Charge-changing Collisions Research Articles

Stochastic treatment of charge states for ion stopping in solids

We describe the evolution of the charge state of a fast atomic projectile under the equilibrium conditions as a stationary continuous-time Markov process with discrete values. The time scale of the charge-state autocorrelation function is governed by the rate of charge-changing collisions in the target. Describing the collective electron excitations in the target by a linear-response formalism, we show that the finite response time of the electron gas makes the mean stopping power and the mean self-energy of the projectile sensitive to the charge-state correlation effects. We evaluate these effects for light projectiles by using a two-level model for the Markov process and simple models for the dielectric function of the electron gas.

Saturation Effects in Charge-changing Collisions with Multiply-charged C and O Ions

The electron loss of multiply-charged dressed ions by heavy neutral atoms can have a significant contribution from collisions with small impact parameters. This can render one of the two competing mechanisms which govern the electron loss, i.e. the screening, highly non-perturbative. The other mechanism (antiscreening) is due to electron-electron interactions and its contribution can be treated perturbatively. The dependence of the total electron loss cross sections on the target atomic number, Z2, presents a strong saturation as the value of Z2 increases. Calculations based on the Plane Wave Born Approximation present such a behavior for the antiscreening but not for the screening, since this saturation is related to a non-perturbative regime. In this work we compare data for the total electron loss cross sections of C3+ and O5+ ions by H, He, Ne, Ar, Kr and Xe targets, with energies ranging from 1.0 to 3.5 MeV, with calculations for the screening contribution based on the free-collision model. This comparison shows that, for highly-charged ions, the electron capture and direct target ionization channels play a major role in the description of experimental electron loss data.

Anisotropy and polarization in charge changing collisions of with Na(3s) and laser aligned Na(3p)

Absolute cross sections for 5) emission at 465.7 nm after collisions of ions with ground state Na(3s) and laser excited aligned Na(3p) atoms are measured over the collision energy range of . For Na(3s) polarizations are observed by measuring the linear polarization of the emitted light. In these collisions the polarization is found to be 0.33 and almost independent of the collision energy. Cross sections for collisions with the Na(3p) target are up to five times larger than for collisions with Na(3s). Slightly larger cross sections are measured when the laser excited Na(3p) target is -aligned as compared to a -alignment. Classical trajectory Monte Carlo (CTMC) calculations agree excellently with the experimental findings and yield predictions of strong positive polarizations for electron capture also from aligned Na(3p), indicating preferential capture to states.

Electronic stopping in a He-H2 mixture substantially exceeds Bragg's rule value.

We measured the energy loss of 8 keV deuterons in mixtures of He and H{sub 2} gases using a time-of-flight technique. According to Bragg{close_quote}s rule the stopping cross section {var_epsilon} of a mixture of nonreacting gases should be the weighted average of the stopping cross sections of its constituents. Experimentally we find {var_epsilon} to exceed the Bragg value by more than 50{percent}. This can be traced to large differences in the electron capture and electron loss cross sections of He and H{sub 2}, respectively, that strongly enhance energy losses due to charge-changing collisions. {copyright} {ital 1996 The American Physical Society.}

Projectile fragmentation of 16O and 32S at 3.65 GeV/n on several targets

We investigate the charge changing collisions for 16O and 32S beams at 3.65 GeV/n on several targets, by using Solid State Nuclear Track Detectors (SSNTD). Track measurements were performed by an automatic measurement system. We determined the total charge changing cross sections and the elemental cross sections for the production of fragments of charge 9 ≤ Z ≤ 14. Comparison with theoretical models as well as with experimental data are given.

