Projectile Electron Research Articles

Free-collision model calculations for projectile electron loss by the H2 molecule

An extended version of the free-collision model was employed for the calculation of the electron-loss cross sections of projectiles carrying one to five electrons in collisions with the H2 molecule in the low- to intermediate-velocity regime (0.2–12 au). The model presented by Sigaud (2008 J. Phys. B: At. Mol. Opt. Phys. 41 015205) has been modified in order to calculate total electron-loss cross sections of projectiles with two or more electrons in the same subshell. The electron–electron contribution to the projectile electron loss was included within the independent particle model. The comparison of the calculated cross sections with the available experimental data shows very good agreement in almost all cases.

Spin-Dependent Low-EnergyHe+4Ion Scattering from Nonmagnetic Surfaces

We investigated electron-spin-polarized (4)He(+) ion scattering on various nonmagnetic surfaces at kinetic energies below 2 keV. It was observed that the scattered He(+) ion yield depends on the He(+) ion spin. We interpret this spin-dependent scattering in terms of the spin-orbit coupling that acts transiently on the He(+)1s electron spin in the He(+)-target binary collision. This interpretation qualitatively explains the relationship between the spin-dependent scattering and the scattering geometry, incident velocity, and magnetic field arrangement. This is the first study to report spin-orbit coupling caused by projectile electron spin in ion scattering.

Coulomb excitation of highly charged projectile ions in relativistic collisions with diatomic molecules

We investigate the Coulomb excitation of highly charged ions colliding with diatomic molecules. In this process, the coherent interaction between the projectile electron and two molecular centers may cause clear interference patterns in the (collision) energy dependencies of the total cross sections and alignment parameters. We discuss such a Young-type interference for the particular case of the $K\ensuremath{\rightarrow}L$ excitation of hydrogen- and helium-like projectile ions. Calculations, performed for the scattering of these ions on nitrogen molecules, indicate that the interference effects are extremely sensitive to the collisional geometry and are pronounced only if the molecular axis is aligned almost parallel to the incident beam trajectory.

Electron loss from fast heavy ions: Target-scaling dependence

The target dependence for projectile electron loss is investigated using experimental data taken from the literature. Impact energies range from a few tens of eV/u to tens of MeV/u. For energies less than several MeV/u, the target dependences are shown to be very similar, independent of projectile species and charge state. Overall, however, with increasing impact energy the cross-section dependence on the target nuclear charge systematically increases. It is shown that none of the existing cross-section target scaling models reproduce these features. A model, based on Born scaling and including both the antiscreening and screening contributions to projectile electron loss, is developed. With the inclusion of relativistic effects, which increase the contribution from both channels at high energies, and ''target saturation'' effects, which reduce the contribution from the screening term for heavy targets and lower impact energies, this model describes quite reasonably all available experimental data. A simple scaling formula that reproduces the measured atomic number and impact velocity dependences is provided. This formula is applicable for projectile electron loss in collisions with either atomic or molecular targets and for impact energies ranging from a few to tens of MeV/u.

Cross sections of Oq+ (q = 1 — 4) electron loss in collisions with He, Ne and Ar**Project supported by the Special Foundation for Key Programs of Basic Research at its earlier stage, Ministry of Science and Technology, China (Grant No. 2002CCA00900) and by the Foundation for the Doctors of University of South China (GrantNo. 5-2007-XQD-001).

Electron-loss cross sections of Oq+ (q = 1 — 4) colliding with He, Ne and Ar atoms are measured in the intermediate velocity regime. The ratios of the cross sections of two-electron loss to that of one-electron loss R21 are presented. It is shown that single-channel analysis is not sufficient to explain the results, but that projectile electron loss, electron capture by the projectile and target ionization must be considered together to interpret the experimental data. The screening and antiscreening effects can account for the threshold velocity results, but cannot explain the dependence of the ratio R21 on velocity quantitatively. In general, the effective charge of the target atom increases with velocity increasing because the high-speed projectile ion can penetrate into the inner electronic shell of target atom. Ne and Ar atoms have similar effective charges in this velocity regime, but He atoms have smaller ones at the same velocities due to its smaller nuclear charge.

Signatures of the projectile electron–target core elastic scattering in Ar (e, 2e) reactions at low and intermediate impact energies

The momentum distributions of the recoil ions Ar+ in Ar (e, 2e) reactions are obtained at incident electron energies from 80 to 220 eV by using the reaction microscope. The contributions of large longitudinal momentum corresponding to the large scattering angle are significant. By comparing the single differential cross sections of the Ar (e, 2e) with those of the electron–argon elastic scattering process, it is concluded that the elastic scattering between the incident electron and the target core plays an important role in the large angle ionization scattering process, and the additional phase shift introduced by the ionized electron results in the disappearance of the sharp minimum that was observed in the elastic scattering. A qualitative explanation of the bigger relative contributions of recoil ions with large longitudinal momenta in Ne (e, 2e) compared to the case in Ar (e, 2e) is given.

