W. S. Melo

Electron loss mechanisms in collisions of He+ ions with various targets☆

Abstract The electron loss of high-velocity ions by neutral atoms is due to two different and competing mechanisms. In the screening mode, the electron loss is basically due to the nucleus-electron interaction, with the target electrons assuming the passive role of decreasing the Coulomb field of the target nucleus in the vicinity of the projectile active electron. For a fixed projectile velocity, this contribution is expected to give a non-linear dependence with the target atomic number Z2 due to the incomplete screening at the impact parameter region where the projectile ionization is more likely to occur. Within first-order theories, if the screening is completely absent, the expected dependence would be Z22; with screening, it should scale between Z2 and Z22. On the other hand, in the antiscreening mode, where the loss is due to the action of the target electrons and the target nucleus plays no active role, the expected dependence would be approximately linear with Z2. Thus, for first-order theories, the expected overall dependence with Z2 would be dominated by the screening mode as Z2 increases. We have measured total electron-loss cross sections of He+ ions impinging upon He, Ne, Ar, Kr and Xe targets in the energy range from 1.0 to 4.0 MeV to complement previous measurements and the results point towards a much smaller contribution from the screening mode than expected from first-order theories, possibly due to a saturation effect manifested only in the screening channel.

Time-of-flight spectrometer for absolute measurements of multiply charged ions and ionic molecular fragments in the gas phase

A time-of-flight spectrometer using an extended target gas cell was built to perform absolute measurements of the yields of ionic species in the gas phase produced by charged particles. A detailed description of the spectrometer design and of an independent, absolute calibration procedure is given. The performance of the spectrometer was verified through absolute measurements of multiple ionization of noble gases by charged particles.

Absolute measurements of electron capture cross sections of C3+ from atomic and molecular hydrogen

Absolute measurements of single- and double-electron-capture cross sections by C3+ projectiles on atomic and molecular hydrogen targets were performed for projectile energies between 1.0 and 3.5 MeV for the single- and 1.0 and 2.0 MeV for the double-capture processes. The H / H2 cross section ratios were measured using an absolutely calibrated tungsten-tube furnace for the production of atomic hydrogen. The single-capture data are compared with calculations based on the boundary-corrected first Born approximation, the eikonal approximation and a semiclassical model, presenting a good overall agreement. Calculations for the double capture using an analytical expression, obtained within the independent electron approximation and based on the same semiclassical model, give a reasonable qualitative description of the data.

Saturation effects in projectile electron loss

Abstract The behavior of the projectile electron-loss cross sections with the target atomic number, Z2, presents a saturation as the value of Z2 increases. This fact, which had elready been observed for the excitation of highly charged projectiles, appears in the first-order calculations for the contribution of the projectile electron-target electron interaction but not for the projectile electron-target nucleus interaction. In this work, we compare total electron-loss cross sections with Electron Loss to the Continuum (ELC) yields, for 1.0 MeV He− projectile on various targets, with atomic numbers ranging from 2 to 54. Besides a similar saturation with Z2 observed in both cases, some differences between the data appear, such as enhanced values of the total loss for all targets when compared to the ELC and a structure between the Ar and Xe targets also in the total loss data. These differences may be assigned to the contribution due to the interaction between the projectile active electron and the target electron cloud.

Absolute cross sections for electron loss, electron capture, and multiple ionization in collisions of Li2+?with argon

Exclusive absolute cross sections for the electron loss and capture processes, accompanied by target multiple ionization and pure target multiple ionization, as well as total electron loss and capture cross sections, in collisions of Li2+?with Ar have been measured in the 0.5?3.5 MeV energy range. The experimental data of the total electron loss cross section are compared with theoretical results based on the plane-wave Born approximation and the free-collision model, and with the available experimental data. Some discrepancies are observed when comparing the experimental data with the theoretical models, which can be attributed to the competitive mechanisms that lead to electron loss. The dependences of the single-capture and transfer-ionization processes on the projectile charge state are similar to those observed for collisions between other low-charged light ions and noble-gas targets. The same behaviour is observed when one compares the present data for the single- and double-ionization cross sections with those for He2+?projectiles on Ar. These facts indicate that the dynamics of the collision does not seem to depend on the projectile species, so that few-electron projectiles may act as structureless point charges in the intermediate- to high-velocity regime.

Dissociative and non-dissociative ionization of the O2 molecule by He+ impact

Absolute cross sections for the dissociative and non-dissociative ionization of the O2 molecule, accompanied or not by the projectile electron loss, have been measured in coincidence with the final projectile charge states for the impact of 0.75–3.5 MeV He+ ions. The fragmentation patterns are analyzed in comparison with data for photon, electron and heavy-ion impact available in the literature for different collision channels. This analysis shows that the fragmentation pathways depend not only on the projectile energy and charge state, but also on the particular collision channel involved. It is also shown that the fragmentation fractions follow the same scaling rule obtained previously for the N2 molecule by Melo et al (2006 J. Phys. B: At. Mol. Opt. Phys. 39 3519).

Dissociative and non-dissociative ionization of the N2 molecule by the impact of 0.75-3.5 MeV He +

Total projectile electron loss and absolute dissociative and non-dissociative ionization cross sections of the N2 molecule in coincidence with the final projectile charge states have been measured for the impact of 0.75–3.5 MeV He+. Comparison is made with proton, He+ and electron impact data available in the literature for the direct ionization channels. The fragmentation fractions for singly-charged products follow a scaling law indicating that the main dynamical variables behind the fractions are the momentum transfer at intermediate velocities and the energy transfer at high velocities.

Saturation effects in electron loss

The electron loss of dressed ions by heavy neutral atoms can be highly non-perturbative, in which concerns one of the two competing mechanisms which govern electron loss, namely the screening contribution. The behavior of the total electron loss cross sections with the target atomic number, Z2, shows a strong saturation as Z2 increases. PWBA calculations present such a behavior for the electron-electron contribution (antiscreening) but not for the screening, since this saturation is related to a non-perturbative regime. In this work, we compare measured total electron loss cross sections of He+, C3+ and O5+ ions, with energies ranging from 1.0 to 3.5 MeV, by H, He, Ne, Ar, Kr and Xe atoms, with calculations for the screening contribution based on the free-collision model, as well as with other models, showing that the inclusion of other competitive channels is needed for a better description of this process.

Projectile electron loss in nitrogen

The projectile electron loss channel plays an important role in modeling several processes connected to the penetration of swift ions in gases, such as radiation damage, energy loss, upper atmosphere studies, storage lifetimes of low-charge state heavy ions, etc. In this paper we have used recent measurements of projectile electron loss of He+ ions in N2 together with previous data for higher charged ions in order to shed light on the role played by the projectile electron loss in the heating and ionization of Titans atmosphere.

Absolutely calibrated atomic hydrogen cell for high energy atomic collisions

Abstract A gaseous cell to study heavy-ion collisions with atomic hydrogen was made with the aim of investigating charge-exchange collisions in the intermediate-to-high velocity regime. The method of thermal-dissociation in a tungsten-tube furnace was used. The operating characteristics of the partially dissociated hydrogen target, which depend not only on the design of the furnace but also on the collision-chamber geometry and pumping system, are described. The absolute calibration of the target cell was performed using double capture of C3+ at 1.1 MeV and elastic scattering of the He0 at 1.0 MeV in the mixture of H and H2. At temperatures of about 2600 K, a degree of dissociation of 80% was obtained with the tungsten filament lasting typically more than 300 h of operation.