Electron Capture To The Continuum Research Articles

Contribution of transfer ionization to cusp electron production in proton on argon collisions

Cusp-electron emission in proton on argon collisions has been investigated at impact energies 33, 50 and 75 keV. By detecting the electrons in coincidence with the charge-state analysed outgoing H+,0 projectiles, the process of electron capture to the continuum (ECC), as well as the process of simultaneous ECC and bound-state capture (transfer ionization, TI) were identified. The ratios of the yields for cusp-electron production by TI and ECC are found to decrease with increasing projectile energies. At the lowest proton energy (33 keV) the measured TI/ECC ratio is about four times smaller than the corresponding ratio for incident He+ measured in earlier work. Significant is the finding that there exists a cusp associated with TI events, and, moreover, that this cusp is present for neutral outgoing projectiles.

Theory of longitudinal recoil-ion momentum distribution in ion-atom ionization.

The longitudinal recoil-ion momentum distributions of single ionization in collisions between fast heavy ions and atoms are analyzed by exploring the three-body kinematics and its relation to dynamic processes. It is found that there is a kinematic threshold for the longitudinal recoil-ion momentum distributions that can be ascribed to electron capture to the continuum (ECC). The finite cross section at threshold provides unambiguous evidence of the divergence of the ECC peak when the electron velocity is equal to the projectile velocity. Other aspects associated with the ionization process, such as soft-electron emission and binary-encounter collision, are also studied. Theoretical longitudinal recoil-ion momentum distributions are also compared to existing experiments.

Projectile and electron spectra resulting from positron-argon collisions

A three-body classical trajectory Monte-Carlo calculation is used to evaluate the differential cross sections for the ejected electron, as well as the projectile scattering for 50, 100 and 150 eV e++Ar0 electron removal collisions. In the ionized electron spectra both the electron capture to the continuum (ECC) and binary peak structures are observed. The theoretical ECC peak is diffuse and shifted to lower energies than expected from simple arguments. The calculated double differential cross sections for electron ejection have been compared to experiment. When the experimental apparatus function of Moxom et al. (1992) is used to convolute the theoretical spectra, good agreement with their data is found that confirms a sharp ECC structure is not present in this system. Moreover, comparisons made at non-zero angles show substantial agreement between theory and experiment. The scattered positrons agree well with measurements made at 3 degrees by Kover et al. (1993), but differ at larger angles. In addition, the differential cross section for positronium formation is presented; no evidence of an observable Thomas peak is present.

Measurement and analysis of the ECC cusp structure

We measured the electron capture to the continuum (ECC) peak in the ionization of He atoms by the collision of 50, 100 and 200 keV H+ and 100 keV amu-1 He2+. We exhaustively scanned the double differential cross section between the angles -1.5 degrees to 10 degrees and within the velocities (1+or-0.15) upsilon i where upsilon i is the impact velocity. We propose a new method for analysing the ECC structure which is independent of the spectrometer transmission function modellization and enables a direct comparison with the available theories. Furthermore, we distinguish two regimes for the ionization process, depending on whether the electron velocity is larger or smaller than the projectile velocity.

Projectile energy dependence of the contributions of different mechanisms to cusp electron production in He + → Ar collisions

The energy spectra of cusp electrons from He + → Ar collisions have been measured in coincidence with the outgoing charge state selected He 0,1+,2+ projectiles in the 18.75–75 keV/u energy range. The electron capture to the continuum (ECC), electron loss to the continuum (ELC), and transfer ionization (TI) reaction channels have been identified. In the energy range studied ECC decreases from 86% to 80% and ELC increases strongly from 2.2% to 19.4% with increasing energy. At 18.75 keV/u TI contributes about 12% to the total cusp electron yield whereas at 75 keV/u this fraction decreases to 1.2%. A strong target gas pressure dependence was found for the TI fraction.

