattering angles were also derived from this experiment and given here as preliminary results.

Stabilization of multiple electron capture by the post-collisional increase of angular momentum of Rydberg electrons

Considerable work has already been devoted to multiple electron capture by highly charged ions colliding on rare gas targets in the keV energy range. In such collisions, several electrons are captured by the projectile ionic core with a binding energy which, according to some simple models, is of the order of the successive ionization potentials of the target. This produces a unique situation in which several electrons are captured in highly excited shells. The large density of levels available at such a degree of excitation opens the possibility of interesting dynamical evolution which leads to the population of doubly excited states with one of the electrons in a high lying Rydberg state (e.g. n=10-30). In this paper we focus on the evolution of the Rydberg electron in the field of the two receding nuclei. In particular, the increase of angular momentum of the Rydberg electron is shown to be fast enough to quench the autoionization process resulting in a true capture of the two electrons.

A coincidence study of multiple-electron capture in 15N7+-Ar collisions

We present energy spectra of autoionization electrons resulting from collisions between fully stripped 15N7+ ions aid Ar in the energy range of 1-10 keV amu-1. These spectra have been measured in coincidence with charge-state analysed Ar ions. Observed populations of the various configurations are discussed in the framework of the classical overbarrier model. Our measurements suggest that target-ion excitation together with two-electron capture is an important process in collisions between bare N7+ ions and Ar. We find evidence that radiative stabilization of doubly excited N5+ (4l, 4l') states is even stronger than was thought up to now.

Simultaneous electron capture and target ion excitation in collisions of C4+ and N5+ on He

Cross sections for simultaneous electron capture and target ion excitation have been measured for impact of slow He-like C4+ and N5+ ions on He. The energy of the primary ion beams has been varied over more than two orders of magnitude: 0.05–7 keV/amu. The results are discussed on basis of a slightly modified version of the “dynamic” classical over-barrier model for multiple electron capture. The differences in the energy dependences of the experimental results of C4+ and N5+ — He can be explained qualitatively by assuming that for N5+ binding energy sharing between the two participating electrons is of importance, particularly at the lower impact energies.

Multiple electron capture by highly charged ions (electron-nucleus versus electron-electron interactions)

Basic Double Capture mechanisms are reviewed for collisions of highly charged ions with atomic target in the keV energy range. It is the main goal of this paper to clarify the relative role of the electron-nucleus and electron-electron interactions. First two independent electron models (IEM) are briefly described: the classical over barrier model and the quasi-molecular model. They account well for the population of configuration with two equivalent electrons. It is then shown that e−-e− interaction is necessary to account for the population of doubly excited states with two highly non equivalent electrons. A special emphasis is given to a recent work on the stabilisation mechanisms responsible for true double electron capture.

Evolution of ions after multiple electron capture from surfaces

A comparison is made of the electronic processes which occur when a multiply charged ion is approaching an atomic target on the one hand or a metal surface on the other hand. In both cases three collision phases can be identified: those of attraction, of electron capture and of decay in the vacuum; in case of surface collisions two more phases show up: the touch down on the surface and the decay inside the solid. Results from various groups, especially regarding the analysis of ejected electrons, are discussed to clarify similarities and differences between the different collision systems.

Multiple electron capture in slow Ar9+ and 13C5+ on Ne, studied by e--Neq+ coincidences

The authors have investigated multiple electron capture into autoionizing projectile states during 108 keV Ar9+ and 80 keV 13C5+ collisions on Ne by measuring autoionization electrons in coincidence with charge state selected target ions. They have determined partial electron energy spectra, corresponding to a well defined number of initially captured electrons. The set-up of the ion and electron spectrometer is described. Trajectory calculations for the determination of detection efficiencies are presented. The partial spectra are discussed in terms of the 'extended classical barrier' model.

