Charged Argon Ions Research Articles

Total cross sections for charge transfer by multiply charged neon and argon ions colliding with various hydrocarbons at keV energies

We have measured single- and multiple-charge transfer cross sections of 2q keV Neq+ (q=2–6) and Arq+ (q=2–9) ions in collisions with CH4, C2H2, C2H4, C2H6, C3H4 [allene, propyne], C3H6, (CH2)3, and C3H8 molecules. Cross sections for total charge transfer were derived by summing up single- and multiple-charge transfer cross sections. Their gross features and scaling properties for the present polyatomic molecules as functions of the incident charge number of primary ions and first ionization potentials of target particles are found to be able to be represented by a scheme of a classical over-barrier model for atomic and simple molecular targets. Apart from the ionization potential, dependences of the present total charge transfer cross sections are found on total number of electrons and the number of bonds in target hydrocarbons.

Physics with Highly-Charged Ions in an EBIT

After the commissioning of the Freiburg electron beam ion trap, experiments on dielectronic recombination of the low-lying resonances in He-like Ar16+ have been carried out at high resolution. Forbidden transitions (“coronal lines”) of highly charged argon ions in the optical range have been measured with an accuracy around 1 ppm. Ions extracted from FreEBIT have been used to perform collision experiments using the Cold Target Recoil-Ion Momentum Spectroscopy (COLTRIMS) technique.

Transfer ionization cross-sections measured in collisions of highly charged argon ions with neon target

Multiple electron transfer processes are studied for Arq++Ne (q=8, 9, 11, 12) collisions by using multi-parameter coincidence techniques. Various electron transfer processes are identified experimentally and the related cross-sections are measured. The dependence of transfer ionization cross-sections on the recoil charge states is compared with the results from the modified molecular classical overbarrier model. It is found that the modified model described the experimental results reasonably.

Production of bare argon, manganese, iron and nickel nuclei in the Dresden EBIT

The production of highly charged argon, manganese, iron and nickel ions in a room-temperature electron beam ion trap (EBIT), the Dresden EBIT, has been investigated by means of energy dispersive X-ray spectroscopy of the direct excitation (DE) and radiative recombination (RR) processes. To derive the charge state distributions of the ions in the trap, direct excitation and radiative recombination cross-sections were calculated at electron energies of 8 and 14.4 keV. Based on these theoretical cross-sections and the measured X-ray spectra, the ion densities and the absolute number of ions, which are trapped in the electron beam, are determined for argon, manganese, iron and nickel. Emphasis has been paid to the highly charged ions, including the helium-like and hydrogen-like ions and bare nuclei. In the case of iron we also determined the contributions from lower ionization stages from DE transition lines. It is shown, that in the Dresden EBIT elements at least up to nickel can be fully ionized. Beside energy dispersive spectroscopy it is shown for iron by wavelength dispersive X-ray spectroscopy that with a comparably high gas pressure in the order of 10 −8 mbar carbon-, boron-, beryllium-, lithium- and helium-like iron ions can be produced.

Charge transfer and ionization in collisions between multiplycharged noble gas ions

Using the crossed-beams technique we have measuredabsolute total cross sections for charge transfer andionization of fourfold charged ions of argon, krypton andxenon. In the centre-of-mass energy range from 10 to60 keV total cross section values around10-16 cm2 have been obtained. In addition, usingnear-threshold electron-impact ionization, the fractionof metastable ions was estimated to be of the order of12%-29% for Ar to Xe, respectively. Based on themeasured cross sections, expected particle losses infuture storage rings, which are currently underdiscussion, have been calculated.

