Capture Collisions Research Articles

Ion optics for multiple charge exchange collisions determined by means of Monte Carlo simulation

A novel type of polarized He3 ion source based on “electron pumping” was recently proposed. This ion source requires multiple electron capture and stripping collisions of He+3 ions with polarized alkali atoms under a strong magnetic field (2–3 T). Since these processes may induce a serious increase in the emittance and reduction of polarized beam intensity, we investigated them in detail by using the Monte Carlo simulation. It was found that there is (1) increase in the emittance results not from angular broadening but from a beam size enlargement, (2) the phenomena under magnetic field B are simplified by a collisionless phenomenon under a weaker magnetic field (effective magnetic field) Beff defined by Beff≃(λ+0/(λ+0+λ0+))B, where λ+0 and λ0+ are mean free paths for He+3 ions and He03 atoms traveling in the alkali vapor.

Development of RCNP polarized 3He ion source based on electron pumping

We constructed a new polarized 3He ion source in order to establish a method of principle of “electron pumping” and experimentally prove its validity. Electron pumping utilizes multiple electron capture and stripping collisions of 3He + with rubidium atoms whose vapor thickness is much thicker than that used in an ordinary optical pumping polarized ion source. Special care was paid in the design of a rubidium vapor cell, a pumping laser and a beam transport system. We present details of the device and report the present status of the development.

Dressing modification in the laser-assisted proton-helium charge exchange collision

A first Born approach to intense laser modified proton-helium electron capture collision is given. Theoretical results show that the angular distribution peaks forward sharply. For a geometry of laser polarization vector parallel to the incident direction, the dressing effect on the differential cross section covers a relatively wide angular range; while for a perpendicular geometry, the result is modified only in a very small angular region. The integral cross sections get highly promoted by laser with the impact energy increasing. The modified cross sections are increasing functions of field strength, but decreasing functions of field frequency. Comparisons with the laser assisted electron capture from atomic hydrogen are made.

Ejected electron spectra from doubly-excited states He (2lnl') produced by He2+-Mg, Ca and Ba collisions

Ejected electrons from doubly-excited helium atoms, produced by double electron capture collisions of He2+ with Mg, Ca and Ba atoms at low energies, have been measured using zero-degree electron spectroscopy with high energy resolution. Electrons from the configurations (2lnl') (n ⩾ 2) were observed. Electron spectra from the terms of higher angular momenta are dominant in contrast to the case of ion or electron-impact excitation of neutral He.

Production of state-prepared beams of multiply charged ions for studies of state-selective electron capture collisions

A new apparatus which utilizes double translational energy spectroscopy (DTES) is described. This allows studies of state-selective electron capture by multiply charged ions in well defined states. This approach avoids the ambiguities inherent in the results of many previous measurements carried out with beams containing metastable ions. The considerable potential of DTES has been illustrated by measurements of one-electron capture by C2+ ions in collision with Ar at 4 keV.

Ion/Ion Reactions in the Gas Phase: Proton Transfer Reactions Involving Multiply-Charged Proteins

Multiply-charged cations derived from electrospray of bovine ubiquitin and horse skeletal muscle apomyoglobin have been subjected to reactions with anions derived from glow discharge ionization of perfluoro-1,3-dimethylcyclohexane. The results are compared with data obtained from proton transfer reactions with strong gaseous neutral bases. Ion/ion reaction rates are shown to be linearly related to the square of the charge on the protein ion, as expected based on a simple capture collision model. Cationic products with charge as low as +1 could be readily formed via ion/ion reactions, whereas efforts to produce such low charge states via ion/molecule reactions have proved unsuccessful. Ion/ion proton transfer reactions appear to be an effective means of reducing charge on highly charged proteins to arbitrarily low charge states. In addition to proton transfer, ion/ion recombination has also been observed. The propensity for proton transfer versus anion attachment to the cation is highly dependent upon the identity of the anion.

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.

Complex substate amplitudes formed in double electron capture.

Complex amplitudes and their dependence upon the projectile scattering angle have been measured for $|L,M〉$ substates of doubly excited $1s[2s2p^{1}P]^{2}P$ and the ${1s2p}^{2}^{2}D$ terms of ${\mathrm{C}}^{3+}$ formed in double electron capture by 25 keV ${\mathrm{C}}^{5+}$ ions from He atoms. These are extracted from Auger electron anisotropy measured with respect to the incident beam and scattering plane by a coincident electron--scattered ion technique. The results are the most complete experimental description of a low energy double capture collision.