Electron Emission Cross Sections for Collisions of Dressed Ions with Atomic and Molecular Targets

As ions pass through matter at sufficiently low impact energies, less than a few hundred keV.u-1, electrons are captured by the ion. Subsequent collisions can remove these, or other, projectile electrons from the ion. Although the actual energy lost by the projectile during charge changing collisions may be quite small, captured electrons may have a profound affect on the ionisation efficiency of the moving particle. For distant collisions involving high energy ions, the primary effect of bound projectile electrons is to screen the projectile nuclear charge and therby reduce the probability for energy loss. As the ion slows to below a few hundred keV.u-1, however, rather than simply screen the projectile nuclear charge, the 'incident' bound electrons tend to contribute to the target ionisation and the probability for target ionisation may actually be enhanced with respect to that expected for an incident fully stripped ion. This can be particularly important for neutral particle impact because of the larger number of bound electrons involved. Studies with hydrogen, helium and carbon ions and neutrals show that the cross sections for ejection of low-energy electrons (1-20 eV) by neutral atoms is comparable to, or greater than, those for equal velocity proton impact and that the cross sections for ejecting fast electrons (binary encounter electrons) are also increased by the presence of bound projectile electrons.

Electron Emission Cross Sections for Collisions of Dressed Ions with Atomic and Molecular Targets

Abstract As ions pass through matter at sufficiently low impact energies, less than a few hundred keV.u-1, electrons are captured by the ion. Subsequent collisions can remove these, or other, projectile electrons from the ion. Although the actual energy lost by the projectile during charge changing collisions may be quite small, captured electrons may have a profound affect on the ionisation efficiency of the moving particle. For distant collisions involving high energy ions, the primary effect of bound projectile electrons is to screen the projectile nuclear charge and therby reduce the probability for energy loss. As the ion slows to below a few hundred keV.u-1, however, rather than simply screen the projectile nuclear charge, the 'incident' bound electrons tend to contribute to the target ionisation and the probability for target ionisation may actually be enhanced with respect to that expected for an incident fully stripped ion. This can be particularly important for neutral particle impact because of the larger number of bound electrons involved. Studies with hydrogen, helium and carbon ions and neutrals show that the cross sections for ejection of low-energy electrons (1-20 eV) by neutral atoms is comparable to, or greater than, those for equal velocity proton impact and that the cross sections for ejecting fast electrons (binary encounter electrons) are also increased by the presence of bound projectile electrons.

Electron capture into He+ (41) states in collisions of He2+ on Li

Absolute cross sections for He II (n=4 to n=3) visible light emission subsequent to charge changing collisions (0.8-9.75 keV amu-1) of He2+ with Li have been measured. The respective l state selective He+ (41) electron capture cross sections have been determined from the intensity of the He II (n=4 to n=3) transition along the ion beam path by means of a deconvolution technique based on the lifetimes of the He+ (41) states.

Absolute visible light emission cross sections for electron capture from Li atoms by slow, highly charged ions

Absolute cross sections for visible light emission subsequent to charge changing collisions (approximately 1-9 keV amu-1) of C6+ and Neq+ (q=6, 7, 8 and 9) with Li have been measured. The emission cross sections for different 6+ ions (C6+, N6+, O6+ and Ne6+) have been found to be of similar magnitude indicating that for these systems core effects are of minor importance. Comparison with calculations based on the classical overbarrier model and the assumption of conservation of classical angular momentum showed that the predicted, near statistical distribution is not in accordance with the actual final distribution over l states within a given n shell.

Charge-changing Collisions of Stored, Multiply-charged Ions

Abstract The results of measurements of rate coefficients for electron transfer from several target gases to multiply-charged ions are summarized and discussed. These measurements were carried out primarily in Penning ion traps. The stored ions were produced at low energy by several methods, including electron impact multi-ionization of atoms and molecules, recoil ions from impact of fast, stripped, heavy ions with target gas atoms, and inner shell photoionization of atoms using synchrotron radiation. The measurements typically do not exhibit any simple variation of the rate coefficients with ion charge state at these low energies, consistent with the expected mechanism of electron transfer at avoided crossings of the equipotential surfaces of the quasi-molecule formed during the collision. The magnitudes of the rate coefficients, and their trends with charge, are compared for different ions interacting with the same target, and with calculations of the rates using Langevin theory, which predicts a linear...

