Cross Sections For One-electron Capture Research Articles

Polarization degrees of 3p 2P3/2–3s 2S1/2 transition in O5+(1s 23p) produced in collisions of O6+ with He and H2

Electron capture processes in collisions of O6+ with ground state He and H2 are investigated using the two-centre atomic orbital close-coupling method. Total and state-selective one-electron capture cross sections are obtained for collision energies between 0.5 and 300 keV/u. The comparison with the available experimental state-selective capture data in the overlapping energy range (0.5–100 keV/u for O6++He and 0.5–8 keV/u for O6++H2) shows a good overall agreement. The polarization degrees of 3p 2P3/2–3s 2S1/2 radiation from O5+(3p 2P3/2) produced in O6++He and O6++H2 collisions are calculated from the magnetic substate-selective cross sections with inclusion of cascade contributions from higher n = 4 and n = 5 states. Good agreement is obtained with the experimental data available in the energy range 3–8 keV/u. Below ∼10 keV/u, the polarization degrees of O5+(3p 2P3/2) in both collision systems exhibit an oscillatory structure and above this energy they steadily increase with the increase of collision energy, reaching the values of about 0.37 at 300 keV/u. The energy behaviour of the polarization degree of O5+(3p 2P3/2) in the O6++He collision system is determined almost exclusively by the direct electron capture to 3p0 and 3p1 states of O5+, while in the case of the O6++H2 collision system in the energy region below ∼40 keV/u it is strongly affected by the cascade contributions from the 4l states, which are the dominant capture states in this system.

Electron capture in collisions betweenO6+ions andH2Omolecules

By means of photon emission spectroscopy, state selective electron capture cross section for low energy (0.1-7.5 keV/amu) collisions of O6+ on H2O molecules have been measured. Over the range of interaction energies the state selective cross sections change strongly, i.e., by factors up to 5, while the total one-electron capture cross section turns out to increase by approximately 40%. Possibilities of using O VI line emission ratios as a remote diagnostics of astrophysical plasma are indicated.

State-selective one-electron capture from He, Ne and Ar by 4 keV state-prepared ions

A new instrument to carry out double-translational energy spectroscopy (DTES) recently developed in this laboratory has been described in detail. This permits studies of state-selective electron capture by slow state-prepared multiply charged ions using translational energy spectroscopy (TES). The effectiveness and future potential of this approach has been illustrated by studies of one-electron capture in helium, neon and argon by 4 keV beams of pure ground-state and pure metastable ions. In each case the main product channels have been identified by TES and analysed quantitatively without the ambiguities associated with previous measurements which used primary ion beams containing a mixture of ground and metastable species. A simple beam attenuation technique has also been used in conjunction with our state-prepared beams to provide estimates of the ratios of the total one-electron capture cross sections which, in each case, show that is considerably larger than . These ratios have in turn been used to show that the metastable content of ions from the present ECR ion source and sources used in other laboratories can be substantial.

One-electron capture in collisions of 6 - 100 keV protons with oxygen atoms

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study one-electron capture by 6 - 100 keV protons in collisions with oxygen atoms. In these measurements, ground-state oxygen atoms were provided by an iridium tube furnace dissociation source. The measurements extend the energy range of previous experiments and, for the first time, provide separate cross sections for the simple charge transfer process (which is dominated by accidentally resonant one-electron capture) and for the transfer ionization process leading to formation. The present cross sections were obtained using a technique and normalization procedure different from all previous measurements. In particular, our measured values of are in good accord with previous 0.04 - 10 keV data by Stebbings et al based on a modulated crossed-beam technique but at variance with the considerably smaller values in the range 2.5 - 25 keV obtained by Williams et al using a furnace target method. While our values of decrease with increasing energy over the range considered, transfer ionization cross sections pass through a peak value at 34 keV where this process accounts for about 9% of the total one-electron capture cross section rising to 17.5% at 100 keV.