Electron capture by slow multiply charged ions from H(1s)

Over the last 11 years a considerable experimental and theoretical effort has been devoted to electron capture collisions involving slow ( v < 1 a.u.) multiply charged ions and atomic hydrogen. These collisions result in the selective population of a relatively small number of excited projectile states. Translational energy spectroscopy (TES) and the complementary technique of photon emission spectroscopy (PES) have been developed to provide state selective cross sections. In the last 5 years the merged beams technique has been used to measure total one electron capture cross sections in the energy range 1–1000 eV amu −1 enabling comparison with theory at much lower energies than was previously possible. The majority of the experimental measurements have been for fully stripped, Li-like or He-like ions. Theoretical models such as the semiclassical close coupling methods and fully quantal molecular orbital methods have been developed in parallel with experiment. In general the agreement is good although there is a need for state selective data at lower energies and for differential experimental measurements. Also experiments are needed with pure ground state or metastable ion beams.

Line emission and anisotropy effects in C 6+ electron capture from Li(2s), Li ∗(2p) and [formula omitted

Line emission cross sections for visible transitions after electron capture collisions of C 6+ with Li(2s), Li ∗(2p), and H ∗(n = 2) are calculated for energies up to 50 keV/u; the values agree with available Li(2s) data. Anisotropy parameters are also given for aligned Li ∗(2p). The results were obtained by using the three-body classical trajectory Monte Carlo method with the application of a model potential based on Hartree-Fock calculations for the Li target. The visible line emission cross sections for Li ∗(2p) and H ∗(n = 2) targets are similar, indicating that experiments on Li ∗(2p) will provide needed plasma information for the difficult to measure H ∗(n = 2) case.

Core-electron effect in state-selective electron capture collisions of highly charged ions with alkali-metal atom targets

We study both experimentally and theoretically the influence of the projectile core electrons on the single electron capture process, in medium-energy collisions of partially stripped ions with ground-state alkali-metal atom targets. The effect is clearly demonstrated in the final nl-distributions of the capture electron in Ar 8+- and Kr 8+Li(2s) collisions, by comparisons of experimental data with three-body classical trajectory Monte Carlo calculations performed for these ions, and also for Ne 8+ and O 8+. The effect manifests itself by a significant population of low l-values which does not show up in the case of fully or nearly fully stripped ions; it is strongly energy dependent and vanishes at high energies. The CTMC calculations show also a clear influence of the core electrons on the m-distribution of a given final n-level.

Dipole moments and anisotropies for electron capture collisions between H +, He +2 and Na(3s,3p)

Using the classical-trajectory Monte Carlo method (CTMC), we have investigated state-selective electron capture by charged ions colliding with ground-state Na(3s) and aligned Na(3p) in the collision energy range 1–15 keV/amu. Emission cross sections and state-selective anisotropy effects are benchmarked with experimental results. The z-component of the dipole moment is calculated, and the coherence of the collision product is discussed in relation to the pre-collision electronic state.

Total and differential cross section calculations in Li3++Li single-electron capture collisions

We have extended our previous work on electron-capture total cross sections for the Li3++Li(2s) system by considering a larger molecular basis and taking into account the electron-translation effects. In the present paper we also include differential cross section results, computed in the energy region of the total cross section maximum. The modifications considered do not introduce major changes in the general trend of the total cross section, but its magnitude is enhanced by about 10%. These results are in general good agreement with those obtained by Kumar et al. (1991) except for the oscillatory structure taking place in the lower part of the energy range considered here. The origin of these oscillations and their effects on the differential cross section are discussed.

Xenon recoil-ion charge distributions produced in electron capture and loss collisions of 8 MeV/u Kr

Recoil-ion charge distributions produced in single collisions of 8 MeV/u Kr13+ and Kr32+ projectiles with Xe atoms have been measured using time-of-flight spectroscopy. The post-collision charge states of the projectile ions were determined by magnetic dispersion onto a position sensitive microchannel plate detector. The recoil-ion distributions for ionization accompanied by electron loss from Kr13+ projectiles were bell-shaped with averages that ranged from 7.4 for 1-electron loss to 13.9 for 5-electron loss. The recoil-ion distributions for ionization accompanied by electron capture to Kr32+ projectiles were also bell-shaped, but had much higher average charges that ranged from 20.4 for 1-electron capture to 28.5 for 4-electron capture. The large difference in the average charges produced in the two types of collisions is mainly attributable to charge magnification by Auger decay. A simple model quantitatively explains the variation of the capture-ionization charge distribution width and average charge as a function of the number of captured electrons.