Charge-changing contribution to energy loss of 32 MeV 3He + in the charge state non-equilibrium region

Energy losses of 32 MeV 3He ions by carbon foils were measured in the charge state non-equilibrium region. The mean energy losses exhibited an abrupt change as a functiion of the foil thickness around 40–50 μg/cm 2. This behavior is quantitatively explained by considering the contribution from charge-changing collisions.

L3-subshell alignment of argon following charge-changing collisions with protons

The alignment of the L3 subshell of Ar induced by electron capture has been studied experimentally for proton impact in the energy range 0.3-1.0 MeV. The experiment is based on the measurement of the angular distribution of the L-MM Auger electrons detected in coincidence with the outgoing neutral H atoms. The results have been compared with the predictions of the Oppenheimer-Brinkmann-Kramers approximation, the strong potential Born approximation, the continuum distorted wave theory and the impulse approximation. The tendency of the measured data towards large negative alignment at high collision energies is in accordance with the asymptotical behaviour characteristic of the capture theories. At lower energies large deviations have been found between the experiment and the theories which can only be partly explained by experimental effects. This finding indicates the failure to understand the capture-induced alignment by the existing theories.

Collisions of polyatomic ions with surfaces

Surface-induced dissociation (SID) of polyatomic ions is reviewed. Instrumentation is described, including hybrid, tandem quadrupole and tandem time-of-flight instruments built especially to study polyatomic ion—surface collision phenomena. In addition, in-line devices that allow SID to be performed on conventional tandem mass spectrometers are noted, as are experiments which utilize ion cyclotron resonance instruments. The extent of energy deposition accompanying surface collisions is characterized and compared with data from gaseous collisions at non-zero scattering angles. The large internal energies available through SID are shown to facilitate structural characterization of molecular ions of polynuclear aromatic hydrocarbons and peptides. Applications to ion strucutre and chemistry are also illustrated. Emphasis is given to competing ion—surface collision phenomena, including charge-changing collisions and ion—surface reactive collisions. These latter processes are shown to provide information on adsorbates and to be thermochemically controlled. When certain projectile ions, especially small fluorocarbons, undergo charge exchange with surface adsorbates, they are released from the surface as ions. The influence of the nature of the surface on these processes and on SID is discussed. Current trends in research on ion—surface collisions are identified.

Electron-loss cross-section measurement of 3He ions by the attenuation method

We have measured single-electron loss cross sections of 3He1+ ions for various solid targets such as C, Al, V, Cr, Cu, Ge, Nb, Ag, Sn and Au at 72 MeV by using the attenuation method. In this method residual 3He1+ fractions after charge-changing collisions were measured with a 3He1+ incident beam. Concentrations of contaminating atoms in the target were determined by an independent RBS measurement. It was found that surfaces of almost all the targets were covered with thin oxygen layers by which 3He1+ fractions were reduced by about 10%. Corrections of the observed loss cross sections are made for the surface oxygen layers.

Heavy ion separation with a gas-filled magnetic spectrograph

Heavy ions passing through a magnetic field region filled with gas experience atomic charge-changing collisions and follow trajectories approximately determined by the mean charge state in the gas. The properties of a gas-filled Enge magnetic spectrograph are studied in detail by measuring focal-plane position spectra of fast heavy ions and their evolution as a function of gas pressure. The method allows physical separation of pairs of isobaric ions in the focal plane. Applications in accelerator mass spectrometry 0experiments are described. At intermediate low pressures, single atomic charge-changing processes can be identified. A Monte Carlo simulation program of the ion transport through the gas-filled magnet is developed and reproduces closely the experimental behavior.

Electron-transfer collisions of low-energy multicharged nitrogen ions with H2 and N2.

