R. A. Phaneuf

Measured cross sections for ionisation of C3+, N4+ and O5+ ions with contribution due to excitation-autoionisation

Measured cross sections for electron impact ionisation of Li-like ions of C3+, N4+ and O5+ are reported for energies from threshold to 1500 eV. For electron energies from near threshold through the first peak in the cross sections, the measured values are in reasonable agreement with Coulomb-Born calculations. However, in each case an abrupt increase in the measured ionisation cross section is observed at the electron energy associated with excitation of an inner-shell electron, a process which has not been included in the theories. The 1s22s to 1s2s2l excitation followed by autoionisation is shown to give an abrupt and significant contribution to the total ionisation cross section. Further, the relative contribution of the excitation-autoionisation process increases with increasing ionic (or nuclear) charge for the three cases tested along the Li isoelectronic sequence.

Electron‐impact dissociation of H3O+

Fragment cross sections for electron impact dissociation of H3O+ into OH+ and O+, and for dissociation of D3O+ into D2O+ have been measured in a crossed beam appartus for electron energies from threshold to 1000 eV. The cross sections for O+ and OH+ rise abruptly at threshold indicating that the mechanism for dissociation is via production of an electronically excited state of H3O+ and direct dissociation into O+ and OH+. At high energy the cross section has a falloff σ∝ln(E)/E expected in the Bethe‐Born approximation. At 200 eV electron energy the cross sections for fragments are: D2O+, σ=6.7±1.8×10−17 cm2; OH+, σ=3.0±0.6×10−17 cm2; O+, σ=7.9±1.6×10−18 cm2. The observed threshold energies for OH+ and O+ production are 14±3 and 19±3 eV, respectively.

Electron-capture and impact-ionisation cross sections of Nq+ in atomic hydrogen

Cross sections for electron capture by Nq+(1<or=q<or=5) incident on atomic hydrogen have been measured in the velocity range (0.3-5.2)*108 cm s-1. A classical trajectory Monte-Carlo method was used to calculate the electron-capture and impact-ionisation cross sections for incident-ion charge state 3<or=q<or=7. The experimental and theoretical electron-capture cross sections are in good agreement and are found to scale as qeff2, where qeff may be obtained from spectroscopic energy levels. The impact-ionisation cross sections rise to a maximum around v=4-6*108 cm s-1, above which impact ionisation dominates electron capture as the principal H electron-loss process.

Studies of electron correlation effects in multicharged ion atom collisions involving double capture

We review measurements of L-Coster Kronig and Auger electron production in slow, multicharged collision systems to study electron correlation effects in the process of double electron capture. The n/sup /minus/3/ law was confirmed for the production of the Coster-Kronig configurations 1s/sup 2/2pn/ell/ (n greater than or equal to 6) in O/sup 6 +/ + He collisions. Enhancement of high angular momentum /ell/ in specific 1s/sup 2/2pn/ell/ configurations was observed by means of high-resolution measurements of the Coster-Kronig lines. The importance of electron correlation effects in couplings of potential energy curves leading to the 1s/sup 2/2pn/ell/ configurations is verified by means of Landau-Zener model calculations. 32 refs., 4 figs.

Collision experiments with highly ionized atoms

Low-energy inelastic collision processes involving highly ionized atoms have become the subject of a steadily growing number of experimental and theoretical investigations. The majority of these studies have focused on collisions which produce a net change in the charge of the ion. Summarized here are results of some recent investigations of multiply charged ion interactions with neutral atoms and with electrons conducted at Oak Ridge National Laboratory. Special emphasis is given to systematic studies of interaction mechanisms, and to two experiments which show evidence for the correlated motion of electrons during the collision.

Backscattering in electron‐impact excitation of multiply charged ions

For the first time, evidence for strong backscattering of electrons during near threshold excitation of highly charged ions has been observed. By measuring the apparent cross section for the electron‐impact excitation of Ar7+(3s→3p) as a function of center‐of‐mass velocity, over 90% of the inelastically scattered electrons were found to be deflected by more than 90 degrees in the center‐of‐mass frame. This unusual angular scattering has not been previously observed for atoms or ions, but may be typical for multiply charged ions. Our observation of backscattering has been confirmed recently by close‐coupling calculations and distorted‐wave calculations. Absolute total excitation cross sections measured in an energy range extending to 2.2 eV above threshold are also reported and are in agreement with close‐coupling calculations.

Electron capture by multicharged ions from hydrogen atoms at eV energies

Abstract To quantitatively study electron capture during collisions of multiply charged ions with neutral atoms at near-thermal energies, keV-energy multicharged ion beams are merged with ground-state beams of H or D atoms of chosen velocity such that collisions in the relative energy range 1–1000 eV/amu result. Recent data for O 3+ , O 4+ +H(D) are presented and compared with theoretical predictions. Recently completed modifications to the apparatus are described that will provide a significant improvement in signal-to-background and angular collection. These improvements will allow measurements to be extended to lower energies, where effects due to the ion-induced dipole attraction may be evident.

Electron capture in slow collisions of multicharged ions with hydrogen atoms using merged beams

Absolute total electron-capture cross-section measurements are reported for collisions of O3+ and O4+ with atomic hydrogen in the energy range 1–1000 eV/amu using merged beams. The data are compared with available coupled-states theoretical calculations.