Electron capture by fully stripped high-Z projectiles from the hydrogen atom.

A single-channel distorted-wave approximation is used to calculate the one-electron capture cross section into an arbitrary state (nlm) of ${\mathrm{Ti}}^{22+}$, ${\mathrm{V}}^{23+}$, and ${\mathrm{Fe}}^{26+}$ from the ground state of a hydrogen atom. Since the interaction between the heavy projectile and the target electron is stronger, we represent the initial-channel wave function by a continuum distorted wave while the wave function in the final channel is taken to be a traveling atomic orbital. The nth partial cross sections are found to be in qualitative agreement with previous calculations for some other systems. It is found that at high energies the value ${\mathit{n}}_{\mathrm{max}}$, where the nth partial cross section is maximum, is larger by a few steps than obtained from the ${\mathit{n}}_{\mathrm{max}}$=${\mathit{Z}}^{3/4}$ model. However, for a fixed projectile ${\mathit{n}}_{\mathrm{max}}$ moves towards the smaller values as the energy increases. The l dependence of the cross sections are also studied at different energies at the corresponding ${\mathit{n}}_{\mathrm{max}}$. We have further studied the mth partial cross sections at various energies and at the corresponding ${\mathit{n}}_{\mathrm{max}}$ for several l values. It is found that the contributions from higher m values are decreasing rapidly for m\ensuremath{\gtrsim}5.

State-selective electron capture by slow N4+ ions in collisions with helium

Translational energy spectroscopy in an apparatus previously developed in this laboratory has been used to study one-electron capture by 4-28 keV N4+ ions in collisions with helium. The main excited N3+ product channels have been clearly identified and the relative cross sections determined. The two main channels are found to involve core rearrangement. While the N3+ 2p2 1 S product channel is dominant, it is shown that other channels involving large energy defects contribute significantly at the higher energies thereby resulting in an essentially energy invariant total one-electron capture cross section: MCLZ calculations, which have also been carried out, satisfactorily predict the main 2p2 1 S product channel but fail to account for the other channels arising from curve crossings at small internuclear separations.

Absolute total and one- and two-electron transfer cross sections for Arq+ (8

Absolute values for the total and one- and two-electron transfer cross sections for ${\mathrm{Ar}}^{\mathit{q}+}$ ions (8\ensuremath{\le}q\ensuremath{\le}16) colliding with helium and molecular hydrogen at 2.3q keV laboratory energy were measured by the growth-rate method. The He and ${\mathrm{H}}_{2}$ total cross sections as a function of Ar-projectile L-shell occupation number increase monotonically from ${\mathrm{Ar}}^{8+}$, 2p shell full, to ${\mathrm{Ar}}^{16+}$, 2s shell empty. The ${\mathrm{H}}_{2}$ one-electron capture cross section scales approximately as [${\mathit{E}}_{\mathrm{IP}}$ (He)/${\mathit{E}}_{\mathrm{IP}}$(${\mathrm{H}}_{2}$)${]}^{2}$ times the corresponding He cross section, but the shapes of the two cross sections differ in detail. The ${\mathrm{Ar}}^{\mathit{q}+}$ ions were produced by the Cornell superconducting-solenoid, cryogenic electron-beam ion source (CEBIS) and extracted at 2.3 kV. Selected charge states traversed a gas cell, after which they were detected and charge-state analyzed by the energy-retardation method and by a \ensuremath{\pi}/ \ensuremath{\surd}2 cylindrical electrostatic analyzer. The target-gas pressure in the cell was measured directly by the orifice-flow method used for absolute-pressure gauge calibration. The overall error in the ${\mathrm{Ar}}^{\mathit{q}+}$ on ${\mathrm{H}}_{2}$ cross-section measurements is \ifmmode\pm\else\textpm\fi{}10%, and \ifmmode\pm\else\textpm\fi{}15% in the He measurements.

Simultaneous single-electron capture and projectile-core excitation enhanced through configuration interaction in very slow Ar6+-He collisions.