Electron capture to the continuum

The second-order Oppenheimer-Brinkman-Kramers approximation is used to derive simple analytical formulae, evaluated to the lowest order in the fine-structure constant alpha in the numerator, for the differential cross section for electron capture to the continuum (ECC) by incident bare ions having velocity nu from target hydrogenic atomic systems. Relativistic as well as non-relativistic forms are derived. Comparison of the theory with the experimental data of Dahl (1985) and Andersen et al. (1986) for H+, He2++He collisions is fairly satisfactory for energies >50 keV amu-1. However, although the velocity dependence obtained by Andersen et al. is nu -11.3+or-0.2 in the range of impact energies 1-2.6 MeV amu-1, this does not imply that the asymptotic nu -11 velocity dependence given by the non-relativistic second-order OBK cross section is almost attained. It is shown that this cannot happen until an energy >500 MeV amu-1 is reached where allowance for relativity produces a considerable change in the energy fall off.

ECC cusp shapes from molecular and atomic gas targets

Electron capture to the continuum (ECC) cusp shape variations (relative to CH 4/Ar) have been examined for 26 molecular gas targets, including hydrocarbons, fluorocarbons, fluorinated hydrocarbons, carbon oxides and SF 6. These molecular species were chosen to examine the dependence of the cusp shape on molecular “size”, shape, valence electron density, number of electrons, ionization potentials, or simultaneous electron transfer. To date the only molecular parameter correlated with the appearance of the cusp shape change called the “W” peak is the number of electrons in the molecule — molecules with more than 26–30 electrons all show evidence of the “W” peak. A similar cusp shape variation among He, Ne, Kr and Xe targets relative to Ar can be understood as arising from different shell contributions to the cusp. Different shell contributions is also a likely explanation for the W peak appearance in target molecules with constituent atoms whose ECC cross sections are similar. In the case of hydrocarbon targets, however, no mechanism presently explains the experimental results.

Electron loss and capture to continuum from He and Ne atoms bombarded with He+-Ions

We have measured the cusp electron yield in coincidence with the transmitted charge state (He0, He+ and He++) when3He+ collides with He and Ne under single collision conditions. For the first time this enables the electron capture to the continuum (ECC) yield to be directly compared with that from electron loss to continuum (ELC). While the ECC contribution is smaller than that from ELC at high projectile velocities (Vp >3 au) the data suggest that ECC will dominate belowVp=2.8 au. The relevance of the results to the projectile velocity dependence of existing capture theories is discussed.

Electron capture to Rydberg and low-lying continuum states

The authors present a comparison of theoretical cross sections for electron capture to Rydberg states (ECR) with the corresponding experimental quantities for electron capture to the continuum (ECC).

On the ultimate sharpness of the ECC cusp

In electron capture to the continuum (ECC) the distribution of scattered electrons is sharply peaked when the velocity of the electron v e matches the velocity of the projectile v p. We discuss the significance of this cusp and present a closed form expre this cusp in the fully peaked strong potential Born approximation (SPB-FP). This cusp diverges at v e = v p for scattering with unscreened Coulomb potentials. In reality this divergence is removed by various effects, including screening of Coulomb potentials in atomic collisions.

Forward electron ejection in H+-He, He+-He and He2+-He collisions

The dependence of the parameters (width, intensity and shape) of the cusp in the processes of electron capture to the continuum (ECC) and electron loss to the continuum (ELC) has been studied as a function of impact velocity (from 2 to 5 au) for the simple collision systems H+-He, He+-He and He2+-He. An approximately linear increase in the width (FWHM) is found in both cases. There is no difference in this respect within the error limits for He2+ (ECC) and He+ (ELC). Regarding cusp shape, similar symmetry (ELC) and asymmetry (ECC) have been observed as in the case of heavy-ion projectiles. None of the theories satisfactorily describes the experimental absolute cross sections of ECC cusp (He2+), while in the case of the He+ projectile (ELC) rather good agreement with theory can be seen in the higher-velocity region. The deviation at lower velocities in the latter case could be explained by an ECC effect present even in the case of He+. Concerning the dependence of the cusp yield on the Zp value of the projectile, Z02.5+or-0.3 was obtained from the yield in H+-He and He2+-He collisions.