Absolute cross sections for projectile electron loss accompanied by target multiple ionization in collisions of He+ with noble gases

Absolute cross sections for projectile electron loss accompanied by target multiple ionization in collisions between He+ ions and noble gases have been measured for energies between 1.0 and 3.5 MeV. The data have been compared with other absolute cross sections that exist in the literature for the same projectile, and with calculations for the screening mode (nucleus–electron interaction) using both perturbative (plane-wave Born approximation (PWBA)) and non-perturbative (extended classical-impulse free-collision model, sudden approximation and coupled-channel method) approaches, and for the antiscreening mode (electron–electron interaction) within the PWBA. The energy dependence of the average number of active electrons for the antiscreening has been described by means of a simple function, which is ‘universal’ for noble gases but projectile dependent. A previously developed method has been employed to obtain the number of active electrons for each target subshell in the high-velocity regime.

Negative ions and their behavior in collisions

AbstractWe present a review of electron detachment cross sections of atomic, molecular, and cluster anionic projectiles colliding with atoms and molecules, the main focus being on reporting systematic experimental studies. These studies use an experimental method developed at our laboratory, which uses the gas cell of standard tandem accelerators as a collision target. The experimental data show that the velocity dependence of electron detachment cross sections can be described modeling negative ions as a quasi‐free electron bound to a neutral core. Using this model, a velocity‐independent contribution to cross sections is associated with the neutral projectile core, while a velocity‐dependent curve is associated to the projectile quasi‐free electron. This latter curve is the convolution of experimental free‐electron cross sections, for the same target, with the momentum distribution of the quasi‐free projectile electron. This model is useful to obtain cross section estimates, as exemplified by its application in several collision systems relevant for inertial fusion and plasma diagnostics. It is also a powerful tool for verifying the accuracy of anion structure calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Cross sections of Cq+(q=14)electron loss in collision with He, Ne and Ar investigating

Electron-loss cross sections in Cq+(q=14)collisions with He,Ne,Ar are measured in the intermediate-velocity regime, and the ratio of the two-electron-loss cross section to the one-electrom-loss cross section, R21, are calculated. It is shown that a single-channel analysis is not sufficient to explain the results, and that projectile electron loss, electron capture by the projectile, and target ionization must be considered together to interpreter the data. The screening and antiscreening theory can annalyse the threshold velocity, but cannot totally explain the data of R21 with velocity increasing. The effective charge of target increases with velocity increasing. Ne and Ar have the same effective charges in the velocity regime, but He has a smaller one at the same velocity. The correlation between loss electrons is also analyzed.

Evidence of strong projectile–target-core interaction in single ionization of neon by electron impact

The momentum distributions of recoil ions were measured in the single ionization of neon by electron impact at incident energies between 80 and 2300 eV. It was found that there are a noticeable number of recoil ions carrying large momenta, and the relative contributions of these ions becomes more pronounced with the further decrease of incident electron energy. These observed behaviors indicate that there is a strong projectile--target-core interaction in the single-ionization reaction. By comparing our results with those of electron-neon elastic scattering, we concluded that the elastic scattering of the projectile electron on the target core plays an important role at low and intermediate collision energies.

Single electron loss in the collisions of C3+ and atomic hydrogen

This paper studies the projectile electron loss cross sections of C3+ colliding with atomic hydrogen in the frame work of extended over-barrier model at intermediate velocities (25 keV/u–600 keV/u). The electron loss is calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between projectile electron and target electron. Compared with the convergent close-coupling calculations, screening and anti-screening calculations, this model satisfactorily reproduces the experimentally obtained energy dependence of the electron-impact ionisation cross sections and the single electron loss cross sections over the energy range investigated here.

Absolute cross sections for electron loss, electron capture, and multiple ionization in collisions ofC3+with noble gases

Absolute charge-state-correlated cross sections for projectile electron loss, electron capture, and target multiple ionization in collisions between ${\mathrm{C}}^{3+}$ ions and noble gases have been measured for energies between $1.3$ and $3.5$ MeV. The data have been compared with other similar absolute cross sections existent in the literature for several projectiles. Calculations for the single-loss$\ensuremath{-}$multiple-ionization channel have been performed for the screening mode, using both an extended version of the classical-impulse free-collision model and the plane-wave Born approximation (PWBA), and for the antiscreening mode within the PWBA. The energy dependence of the average number of target active electrons which contribute to the antiscreening has been described by means of a simple function, which is ``universal'' for noble gases but, in principle, projectile dependent. A method has been developed to obtain the number of active target electrons for each subshell in the high-velocity regime, which presented physically reasonable results. Analyses of the dependences of the single-capture and transfer-ionization (SC and TI, respectively) processes on the projectile charge states showed that, for He, equally charged bare and dressed projectiles have very similar cross sections; the latter thus acting as structureless point charges. A behavior similar to that in the SC has been observed for the pure single ionization of He by projectiles with different charge states and of the other noble gases by singly charged projectiles. It has been shown that the ${q}^{2}$ dependence of the pure-single and total-ionization cross sections, predicted by first-order models, is only valid for high-collision velocities. For slower collisions, the electron capture process becomes more relevant and competes with the ionization channel, a feature which grows in importance as the projectile charge state increases.

