M. M. Sant’Anna

Post-collisional effects in multiple ionization of neon by protons

Absolute cross sections for single and multiple ionization of Ne by fast (0.75–3.50 MeV) protons have been measured. The comparison with available theoretical calculations shows that the inclusion of the contribution from time-delayed post-collisional mechanisms is needed for a good description of the experimental results for double and triple ionization at high velocities.

Ionization and dissociation of the formic acid molecule by protons and electrons

Ionization and dissociation of the formic acid molecule (HCOOH) by energetic charged particles have been studied for 1 keV electrons and nearly equivelocity 2 MeV protons. Whereas the fragmentation pattern induced by impact of electron and proton show close similarities, some distinctions in the fragmentation yields were observed for these equivelocity projectiles.

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 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.

Alternative uses of a megavolt tandem accelerator for few-keV studies with ion-source SIMS monitoring.

We increase the versatility of a tandem electrostatic accelerator by implementing simple modifications to the standard operation procedure. While keeping its ability to deliver MeV ion beams, we show that the experimental setup can (i) provide good quality ion beams in the few-keV energy range and (ii) be used to study ion-beam surface modification with simultaneous secondary ion mass spectrometry. This latter task is accomplished without using any chamber connected to the accelerator exit. We perform mass spectrometry of the few-keV anions produced in the ion source by measuring their neutral counterparts at the accelerator exit with energies up to 1.7 MeV. With an additional modification, a high-current few-keV regime is obtained, using the ion source as an irradiation chamber and the accelerator itself only as a mass spectrometer. As an example of application, we prepare a sample for the study of ion-beam assisted dewetting of a thin Au film on a Si substrate.

Electron loss and capture from low-charge-state oxygen projectiles in methane

Absolute cross sections for single- and double-electron loss and single- and multiple-electron capture of 15?1000?keV oxygen projectiles (q =??1, 0, 1, 2) colliding with the methane molecule are presented. The experimental data are used to examine cross-section scaling characteristics for the electron loss of various projectiles. In addition, a modified version of the free-collision model was employed for the calculation of the single- and total-electron-loss cross sections of oxygen projectiles presented in this work. The comparison of the calculated cross sections with the present experimental data shows very good agreement for projectile velocities above 1.0 au. The comparison of the present single-electron-capture cross sections with other projectiles having the same charge shows good agreement, and a common curve can be drawn through the different data sets.

Surface damage in cystine, an amino acid dimer, induced by keV ions

We have studied the interaction of an ion beam (17.6 keV F-) with cystine, a dimer formed by the binding of two cysteine residues. Cystine can be considered as an ideal prototype for the study of the relevance of the disulfide (-S-S-) chemical bond in biomolecules. For the sake of comparison, the amino acid cysteine has also been subjected to the same experimental conditions. Characterization of the samples by XPS and NEXAFS shows that both pristine cystine and pristine cysteine are found as a dipolar ion (zwitterion). Following irradiation, the dimer and the amino acid show a tendency to change from the dipole ion form to the normal uncharged form. The largest spectral modification was observed in the high resolution XPS spectra obtained at around the N 1s core level for the two biomolecules. The 2p sulfur edge spectra of cysteine and cystine were much less sensitive to radiation effects. We suggest that the disulfide bond (-S-S-) remains stable before and after irradiation, contributing to the larger radiation stability of cystine as compared to the amino acid cysteine.

Atomic versus molecular Auger decay in CH2Cl2 and CD2Cl2 molecules

Autoionization spectra of CH2Cl2 and CD2Cl2 molecules after Cl 2p excitation are studied. The two molecular and atomic Auger transitions are examined and assigned. The contribution of atomic Auger transitions is lower in the deuterated molecule. In addition, to support the presence of the ultrafast dissociation mechanism in the dichloromethane molecule, a series of high-level ab initio quantum mechanical calculations were performed at multiconfigurational self-consistent field (MCSCF) and multireference configuration interaction (MRCI) levels of theory. Minimum energy pathways for the dissociation of the dichloromethane molecule have been calculated by taking into account the spin-orbit splitting between the singlet and triplet transitions in the Cl 2p edge.

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.

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.