Modified distributed electron cyclotron resonance ion source for the production of large diameter ion beams

The work presented in this article concerns the conception and the study of a multiantenna electron cyclotron resonance (ECR) ion source at 2.45 GHz dedicated to the surface treatment of large area materials. In this original device, a microwave plasma is created in a 20 cm in diameter ionization chamber. The 15 cm in diameter extracted ion beam has a current density of about 1 mA/cm2. The applicator is an N-way coaxial power divider in which the N=20 antennas are individually magnetized by internal SmCo bars in order to produce ECR zones in their vicinity. Moreover, the microwave plasma is confined by a multicusp magnetic structure surrounding the ionization chamber. The choice of the source geometry has been guided by the study of some theoretical considerations such as the characteristic diffusion length, the minimum breakdown field, and the penetration depth of the wave into the plasma. Simulations both of the electromagnetic field and the static magnetic field distribution have been carried out in order to validate the final choice of the source geometry. A full characterization of the ECR plasma and of the extracted ion beam was made with different experimental techniques. These results allow us to localize the plasma creation zones inside the ionization chamber. A 120 mA singly charged argon ion beam with a profile homogeneity better than ±5% over 10 cm was obtained 5 cm downstream the extraction system with a 300 W microwave power and a neutral argon pressure of 10−3 mbar in the ionizing chamber.

Retention of the potential energy of multiply charged argon ions incident on copper.

The retained fraction of the potential energy of argon ions incident on copper has been measured using stationary calorimetry at charge states up to 9+ and kinetic energies ranging from 75 to 240 eV per ionic charge. An average fraction of 30% to 40% is found with little dependence on the charge number and on the kinetic energy. The retention of the total energy ranges from 60% to 75% and can mainly be accounted for by the retained fraction of the potential energy and the collisional energy lost by reflected ions and sputtered target atoms.

Asymmetric high-energy ion emission from argon clusters in intense laser fields.

Clusters of 2x10(3) to 4x10(4) Ar atoms are Coulomb exploded in intense (up to 8x10(15) W cm(-2)) laser fields. The dependence of multiply charged argon ion energies on the polarization state of light is probed. A directional asymmetry in the ion-explosion energies is observed for the highest charge states. The ion-energy distribution consists of a low-energy isotropic component, and a high-energy anisotropic one. The results are discussed in terms of an asymmetric Coulomb-explosion scenario.

Improvement of Yield and Emittance of Ions from an EBIS/T with rf-Modulated Electron Beam

By rf-modulating the current of an electron beam ion source and trap (EBIS/T) the ion distribution is significantly influenced by the simultaneously modulated radial trapping potential. It has been observed that highly charged ions are preferentially accumulated near the axis, showing a substantial reduction of their emittance. Furthermore, by using the appropriate cyclotron frequency adjusted according to the shift caused by the space charge of the electron beam it is possible to remove ions with specific mass to charge ratio from the trap. Rf-heating of low charged argon ions during stepwise ionization is enhancing the yield of highly charged argon ions.

Differential cross section measurements for elasticscattering of electrons by highly charged argonions

Elastic scattering of electrons by highly charged ions, inparticular the measurements of differential cross sections,is the focus of the present research. By using a150° crossed beam method, we have successfullymeasured the angular distributions of electrons elasticallyscattered from Ar7+ and Ar8+ ions over a wideangular range, employing a novel toroidal electron energyanalyser, which is capable of simultaneous determination ofthe angle and the energy of the scattered electrons over a widerange. Relative differential cross sections for the e + Ar7+ system have been derived by employing results ofe + Ar8+, which can be treated as pure Rutherfordscattering, as a calibration for the detection efficiency overthe angular range. In the angular range studied, between34°-85°, experimental data of the e + Ar7+ system agree with the Rutherford classical crosssections fairly well, indicating that the Coulombic interactiondominates over this angular range, at a collision energy of100 eV. Some small deviations from the Rutherford cross sectionsare considered to be due to the influences of the ionic structureto the scattering cross sections.