Transfer excitation in He++He collisions studied by 0 degrees electron spectroscopy

Spectra of autoionisation electrons from helium projectiles have been measured in collisions of 50-500 keV 3He+ ions with He target atoms. Kinematic line broadenings were substantially reduced by observing electrons at 0 degrees with respect to the incident beam. Lines attributed to states such as 2s2 1S, 2s2p 3P,2p2 1D and 2s2p 1P were resolved. The 2s2p 3P state was found to be predominantly produced exhibiting an asymmetric lineshape (Fano profile) that arises from an interference between transfer excitation and electron capture to the continuum (ECC). Absolute transfer excitation cross sections were deduced from the data. The cross section for the production of the 2p2 1D state exhibits a maximum at about 350 keV projectile energy showing that electron-electron interaction may play an important role in the excitation process.

Electron capture to the continuum from atomic hydrogen

The first known measurement of the differential cross section for electron capture to the continuum (ECC) from atomic hydrogen is presented. A 12 MeV beam of C 6+ ions traversed a static target of atomic hydrogen produced by an electron impact heated dissociation oven. The resulting ECC spectrum was obtained with a channel electron multiplier detector mounted at the exit of a 160° spherical sector electrostatic spectrometer with an angular acceptance of 2°. The ECC spectrum clearly shows the asymmetry generally associated with ECC spectra from gaseous targets. The ratio of the singly differential cross section of H to that of H 2 was found to be 0.80.

Shape of the electron capture to the continuum cusps for H, H2, and He targets in the velocity range 6.3-18.0 a.u.

A general purpose projectile-frame expansion fitting method is used to characterize the shape of the electron capture to the continuum (ECC) cusp for bare carbon, oxygen, neon, and argon projectiles, incident on atomic and molecular hydrogen, and helium targets, over the velocity range 6--18 a.u. The method used to fit the ECC cusp shape allows the direct comparison of spectra taken under differing instrumental conditions in an apparatus-independent manner. The results reveal a substantial similarity in the ECC cusp shape for a wide variety of projectile Z, projectile velocity, and target combinations which has not yet been explained by current theories which predict the shape of the ECC cusp.

Ion-atom differential cross sections at intermediate energies

The classical-trajectory Monte Carlo method has been used to calculate ${\mathrm{H}}^{+}+\mathrm{H}(1s)$ electron-capture and ionization differential cross sections in the range 25-200 keV. The results indicate the importance of including excited product states to describe the small-angle electron-capture scattering. Angular scattering of the removed by the ionization process has been studied as a function of ejected-electron velocity ${v}_{e}$. The classical calculations are in reasonable agreement with coupled-channel results of Shakeshaft [Phys. Rev. A 18, 1930 (1978)] as to the electron capture to the continuum (ECC) component of the ionization process where this term is defined as the ejected being more closely centered to the projectile than the target nucleus after the collision. The ECC cross section ${\ensuremath{\sigma}}_{\mathrm{ECC}}$ was studied as a function of collision energy (50-500 keV/amu) and projectile charge state ($q=1\ensuremath{-}10$). At high energies, ${\ensuremath{\sigma}}_{\mathrm{ECC}}$ scales as $\frac{{q}^{2.3}}{{E}^{2.5}}$. The maximum value for ${\ensuremath{\sigma}}_{\mathrm{ECC}}$ was determined to be an energy ${E}_{max}\ensuremath{\cong}(56 \frac{\mathrm{keV}}{\mathrm{amu}}){q}^{0.4}$. Restricting the ECC component to small electron-scattering angles, ${\ensuremath{\theta}}_{\mathrm{lab}}\ensuremath{\le}5\ifmmode^\circ\else\textdegree\fi{}$, and electron-ejection velocities ${v}_{e}={v}_{p}(1.0\ifmmode\pm\else\textpm\fi{}0.1)$, where ${v}_{p}$ is the projectile velocity, indicates this process is a minor component of the total ionization cross section at intermediate energies.

