C. McGrath

Dissociative Charge Exchange and Ionization of O2 by Fast H+ and O+ Ions: Energetic Ion Interactions in Europa's Oxygen Atmosphere and Neutral Torus

Measurements of electron capture and ionization of O2 molecules in collisions with H+ and O+ ions have been made over an energy range 10-100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiters magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states (electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

Zero degree electron emission in H+collisions with gas targets

A recently constructed apparatus permits measurements of the electron velocity distributions at zero degrees for fast ions in collision with target atoms and molecules. A 260mm diameter hemispherical analyser with a half angle acceptance of 1.5° and an energy resolution ΔE/E of 0.012 is used to measure the velocity distribution of the emitted electrons. The analyser and the surrounding interaction region are double shielded against the earth’s magnetic field by use of μ-metal enclosures. Measurements show that electrons with >2eV pass energy travel through the analyser with no appreciable deflection due to the residual magnetic field. For 40keV H+ projectiles in collision with H2 and He targets the electron energy spectra at zero degree emission angle show that it is dominated by a well-defined electron capture to the continuum peak. Existence of saddle-point electrons at zero degrees is not confirmed.

Ejected electron spectroscopy: Ionization in ion-atom collisions

The continuum-distorted-wave eikonal-initial-state (CDW-EIS) approximation has had much success describing the single ionization of a bare atom by ion impact within the non-perturbative regime [Proc. R. Soc. Lond. A. 452 175–184 (1996)]. This model is first order in a distorted wave series incorporating an eikonal phase distortion in the initial state and a continuum-distorted-wave in the final state. The latter of these accounts for the ejected electron travelling in the presence of two long ranged Coulomb potentials. The single ionization of multielectron atoms shall be discussed, with particular emphasis on heavier targets such as neon and argon. A comparison will be made with other available theoretical models and experimental data.

Fragmentation of 50–100 keV molecular hydrogen ions in collision with a molecular hydrogen target

Dissociation of the molecular hydrogen ion, H2+ in the energy range 50–100 keV in collision with a H2 target has been investigated. Production of one or two fast protons formed from the projectile break-up was distinguished by use of a Si barrier energy detector. Various projectile fragmentation channel cross-sections were determined by coincidence counting techniques between the target ions (separated by time-of-flight analysis) and one or two fast protons. Such data is of importance in the understanding of astrophysical and high temperature laboratory plasmas.