Processes involving electron capture and multiple ionization in collisions of fastH+andHe2+ions with lead atoms

Abstract

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study ${\mathrm{Pb}}^{q+}$ formation with q up to 8 in collisions between ground-state Pb atoms and ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ ions within the range 50--600 keV ${\mathrm{amu}}^{\mathrm{\ensuremath{-}}1}$. The separate cross sections for simple charge transfer, transfer ionization, and pure ionization leading to the formation of ${\mathrm{Pb}}^{q+}$ ions have been obtained and the relative importance of these processes has been established. Accurate measurements and rigorous theoretical descriptions of these multielectron processes in such heavy atoms are difficult and data are still very limited. The present measurements have been designed to extend our previous studies of multiple ionization of a few selected heavy metal atoms and to provide a further check on the extent to which the main collision processes can be described quantitatively in terms of simple models based on an independent electron description. In our previous work with Fe, Cu, and Ga atoms using the same experimental approach, we were able to describe the formation of multiply charged ions through both transfer ionization and pure ionization with a high degree of success using an independent electron model. However, the present results for Pb show that the success of this simple approach is much more limited for these much heavier atoms.