Coherent and incoherent effects on the imaging and scattering process in transmission electron microscopy and off-axis electron holography

The standard treatment for the different plane wave components of incoming electrons in transmission electron microscope imaging is an incoherent superposition. However, projectile electrons in transmission electron microscopes are localized in space, and therefore have to be described as coherent wave-packets. Moreover, recent developments towards ultrafast electron microscopy and dynamic transmission electron microscopy require a description using highly localized wave-packets. Here we will extend the standard stationary modeling of the elastic scattering processes in high-resolution microscopy to a fully time-dependent approach, by using the direct solution of the time-dependent Schrödinger equation. We will draw the connection to the detection of coherent wave-packets, giving explicit implications for the reconstructed waves in off-axis electron holography. Additionally the description of incoherent aberrations is extended to incorporate the influence of the biprism accurately, leading to a modified form of the damping of spatial frequencies.

Single electron loss in the collisions of lowly charged ions and atomic hydrogen

In this paper, the projectile electron loss cross sections of He +, Li +, Li 2+ and C 2+ colliding with atomic hydrogen are studied in the frame work of the extended over-barrier model at intermediate velocities (25–600 keV/u). The electron loss cross sections are calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between the projectile electron and the target electron. Compared with the measurements, this model satisfactorily reproduces the experimentally obtained energy dependence of the single electron loss cross sections over the energy range investigated here.

Elastic scattering of electrons from Rb, Cs and Fr atoms

Differential, integrated elastic, momentum-transfer and total cross sections as well as differential S, T and U spin parameters for scattering of electrons from rubidium, caesium and francium atoms in the incident energy range up to 300 eV are calculated using a relativistic Dirac equation. The projectile electron–target atom interaction is represented by both real and complex parameter-free optical potentials for obtaining the solution of a Dirac equation for scattered electrons. The Dirac–Fock wavefunctions have been used to represent the Rb, Cs and Fr target atoms. The results of differential cross sections and spin asymmetry parameter S for e-Rb and e-Cs have been compared with the available experimental and theoretical results. Detailed results are reported for the elastic scattering of electrons from the ground states of a francium atom for the first time in the wide range of incident electron energies. The results of electron-Fr elastic scattering show the similar features to those obtained in the case of e-Rb and e-Cs elastic scattering.

Experimental observation of the strong influence of crystal orientation on Electron Rutherford Backscattering Spectra

In Electron Rutherford Backscattering Spectroscopy (ERBS) energetic electrons (in our case up to 40 keV) impinge on a target and one measures the energy of elastically scattered electrons. This energy depends on the mass of the scattering atom, due to the recoil effect. This technique thus provides information about the sample composition. For single crystals the interaction of the projectile electron with the crystal potential modifies the angular intensity distribution of the scattered electrons. This leads, for example, to the well-known Kikuchi patterns. Here we investigate if such modified angular distribution has any influence on the intensity ratio of the observed elastic peaks in ERBS. Dramatic effects are found. Implications of these observations for quantitative surface analysis using energetic electrons are discussed.

Electron impact ionization and fragmentation of C2H2

Ionization and fragmentation of C2H2 resulting from 750 eV electron impact is measured as a function of electron loss. Electron losses up to 400 eV are investigated. The interaction kinematics are investigated by measuring ejected electron-C2H2+ ion-scattered projectile coincidences and generating triply and doubly differential yields for the electron emission and projectile scattering respectively.

Single and multiple projectile electron-loss and capture of low charge states of oxygen beams colliding with the methane molecule

The projectile electron loss and capture are important ionization components collision channels in dressed ion-atom and ion-molecule collisions. They play important roles in several processes connected to the penetration of neutral and charged projectiles in gases, in radiation damage and upper atmospheric physics. In this work, cross sections for total single and multiple electron loss and capture of O-, Oo, O+, O2+ projectiles impinging on the methane molecule were measured in the 15 keV ≤ E ≤ 1000 keV energy range. In addition, an extended version of the free-collision model was employed for the calculation of the electron loss cross sections. Multiple electron loss was analyzed within the independent particle model.

X-ray processes associated with projectile charge changing in ~ 1 MeV/u collisions of O5+ on Ar

We are investigating projectile and target x-ray processes associated with projectile electron capture and loss. Processes of interest are resonant-transfer and excitation (RTE) and projectile and target x-ray emission resulting from inner-shell capture. Preliminary measurements have been made for ~ 1 MeV/u O5+ + Ar collisions.

Projectile electron loss in collisions of Ar6+ and Ar8+ ions with He and Ar atoms

Multiple electron loss of fast Ar6+ (10.2 MeV amu−1) and Ar8+ (19 MeV amu−1) ions in collisions with neutral helium and argon target atoms is analysed in the independent particle model. It is found that losses from inner shells are significant such that subsequent Auger electron emission processes cannot be neglected. The Auger processes are taken into account on the level of a statistical model introduced recently. Total cross sections for multiple electron loss are calculated and compared with experimental data. In particular, ratios of q-fold to single ionization are described well.