A stored-ion collision technique has been used to study the charge-changing collisions of multicharged nitrogen ions with ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$ target gases. The ions were produced inside a Penning trap with use of electron-impact ionization. The time constants for exponential ion loss by charge-changing collisions were measured at several target-gas densities, and from these time constants and densities the rate coefficients for electron capture were calculated. For the collision of ${\mathrm{N}}^{3+}$ with ${\mathrm{H}}_{2}$, for which calculations by Gargaud and McCarroll were available, the cross section for electron capture was obtained from the rate coefficient using v\ifmmode\bar\else\textasciimacron\fi{}, where v\ifmmode\bar\else\textasciimacron\fi{} is the rms ion velocity. The relationships of the other measurements to available theory and to ion-beam measurements at higher energies are discussed. Rate coefficients were obtained at mean ion energies near 1.1, 1.65, and 2.3 eV for ${\mathrm{N}}^{\mathrm{q}+}$ charges q of 2, 3, and 4, respectively. The results of the rate coefficient determinations are k(${\mathrm{N}}^{2+}$,${\mathrm{H}}_{2}$)=[7.8(1.2)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s, k(${\mathrm{N}}^{3+}$,${\mathrm{H}}_{2}$)=[5.4(0.8)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/s, k(${\mathrm{N}}^{4+}$,${\mathrm{H}}_{2}$)=[4.2(0.6)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s, k(${\mathrm{N}}^{2+}$,${\mathrm{N}}_{2}$)=[2.6(0.3)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{3}$/s, k(${\mathrm{N}}^{3+}$,${\mathrm{N}}_{2}$)=[3.5(0.4)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s, and k(${\mathrm{N}}^{4+}$,${\mathrm{N}}_{2}$)=[2.0(0.3)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s.

Electron capture in collisions of O 8+ with H: Absolute line emission cross sections

We present the first experimental determination of absolute line emission cross sections, resulting from charge changing collisions of bare oxygen ions with atomic hydrogen. The O VIII lines resulting from the dominantly populated n=5 level confirm to a large extent the almost coinciding theoretical data obtained with an MO approach by Shipsey et al. and with an AO+ approach by Fritsch and Lin. Emission from the non-dominantly populated n = 6 level is found to be stronger than predicted by theory and favours the MO data.

Recent studies of simultaneous ionization and charge transfer in helium ion-atom collisions

In our laboratory, we are interested in ionization of atomic and molecular targets induced by light ion impact. Generally these collisions are studied by monitoring the ejected electron spectra with the assumption that each ionizing event leads to a single secondary electron. By measuring the target ionization charge states produced by directly ionizing and charge changing collisions, we have obtained cross sections for direct ionization, pure charge transfer, and charge transfer plus ionization thus making it possible to distinguish how target ionization occurs. It was found that charge transfer plus ionization can sometimes be more probable than pure charge transfer alone. This is important because one mechanism which has until recently been disregarded as a means of producing free electrons is the possibility of capturing an electron from the target and simultaneously ionizing one or more target electrons. For ion impact velocities comparable to, or less than, the outer shell bound electron velocities, we have found that charge transfer plus ionization can be a substantial, if not principal, means of producing free electrons.

The role of charge-changing collisions with impurity ions in active-beam plasma diagnostics

The presence of multiply charged impurities in a (tokamak) plasma can be very disturbing if one wants to apply active-beam scattering diagnostics, since it is nearly impossible to separate neutrals scattered by the main plasma constituent from those scattered by the impurity ions. In this work it is shown by making use of already published theoretical results of the electron-loss probability as a function of impact parameter, that not only for H, but also for He injected neutrals, the effect of impurity ions on the relevant energy distribution is negligible.

Alignment of H(2p) following H+-He, Ar charge-changing collisions

Experimental results for the integral alignment A20 of H(2p) following charge-changing collisions of protons with helium and argon atoms are presented for incident energies of 1-5 keV and 35-300 keV. In addition, theoretical calculations in the continuum-distorted-wave approximation were performed. The observed discrepancies between experiment and theory are discussed.