DISSOCIATIVE CHARGE EXCHANGE AND IONIZATION OF O_2 BY FAST H~+ AND O~+ IONS: ENERGETIC ION INTERACTIONS IN EUROPA'S OXYGEN ATMOSPHERE AND NEUTRAL TORUS

摘要

Measurements of electron capture and ionization of O_2 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_2~+ 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 Jupiter's 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.