Observation of two components in molecular cusp spectra for ˜˜MeV/U H+, He++,++ projectiles on M ⩾ 4 hydrocarbon molecules

Recently it has been realized that the cusp spectra obtained with bare projectiles and projectiles incident on gaseous targets are different in their widths and symmetries due to origin of the electrons in the cusp. For the case of bare projectiles, the electrons in the cusp are those captured frorn the target to the projectile's continuum, while in the case of the clothed projectiles the dominant contribution to the cusp yield is from projectile electrons that are expelled into the continuum during the collision. The former process is called electron capture to the continuum (ECC), while the latter is called electron loss to the continuum (ELC); together these processes constitute electron transfer to the continuum (ETC).

Electron Capture to the Continuum at Asymptotically High Velocities

The velocity distrubutions of electrons ejected from gaseous He targets into the forward direction by 15-18 au velocity bare O, Ne and Ar nuclei are measured. The shape of the resulting ECC (electron capture to the continuum) cusps is studied in detail by three fitting methods and compared to theory. The results favor theoretical treatments which include higher-order ejected electron partial waves to describe the observed asymmetry.

Experiments concerning electron capture and loss to the continuum and convoy electron production by highly ionized projectiles in the 0.7 - 8.5-MeV/u range transversing the rare gases, polycrystalline solids, and axial channels in gold

Velocity spectra of electrons emitted into the forward direction have been measured for 0.7--8.5-MeV/u projectile ions traversing He, Ne, and Ar targets, polycrystalline solids (C, Al, Ag, Au), and axial channels in gold. Spectral shapes and yields are compared and contrasted with one another and with theories which seek to account for the cusp-shaped peaks observed in terms of electron capture and loss to low-lying projectile-centered continuum states. We report the results of both singles and coincidence measurements, where the dependence of cusp shapes and yields on the emergent-ion charge state are examined. For electron capture to the continuum (ECC), variance is noted with respect to the simple, first-order theory of Dettmann, Harrison, and Lucas. The ECC yields are compared to experimental and theoretical studies of bound-state capture, especially to high Rydberg states. For electron loss to low-lying continuum states (ELC), variance is noted with respect to the corresponding theories of Briggs, Drepper, and Day, and ELC cross sections are compared to total electron-loss cross sections. For convoy electron production in solids, no known theory accounts for the results. As in ELC processes, convoy cusp widths are observed to be velocity independent. Convoy electron yields are also observed to bemore » independent of emergent-ion charge state.« less

Convoy electron production in heavy ion-solid collisions

The properties of the sharp ν c ≌ ν cusps observed in the velocity spectrum of convoy electrons ( υ e) ejected in heavy ion-solid collisions in the ion velocity range (ν) 6–18 a.u. are compared to the properties of analogous cusps observed in binary electron capture to the continuum (ECC) and electron loss to the continuum (ELC) collisions in gases. Apart from a skew toward ν e > ν, the ν-independent convoy distributions observed are very similar to those for ELC and the cusp widths are the same in both cases. While the shape of convoy peaks is approximately independent of projectile Z, ν, and of target material, yields in polycrystalline targets (C, Al, Ag, Au) exhibit a strong dependence on Z and ν. Coincidence experiments in which convoy electrons are allocated according to emergent ion charge-state q e show a surprising independence of q e, mirroring the unweighted statistical emergent charge-state fraction Coincidence experiments on O 6+ ,7+ ,8+ ions traversing 〈 110 〉 and 〈 100 〉 channels in Au show a strong yield suppression and a dependence of yield on the channel chosen. Interpretation of these observations, comparisons to convoy production studies using protons, and a discussion of remaining puzzles is given. The history of ECC, ELC, and “wake-riding” models of convoy electron production is also reviewed.