ELECTRON-CAPTURE AND MULTIPLE IONIZATION IN COLLISIONS OF FAST H+ ANDHE2+ IONS WITH GALLIUM ATOMS

A crossed-beam technique incorporating time-of-flight analysis and coi ncidence counting of the collision products has been used to study ele ctron capture, and pure ionization of ground-state gallium atoms in co llisions with H+ and He2+ ions. Cross sections leading to the formatio n of up to six-fold ionized gallium in electron capture and up to four -fold ionized gallium through pure ionization have been determined wit hin the energy range 38-1440 keV amu(-1). Pure ionization rather than electron capture is found to provide the main contribution to Gaq+ for mation over most of the energy range considered. A notable feature of the electron capture data is the dominance of the Ga2+ transfer ioniza tion contribution which is believed tc be indicative of the important role of Auger ionization processes. Calculations based on an independe nt electron model of ionization (which have provided a good descriptio n of our previous measurements in Fe and Cu) are shown to fit our meas ured pure ionization cross sections in Ga very satisfactorily. In the case of electron capture, when the model is modified to take some acco unt of Auger ionization, Gaq+ formation is described reasonably for H impact but with only limited success for He2+ impact.