We report experimental angular differential cross sections for one-electron capture in ${\mathrm{Ar}}^{6+}$-He collisions at very low energies (1.6--13 eV/u). The cross sections show an expectedly high population of core-excited states of ${\mathrm{Ar}}^{5+}$. An alternative mechanism for enhanced capture excitation through configuration interaction is proposed.

Electron capture by Mg+ and Mg2+ in slow collisions with Mg or Zn

Absolute cross sections for one-electron capture by Mg+ and Mg2+ and for two-electron capture by Mg2+ in single collisions with Mg or Zn atoms have been measured at impact energies ranging from 0.50 to 1000 keV. The cross sections for the two-electron capture reactions Mg2++Mg to Mg+Mg2+ and Mg2++Zn to Mg+Zn2+ exhibit remarkable energy dependences which presumably reflect important details of the molecular dynamics of the collisions.

One-electron capture by slow Al2+ ions in atomic and molecular hydrogen

Cross sections for one-electron capture by Al2+ ions in both atomic and molecular hydrogen have been measured and calculated at energies within the range 10-2000 eV amu-1. Measurements are based on the use of a tungsten tube furnace to provide a target of highly dissociated hydrogen. Translational energy spectroscopy used with a specially developed recoil source of metallic ions is used to show that ground state Al+ ion production is predominant. The calculations, which are based on a model potential description of the orbitals, predict cross sections which are somewhat larger than the measured values.

Time-dependent Hartree-Fock calculations for the He+(1s)+He+(1s) system

Time-dependent Hartree-Fock calculations are performed for the two-electron system He+(1s)+He+(1s) in the centre-of-mass energy range 5-80 keV. One-electron capture cross sections are calculated for the Ms=0 and mod Ms mod =1 channels and compared with experimental results. The Pauli blocking for the n=2 shell is briefly discussed.

Charge Transfer of Ar2+and Kr2+in Their Own Gases Studied by the Beam Guide Technique

By an o cto p ole i on-beam g uide ( OPIG ) technique combined with a tandem mass spectrometer technique, one- and two-electron capture cross sections of Ar 2+ and Kr 2+ in their own gases have been measured in the wide collision energy range from 0.1 to 1000 eV (in center-of-mass systems). By changing electron impact energy to produce incident ions, contributions to cross sections from various ionic states in beams are examined. Two-electron capture cross sections are almost independent of ionic states, but one-electron capture cross sections depend strongly on ionic states. One-electron capture cross sections by \(^{3}\bm{P}\) state are determined at collision energies of 1.0, 10 and 100 eV. In this paper, it is also shown that this OPIG experiment is very useful for low energy collision experiments and spans well the energy gap in previous experiments.

Total one-electron capture cross sections for Ar q+ and I q+ ions in slow collisions on H2 and He

Argon and iodine recoil ions were produced by a 2 GeV U75+ beam and total one electron capture cross sections are measured for 198 eV/q Arq+ (4≦q≦15) and Iq+ (5≦q≦27 on He andH2. The cross section can be approximately reproduced by 1/2 πR2 according to the classical barrier model. Theq-dependences exhibit significant fluctuations even for high charge states.

Ionisation and charge transfer in collisions of H+and He2+with lithium

A crossed-beam coincidence technique has been used to determine cross sections for single and double ionisation of lithium by H+ and He2+ impact within the range 22-2100 keV amu-1. There have been no previous measurements and the results provide as assessment of theoretical predictions based on first Born, close-coupling and classical calculations. Estimates of the separate contributions to single ionisation from electrons in the L and K shells have been obtained from the measured cross sections. Cross sections for one-electron capture have also been determined together with cross sections for transfer ionisation.