Highly charged ion densities and ion confinement properties in an electron-cyclotron-resonance ion source

Absolute ion densities in an electron-cyclotron-resonance ion source (ECRIS) plasma have been measured using high-resolution x-ray spectroscopy of $(n=2)\ensuremath{\rightarrow}(n=1)$ emission lines from highly charged argon ions. Ion densities have been correlated to extracted currents. The evolution of the ion confinement times with charge state has been investigated for various plasma parameters: a linear increase of the confinement time with charge was found. This result leads to a better understanding of the ion confinement mechanism in ECRIS plasmas.

A fourier transform ion cyclotron resonance study of two- and one-electron capture reactions between doubly charged rare gas ions and rare gas atoms

A Fourier transform ion cyclotron resonance (FTICR) mass spectrometer has been used to study near thermal collision energy symmetric charge transfer reactions. The systems Xe n+ /Xe, Kr n+ /Kr and Ar n+ /Ar have been measured at ambient temperature, where n = 1, 2. In the interactions between doubly charged rare gas ions and their corresponding neutral gas atoms the double electron capture (DEC) has been observed to be more efficient than the single electron capture (SEC). From the measured overall rate coefficient of 3.4 × 10 −10 cm 3 s −1 and the branching ratio of 1.1 between the DEC reaction and the SEC reaction the rate coefficients for double and single electron transfer from Xe to Xe 2+ have been calculated to be 1.8 × 10 −10 cm 3 s −1 and of 1.6 × 10 −10 cm 3 s −1. In the experiments with doubly charged krypton and doubly charged argon ions in their parent gases the symmetric double charge transfer is observed to be even more favored in comparison with the Xe 2+/Xe system at near thermal collision energies.

Highly Charged Argon Density Measurements in an Electron Cyclotron Resonance Ion Source Plasma Using High Resolution X-ray Spectroscopy

Absolute ion densities in an Electron Cyclotron Resonance (ECR) ion source plasma have been measured using high resolution x-ray spectroscopy of (n = 2) → (n = 1) emission lines from highly charged argon ions. Ion densities have been correlated to extracted currents and ion confinement times have been deduced. Confinement time evolution with charge state and plasma performances is presented and discussed.

Evolution of Argon K X-Ray Spectra During the Ion Confinement in Electron Strings

An electron string is a state of a one component electron plasma, which can arise when multiply reflected electrons are accumulated in a strong solenoidal magnetic field. Based on the Electron Beam Ion Source Krion-2 in the reflex mode of operation electron strings were used for production of highly charged Argon ions. Evolution of K x-ray spectra during the confinement of Ar ions was obtained. Dielectronic recombination of Ar16+ ions with string electrons was detected. The possibility of K x-ray diagnostic for studies of electron strings is discussed.

Multicharged Ion Impact on Clean Au(111): Suppression of Kinetic Electron Emission in Glancing Angle Scattering

We have measured the number statistics of emitted electrons in coincidence with the projectile scattering angle distribution for multiply charged slow Ar ions scattered off an Au(111) surface at grazing incidence. The comparison of the electron statistic spectra for different projectile exit and incidence angles permits a separation of potential and kinetic electron emission. A molecular dynamics simulation allows to associate the multiple electron emission with specific trajectory histories (planar and subsurface channeling, close collisions with target atoms at steps and imperfections). [S0031-9007(98)06964-6] PACS numbers: 79.20.Rf, 61.85. + p, 79.60.Bm The interaction of slow highly charged ions (HCI) with clean surfaces reveals new phenomena like the transient formation of exotic “hollow atoms” [1 ‐5] and the recently discovered effect of “potential sputtering” [6]. Above the surface the formation and evolution of hollow atoms can be described theoretically by the classical over-the-barrier (COB) model [1,7]. The image energy gain and the distance of hollow atom formation predicted by this model could be verified in several independent experiments [8‐ 10]. Other signatures of hollow atom formation and decay like potential electron emission (PE) [11] are often masked by significant contributions from below surface interaction processes, in particular, at higher projectile energies where kinetic emission phenomena become dominant [12]. So far, most such results could therefore be used only as indirect proof for the COB model. We have developed an experimental approach which enables us to distinguish between electron emission induced above (or at) the surface from below surface emission. This information can, in turn, provide distinction between kinetic and potential emission. In this work we correlate the electrons emitted during grazing incidence scattering of multiply charged argon ions at a clean metal surface with specific projectile trajectories characterized by the resulting scattering angle, to achieve a separation of above and below surface emission. A comparison with electron emission data for normal incidence with the same projectile, charge state, and target species having equal velocity normal to the surface allows the direct identification of potential and kinetic electron emission

