Multiple-electron Capture Processes Research Articles

X-ray emission produced in charge-exchange collisions between highly charged ions and argon: Role of the multiple electron capture

In this work we use the classical trajectory Monte Carlo method within an eight-electron scheme to theoretically study photonic spectra that follow charge-exchange processes between highly charged ions of charge states 10+, 17+, 18+, and 36+ with neutral argon. The energy range considered is 18 eV/amu to 4 keV/amu, covering typical electron beam ion traps and solar wind energies. The role played by multiple electron capture processes for the different collision systems under consideration is explicitly analyzed and its contribution separated as arising from radiative decay and autoionizing multiple capture. For the present collision systems we find that multiple electron capture is responsible for 50%--60% of the resulting x-ray spectra. The present results are of direct relevance to the astrophysical program.

Vacuum ultraviolet spectra in charge transfer collisions of multiply charged Sn ions

We measured the vacuum ultraviolet (VUV) emission spectra following charge transfer collisions of Snq+ (q = 3-6) with neutral rare gas targets at collision energies of 50-100 keV. We identified some of emission lines from reported energy levels. Several lines were observed in collisions of different charge states of ions. This finding means that the multiple-electron capture processes can take place effectively in our experiment. Significant target dependences were observed in the spectra with the projectile of Sn4+ and Sn6+.

On the Significance of the Contribution of Multiple-Electron Capture Processes to Cometary X-Ray Emission

We report laboratory studies of the role played by multiple-electron capture (MEC) in solar wind induced cometary X-ray emission. Collisions of Ne{sup 10+} with He, Ne, Ar, CO, and CO{sub 2} have been investigated by means of the traditional singles X-ray spectroscopy in addition to the triple-coincidence measurements of X-rays, scattered projectile, and target recoil ions for the atomic targets. The coincidence measurements enable the reduction of the singles X-ray spectra into partial spectra originating in single-electron capture (SEC) and MEC collisions. The measurements provide unequivocal evidence for a significant role played by MEC, and strongly suggest that models based solely on SEC are bound to yield erroneous conclusions on the solar wind composition and velocities and on cometary atmospheres. The experimental relative importance of MEC collisions is compared with molecular classical-over-the-barrier model (MCBM), classical trajectory Monte Carlo (CTMC), and multi-channel Landau-Zener (MCLZ), calculations which can qualitatively reproduce the experimental trends.

Dissociation and fragmentation of N2 molecules by slow highly charged ions

Single and multiple electron capture processes are investigated in slow collisions between highly charged Xeq+ (q=15–21) ions and molecular nitrogen. The formation of N2q+ ions by electron capture, dissociation of molecular ion N2+ and Coulomb Explosion of N2q+ (1<q⩽5) were observed. Various dissociation and Coulomb explosion (CE) channels related to multiple electron capture can be identified in the two-dimensional plot. The kinetic energy release of the fragments is derived from the time-of-flight spectra. The results are compared with CE model.

State-selective Charge Transfer Studies Relevant to Solar Wind–Comet Interactions

We report relative experimental and absolute theoretical n-state-selective, nondissociative, single-electron capture cross sections for a number of collision systems typical of the solar wind-comet interactions. The experimental measurements were carried out using the technique of cold-target recoil-ion momentum spectroscopy and involved the N7+ and O7+ projectile ions and, for the first time, the H2O, CO2, and CO molecular targets, as well as the He atomic target. The theoretical calculations were performed using the classical trajectory Monte Carlo (CTMC), the Landau-Zener (LZ), and the classical over-the-barrier (COB) models. The models, with CTMC favored, agree reasonably well with the measurements and can be used in the near term to aid in the modeling of cometary X-ray and EUV emission. The pathways through which multiple-electron capture processes contribute to the cometary X-ray and EUV emission are also discussed.

Production and relaxation pathways of multiply excited states in slow highly charged ion-atom collisions

When a slow multiply charged projectile ion moving at a velocity below that of typical outer-shell atomic electrons collides with a many-electron neutral target, a large number of electrons can become active during the collision. Such collisions are dominated by the transfer of a number of target electrons to the projectile, resulting in the formation of projectile multiply excited states. Substantial progress has been made toward understanding one- and two-electron processes in slow ion-atom collisions during the past three decades ~see, e.g., Ref. @1#, and references therein!. Although slow collisions involving more than two active electrons have also been investigated for over two decades as well, our understanding of multielectron processes is by no means comparable to that of one- and two-electron processes. The vast majority of experimental studies of multielectron processes involved measurements of cross sections for projectile charge-change and recoil-ion production, both in a singles’ and in coincidence modes, and a limited number involved energy gain and visible photon spectroscopy ~see, e.g., @2‐5#, and references therein!. Moreover, since autoionization is a main decay mode of multiply excited states, Augerelectron spectroscopy has been employed in a singles’ mode @6,7# to study such collisions. While such measurements have played a significant role in understanding two-electron processes ~see, e.g., Ref. @8#, and references therein!, the situation is drastically different when many electrons are involved. For example, Benoit-Cattin et al. @6# obtained a singles’ electron spectrum for the 70-keV N 71 1Ar collision system. The analysis of the spectrum was rather difficult since it contained contributions from doubly, triply, quadruply, and quintuply excited states that rendered the interpretation nontrivial. During the last six years, however, Morgenstern and co-workers have made significant contributions @9‐12# toward understanding Auger-electron spectra obtained in multiple-electron capture processes by means of the coincident detection of Auger electrons and target ions. They obtained partial Auger spectra corresponding to the different target ion charge states that are much more informative than singles’ spectra. These spectra can provide further information if the final projectile charge state is also determined. From a theoretical point of view, understanding multielectron processes is a twofold problem. First, the different mechanisms involved in the collision process that lead to the production of the multiply excited states must be recognized and described. Second, the radiative and nonradiative properties of the resulting multiply excited states must be known. Concerning the first problem, quantum-mechanical or semiclassical treatment of collisions involving more than two electrons is prohibitively difficult due to the large number of channels involved. Therefore, extended classical overbarrier ~ECB! models have been developed @13,14# to account for multiple-electron capture processes. These models are limited to giving the capture state distribution on the projectile and possible simultaneous target excitation. There has been, until recently @15‐17#, a severe lack of theoretical work on the radiative and nonradiative properties of multiply excited states, due in part to the extremely large number of states that need to be taken into account, and in part to the lack of experimental data to which the calculations can be directly compared. Therefore, relaxation schemes @6,12# based on simple arguments, such as autoionization to the nearest continuum limits and minimum electron rearrangement ~twoelectron transitions!, have been invoked. This Rapid Communication reports triple-coincidence measurements of Auger electrons, scattered projectile, and target recoil ions in slow multiply charged ion-atom collisions. The measurements provide insights into the relaxation pathways of multiply excited states populated in such collisions.

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.

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.

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.

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.