Electron capture in O8+-He collisions

Single-electron capture in collisions of O8+ with He have been studied theoretically in the energy range 0.2-50 keV amu-1. The molecular orbital expansion method incorporating electron translation factors within the classical trajectory formalism was utilised. The calculated one-electron capture cross section is found to be relatively energy independent with a value of approximately 2*10-15 cm2. Such findings are in general agreement with experimental observations. However, these findings differ substantially from conclusions obtained from an extension of the one electron in a diatomic molecule (OEDM) method to this system.

Total and state-selective capture by slow N2+ions in atomic and molecular hydrogen

Translational energy spectroscopy of a tungsten tube furnace has been used to identify the main collision channels for electron capture by N2+ ions in atomic hydrogen. The results have been used in conjunction with total electron capture cross sections measured in a parallel experiment to determine cross sections for capture into the 3Do, 3Po, 1Do and 3P states of N+ in the range 0.6-8 keV. The observations are shown to be consistent with recent theoretical predictions by Dalgarno et al. (1984) for the 3Do product channel. Total cross sections for one-electron capture by N2+ in both H and H2 have been determined in the range 0.6-200 keV and a simple beam attenuation technique has been used to investigate the influence of metastable N2+ (4P) ions in the primary beam.

Single-electron capture into C3+(n,l) subshells in C4+-Li collisions (20-80 keV)

Absolute emission cross sections have been measured for 29 lines of the C IV spectrum in the wavelength range 20-500 nm resulting from C4+-Li collisions at impact energies between 0.83 and 6.67 keV amu-1. From these emission cross sections one-electron capture cross sections sigma nl have been deduced for all subshells with n<or=7, using known transition probabilities. It is found that electron capture takes dominantly into the n=5 shell, as expected from a classical model. Capture into the n=6,7 shells increases with increasing impact energy, whereas capture into the n=5 shell decreases slightly, resulting in a total capture cross section of about 2*10-14 cm2 which is almost independent of energy. The distributions over l subshells within each n shell are strongly energy dependent, shifting to larger values of l with increasing energy. However, capture into the 5g subshell is dominant over the whole energy range. The results are partly explained qualitatively and are in good agreement with very recent calculations by Fritsch and Lin (1983) who used a modified two-centre atomic orbital expansion.

The dependence of Rcof cross sections for one-electron capture by S11+, S13+and Krq+(q=7-25) ions from He atoms

Total cross section measurements and translational energy spectroscopy have been performed at collision energies of 1q keV for one-electron capture processes by S11+, S13+ and Krq+ (7<or=q<or=25) ions from He atoms. When the measured cross sections are plotted as a function of the crossing radius Rc deduced from the observed energy gain, it is found that the cross sections in general increase with Rc and almost all of them lie between the 1/2 pi Rc2 and pi Rc2 curves. This dependence on Rc is very different from previous results for ions with relatively low q(q<10).

Cross Sections for One-Electron Capture by Highly Stripped Ions of Be, B, and C from H2 and Ar below 10 keV

Cross sections for one-electron capture by highly stripped ions of Be, B and C from H 2 and Ar are measured at low energies below 10 keV. The cross sections are nearly independent of the collision energy investigated. The distinct oscillation with incident ionic charge q in the cross sections are observed.

Collision experiment on highly ionized ions using a vacuum spark source

Cross sections for one-electron capture by Fe 6+ in H 2 have been measured at collision energies below 10 keV by using a vacuum spark ion source. It is found that the cross sections are little dependent on the collision energy and their values are around 6×10 −15 cm 2. This ion source, which has no electrode for ion extraction, can produce multicharged ions from several hundreds of eV to several tens of keV and the maximum charge state of 16 in Fe at the discharge energy of 125 J (capacitor: 10.4 μF, operation voltage: 5 kV). With an ion selection system of 2.7 m time-of-flight and a 127° electrostatic analyzer of 1% resolving power, 10 2−10 3 ions/pulse are obtained. Because of the poor reproducibility of the discharge, ions which have captured an electron from the target molecule and ions which have not reacted, are measured simultaneously during one discharge with a microchannel plate which has two collectors at the back.