Electron capture and multiple ionization in collisions of fast H+ and He2+ ions with gallium atoms

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