Precision measurements of the wavelengths of spectral lines of multiply charged krypton and argon ions formed in a gas target heated by laser radiation

The first precision (with an error of 0.4–1.6 mÅ) measurements were made of the wavelengths of x-ray spectral lines emitted as a result of n=4 — n'=2 transitions in the Ne- and Na-like Kr XXVII and Kr XXVI ions. The spectra were recorded by means of a spectrograph with a spherically bent mica crystal, characterised by a spectral resolution λ/Δλ ≈ 5 × 103. The dispersion characteristic of this crystal was determined with the aid of reference lines, which were emitted by the He-like Ar XVII ion. The wavelengths of a series of dielectronic satellites of the resonance line of the Ar XVII ion were identified and measured. The results obtained were compared with calculated data.

Energy loss of highly charged argon ions at grazing incidence on a graphite surface

We have used the time-of-flight technique to measure the energy loss for 51-keV argon ions scattered grazingly from a smooth highly oriented pyrolytic graphite surface. The energy loss has been measured for several impact angles and two charge states, q=2+ and q=16+. Our data indicate that the image charge acceleration in the front of the surface, which increases the effective impact angle of the projectile, affects the energy loss. After correcting for this effect, the energy loss does not depend on the initial charge state of the projectile but increases with increasing the angle of incidence. This indicates a full neutralization of the projectile before it reaches the distance of the closest approach. The experimental results agree approximately with predictions of a modified Firsov model.

Improvement of RIKEN 18 GHz Electron Cyclotron Resonance Ion Source using Aluminum Tube

We measured the beam intensity of highly charged argon and krypton ions covering the inner wall of a plasma chamber with an aluminum tube. The beam intensity was enhanced strongly and gas mixing was found to be unnecessary.

Neutralization and equilibration of highly charged argon ions at grazing incidence on a graphite surface.

Final-charge-state distributions of argon ions, scattered grazingly from a smooth highly oriented pyrolytic graphite surface, have been measured as a function of initial charge state (q=4--17) and impact velocity (v=0.15--0.62 a.u.) The final-charge-state distribution changes strongly with the impact velocity, but is almost independent of the initial charge state. The neutralization during grazing-angle scattering is compared to the charge-state equilibration experienced by ions passing through a solid (carbon foil), and these two processes seem to have common properties. A K x-ray spectrum from the K-shell vacancy decay of 51-KeV ${\mathrm{Ar}}^{17+}$ projectiles was obtained as a function of the angle between the ion beam and the surface. Measurements of x-ray spectra in coincidence with grazingly scattered ions are reported. A simple model for argon neutralization near and below the surface is proposed. The model assumes a direct side feeding into the Ar M shell followed by Auger and radiative L- and K-shell filling. \textcopyright{} 1996 The American Physical Society.

Light ion heating in electron cyclotron resonance ion sources

The experiments described here were performed at a 5 GHz electron-cyclotron-resonance ion source with a magnetic hexapole. The discharge was run in argon. By launching a fast magnetoacoustic wave from the high-field side (end on) we heated hydrogen ions—present in traces from the residual gas—at their fundamental ion cyclotron resonance (ICR) frequency of 2.2 MHz with 5 W only. As a consequence the currents of Arq+ ions extracted from the source were significantly increased for q≥5 while the currents of lower charged argon ions were decreased, indicating an enhanced plasma confinement. However, a reduction of the hydrogen content in the residual gas resulted in a reversal of the effect of ICR heating.