S. M. Ferguson

Single-electron capture by He2+ ions from triatomic molecules

Abstract State-selective single-electron capture (SEC) processes in low-energy collisions of He 2+ ions with H 2 O and CO 2 have been studied experimentally at impact energies between 0.1 and 1 keV by means of translational energy-gain spectroscopy. At the lowest collision energy, 100 eV, the energy-gain spectra indicate that the dominant reaction channels are dissociative transfer ionization (DTI), due to SEC into the n =1 states of He + product ions with simultaneous ionization of the target products. As the projectile impact energy is increased, DTI channel remains dominant for the He 2+ –CO 2 collision system. However, for the He 2+ –H 2 O collision system, non-dissociative SEC into n =2 states of He + with production of H 2 O + in the ground state is also observed and found to become the dominant process at collision energies in excess of 800 eV. The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the Landau–Zener model and the extended version of the classical over-the-barrier model.

Electron Correlation Leading to Double-K-Shell Vacancy Production in Li-Like Ions Colliding with Helium

Single and double K‐shell vacancies in Be+, B2+, C3+, and O5+ Li‐like ions colliding with neutral helium at intermediate‐to‐high velocities have been investigated. Specific excited states were observed using high‐resolution Auger‐electron spectroscopy. The velocity dependence of the measured cross sections for the formation of doubly vacant K‐shell states was used to determine the mechanisms responsible for producing these so‐called “hollow states”. Electron correlation effects were inferred from the spectral features and the velocity dependence of the observed hollow states. The variation of the correlation strength was investigated as a function of the atomic number of the Li‐like ion.

Single and double ionization of He by 0.5–8 MeV/q Liq+ (q = 1, 2, 3) ions

Abstract Single and double ionization of He by Liq+ projectiles (q = 1, 2, 3) have been investigated in the energy range 0.5–8 MeV/q. Target ionization associated with specific outgoing projectile charge states was identified using time-of-flight techniques. The primary goal of this work was to investigate the ionization of He by “structured” projectiles and to determine the effect of the projectile electrons on the ionization process. For all energies investigated, the double-to-single ionization ratios associated with electron capture and loss are significantly larger than those for direct ionization (no projectile charge change). The direct double-to-single ionization ratios are largely independent of the incoming projectile charge state, apparently depending only on the nuclear charge. For single-electron capture and single-electron loss, the ratios depend quite strongly on the incoming projectile charge state, however.