M. M. Sant'Anna

Anion and cation-yield spectroscopy of core-excited SF6

We report an extensive study on total and partial-ion-yield spectroscopy around both the S 2p and F 1s thresholds in SF(6). All positive and negative single-ion channels have been measured. Below the F 1s threshold we detect a large variation in relative intensity of the resonant structures according to the specific channel monitored, indicating selective fragmentation. Above threshold, at variance with previous cases described by us, we detect high-intensity structures related to shape resonances not only in the cation channels but also for the anions. We discuss the applicability and limits of a model we have developed for the analysis of shape resonances in anion yields as a function of molecular size.

Lifetime of a K-shell vacancy in atomic carbon created by 1s → 2p photoexcitation of C+

Lifetimes for K-shell vacancy states in atomic carbon have been determined by measurement of the natural linewidth of the 1s → 2p photoexcited states of C + ions. The K-shell vacancy states produced by photoionization of atomic carbon are identical to those produced by 1s → 2p photoexcitation of a C + ion: 1s2s 2 2p 22 D, 2 P, and 2 S autoionizing states occur in both cases. These vacancy states stabilize by emission of an electron to produce C 2+ ions. Measurements are reported for the lifetime of the 1s2s 2 2p 22 D, 2 P and 2 S autoionizing states of C + :6 .3± 0.9 fs, 11.2 ± 1.1 fs and 5.9 ± 1.3 fs respectively. Knowledge of such lifetimes is important for comparative studies of the lifetimes of Kshell vacancies in carbon-containing molecules, benchmarking theory, and interpreting satellite x-ray spectra from astrophysical sources such as x-ray binaries. Absolute cross sections were measured for both ground-state and metastable-state ions providing a stringent test of state-of-the-art theoretical calculations. Carbon is ubiquitous in nature and is the building block of life. This atom in its various stages of ionization has relatively few electrons, and is thus amenable to theoretical study. Lifetimes

Electron-loss cross sections for heavy-ion beam probe using gold beams: experiment and theory

We present cross section experimental data required for modeling beam production, attenuation and energy loss in the heavy-ion beam probe technique, namely, electron-loss cross sections for anionic gold projectiles and Ar and N2 targets in the energy range from 30 keV to 1 MeV. The results agree well with low-energy measured values from the literature for the Ar target. The present results are also compared with literature results for the H− projectile and the Ar target, displaying similar velocity dependences. This similarity led to the use of a simple semi-classical model to calculate the projectile-electron-loss cross sections in a wide energy range. This model employs the total scattering cross sections for free electrons and the momentum distribution of the Au− electrons. The present experimental results are in good agreement with this model, allowing a reliable estimate of the cross section values up to 100 MeV. Our results indicate that previously calculated cross sections are overestimated by at least a factor of two. This previous overestimation of electron-loss cross sections leads to significant errors in the modeling of charge-state fractions of gold beams, with consequences in the optimization of beam energy resolution and intensity for HIBP with megaelectronvolts gold beams.

Electron-detachment cross sections of halogen negative-ion projectiles for inertial confinement fusion

Negative-ion beams have recently been suggested as sources of high-energy heavy atoms to be used as drivers for inertial confinement fusion (ICF). Owing to their electron affinities limited to a few eV, anions can be efficiently photo-detached in the vicinity of the fusion chamber, with the resulting high-velocity neutral projectiles following ballistic trajectories towards the hydrogen pellet target. Electron-detachment cross sections are needed as parameters to estimate the beam attenuation in the path from the ion source to the hydrogen pellet. Halogen anions are possible projectile choices. In this paper we present experimental data for total electron-detachment cross sections for F-, Cl-, Br - and I- ions incident on N 2 , in the 0.94-74 ke V u -1 energy range. Our measurements can benchmark theory on anion electron detachment at intermediate to high velocities. Comparison between different projectiles shows very similar collision velocity dependencies. A simple geometrical scaling is presented, providing an estimate for electron-detachment cross sections at the Me V u -1 energy range. The presented scaling indicates that the vacuum requirements due to the use of halogen anions for ICF are less critical than previously suggested.

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.

Valence-shell single photoionization of Kr+ ions: Experiment and theory

Photoionization of Kr

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

^+

Saturation effects in projectile electron loss

ions was studied in the energy range from 23.3 eV to 39.0 eV at a photon energy resolution of 7.5 meV. Absolute measurements were performed by merging beams of Kr

Electron emission from binary collisions with ZProjectile ≤ ZTarget

^+

Valence-shell photoionization of Cℓ-like Ar+ Ions

ions and of monochromatized synchrotron undulator radiation. Photoionization (PI) of this Br-like ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. Resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurements are benchmarked against state-of-the-art theoretical cross-section calculations from the Dirac-Coulomb R-matrix method.