Multiple-electron capture and ionization in collisions of highly stripped ions with Ar atoms

Multiple-vacancy production in Ar atoms by 1.4-MeV/u highly stripped ion impact is studied. From a recoil-ion–projectile-ion coincidence experiment charge-state distributions of Ar ions were obtained distinguishing between pure ionization and capture of up to three electrons by ${\mathrm{N}}^{6+}$,7+, ${\mathrm{Fe}}^{12+}$,15+,20+,21+, ${\mathrm{Kr}}^{18+}$, ${\mathrm{Gd}}^{37+}$, and ${\mathrm{U}}^{30+}$,36+,41+,44+,4 ${}_{}{}^{8+}{}_{}{}^{}$ ions. With increasing projectile charge state the relative abundances of multiply charged recoil ions increase. However, for high-charge-state projectiles, the relative fractions of recoil ions for concomitant electron capture and ionization are found to be independent of projectile charge or species. To interpret the data, classical trajectory Monte Carlo calculations have been made assuming the applicability of the independent-electron model. The calculations indicate the dominant electron capture is from the L-shell of Ar while the ionization process includes both the L- and M-shell electrons. The use of a common ionization potential inherent in the application of the independent-electron model is not adequate for the high degrees of ionization obtained in the investigated collisions; the cross sections for the production of high recoil-ion charge states are overestimated. Nevertheless, the calculated total cross sections for ionization and electron capture are in reasonable agreement with the available experimental data.