Transition-state theory treatment of capture collisions between ions and symmetric top dipolar molecules

Classical microcanonical transition-state theory gives the reduced rate coefficient for the capture of an ion by a symmetric top dipole which is dependent on two dimensionless parameters: ξ, which is defined by molecular constants of the system exclusively and X1 which is additionally defined by energy. Examination of the energy dependence of the microcanonical rate coefficient reveals that at both ξ→ 0 and ξ→∞ the values obtained are larger by ca. 20% than those estimated for the ion–linear dipole capture. However, thermal averaging results in complete removal of this difference. Apparently, the dipole symmetry is of little consequence under the condition of thermal equilibrium.

A classical trajectory study of possible symmetry restrictions in the hydrogen molecular cation/H2 proton/atom transfer reaction

The suggestion that the title reaction may be subject to symmetry restrictions [J. Chem. Phys. 1992, 96, 4387] is investigated by using classical trajectory techniques to determine capture collision rates as a function of reactant isotopic composition, neutral rotational state, and translational temperature. A comparison of the theoretical results with available experimental reaction rates provides no evidence for a symmetry restriction at 300 K. It is suggested that the effect of a possible symmetry restriction may be best observed for reactions of J=0 neutrals at T<10 K where dynamical constraints restrict coupling of orbital and rotational angular momentum

Electron capture from Li by B5+, N5+ and Be4+ ions

The authors present state selective experimental and theoretical results for electron capture in B5+ and N5+-Li collisons at energies in the range of 1-10 keV amu-1. Experimentally they have used photon emission spectroscopy in the VUV and visible spectral range and theoretically they have performed classical trajectory Monte Carlo (CTMC) calculations. The influence of projectile-core electrons on electron capture processes in such highly charged ion-Li collisions has been investigated by comparing experimental data for He-like N5+ ions with both experimental and theoretical results for the fully stripped B5+ ions. They find that in the B5+ and N5+ cases the projectile-core electrons are of no direct importance for electron capture from Li by these ions. This allows them to compare the CTMC results for Be4+-Li electron capture collisions with atomic orbital calculations as well as with experimental data for He-like C4+ colliding on Li.

Capture and confinement of highly-charged beam ions in a Kingdon trap

Slow multiply-charged Ar q+ ions (2 ⩽ q ⩽ 11) extracted in beams from an electron cyclotron resonance ion source have been injected into an electrostatic (Kingdon) ion trap and confined for times up to a second. Ions were detected at the end of the storage interval by ejecting a fraction of them onto a microchannel plate detector. The stored ion numbers were observed to decrease exponentially with time, indicating that product ions from electron capture collisions were not stored. Photons from the decay of metastable ion levels with lifetimes in the millisecond and longer range have also been observed.

Line emission from C6+, O8+ + Li electron capture collisions

Electron capture cross sections to n sublevels have been calculated for 1-10 keV u-1 collisions of C6+ and O8+ projectiles on a Li target. The classical trajectory Monte Carlo method has been employed with the initial phase distributions for the Li(2s) target obtained from Hartree-Fock calculations. The cross sections are found to maximise at n=7 for C6+ and n=8-9 for O8+. The n cross sections were used to calculate Delta n=1 line emission cross sections. Comparison of these cross-sections with the experimental results of Wolfrum et al. (1992) indicates good agreement between theory and experiment.

Hydrogen atom transfer reactions of N+2 with H2, HD, and D2 from thermal to 10 eV center of mass

Reactions of N+2 ions with H2, HD, and D2 are studied under single-collision conditions in a guided-ion beam mass spectrometer over a much broader range of interaction energies than in any previous study, including the low energy region of thermal to 0.1 eV. Reactant ions are formed in a flow tube source to ensure thermalization. Possible reaction mechanism are discussed, and the present results are compared to previous measurements. We find that the reaction proceeds at the rate predicted by the classical ion–molecule capture collision theory at thermal energy, but exceeds this prediction at energies above 0.1 eV. This behavior is discussed and attributed to details of the interaction between the N+2+H2 surface and the N2+H+2 surface along which the hydrogen atom transfer reaction proceeds. Intramolecular isotope effects and product ion dissociation behavior suggest that the reaction occurs via a direct mechanism with no long-lived intermediate at elevated energies.