Cross sections for single and multiple electron capture in low energy collisions of C4+ with H2 O2 and N2

Cross sections for single and multiple electron capture in collisions of C4+ with H2, N2 and O2 have been measured within the energy range 0.33-666 eV amu-1. Where comparison is possible, the results are in good agreement with previous measurements at higher energies. Below 100 eV amu-1 there are significant differences between experiment and the currently available theories.

Electron emission in collisions of slow, highly charged ions with gases and solid surfaces

We measured electron spectra produced in O 6+ + He collisions and by N 6+, O 7+, Ne 9+, and Ar q+ ( q = 9, 13, 14 and 16) incident on solid surfaces of Cu and C under 10 ° with energies from 10 q to 20 q keV. Auger electrons were detected at various emission angles including 0° and 180° with respect to the incident beam. For the He gas target the angular distribution of oxygen L-Auger electrons is found to be isotropic between 10 ° and 140 ° but enhanced at 0 °. In ion-surface collisions, multiple electron capture is studied. By analyzing the Doppler shift of the projectile Auger electrons, the L-Auger electrons of argon were found to be emitted from ions incident on the surface and the K-Auger electrons of neon were observed to be emitted from reflected ions.

A critical study of the molecular coulombic barrier model (MCBM) for multiple electron capture

Multiple electron capture by highly charged ions in collision with rare gas targets is experimentally investigated. Differential cross sections are measured for specific processes in which both the number of electrons lost by the target and captured by the projectile are determined. This set of data is compared with the predictions of the Molecular Coulombic Barrier Model . The good predictive quality of this model is confirmed but deficiencies are pinpointed which are attributed to the neglect of “simultaneous” electron transfers as well as to the intrinsec static character of the model.

Two electron processes in charge exchange reactions involving multiply charged ions

Multiple electron capture mechanisms are discussed in light of differential energy gain measurements. Two electron processes are examined in terms of one electron processes. Special emphasis is brought to transfer excitation processes

Critical study of the molecular Coulombic barrier model for multiple electron capture by highly charged ions

Multiple electron capture by highly charged ions in collision with rare gas targets is experimentally investigated. Differential cross sections are measured for specific processes in which both the number of electrons lost by the target and captured by the projectile are determined. This set of data is compared with the predictions of the molecular Coulombic barrier model. The good predictive quality of this model is confirmed but deficiencies are pinpointed which are attributed to the neglect of 'simultaneous' electron transfers as well as to the intrinsic static character of the model.

Radiative and non-radiative decays of doubly-excited configurations in the Ar7+ spectrum

A configuration-average method is developed to investigate the properties of multiply-excited configurations of many-electron ions produced during multiple-electron capture collisions. For doubly-excited configurations of Ar7+ lying below the 2p55s ionisation limit, the average energy is calculated and a classification into core-excited Rydberg series is presented. The energy range spanned by Wannier-type configurations 2p5N'n with N approximately=n also includes high members of the Rydberg series with N<<n. The radiative and non-radiative decay rates are calculated and the cascade processes from excited configurations are analysed explicitly. The stabilisation ratio R(N', n), defined as the probability of reaching non-autoionising configurations of Ar7+ during the cascades from the configuration 2p5N'n, is calculated for all configurations with N<or=5, n<or=9. Its dependence on the quantum numbers N, n, and on the energy of the configuration is investigated. Highly-excited configurations with very asymmetrical excitation of the two electrons (N<</n, '<<) correspond to high R ratios.

Multiple-electron capture processes by multiply charged ions from rare-gas atoms at low velocities

State-selective single- and multiple-electron capture processes for collisions of multiply charged Krq+ ions (q=3-5) and Xeq+ ions (q=3 and 4) with rare-gas atoms have been studied by means of translational energy spectroscopy, at collision energies between q and 5q keV. The collisional energy gain spectra for single-electron capture by Kr5+ from He, Ne and Ar are interpreted qualitatively in terms of the 'reaction windows', which are calculated using a multichannel Landau-Zener model. The measured energy gain spectra are in good agreement with the calculations.