Multiple ionization of Ar byF−impact: Projectile-electron-loss and direct-ionization collision channels

Abstract

We have measured single- and multiple-target ionization cross sections for the ${\mathrm{F}}^{\ensuremath{-}}+\mathrm{Ar}$ collision system. Measurements of the final target and projectile charge states were performed in coincidence, separating the collision channels for single-, double-, and triple-projectile-electron loss and for direct ionization. The studied velocity region extends from $v=0.46$ to $v=1.45$ atomic units. Results are compared with existing ${\mathrm{H}}^{\ensuremath{-}}+\mathrm{Ar}$ data as well as with Ar multiple ionization by protons, electrons, and antiprotons. For the direct-ionization channel, ratios for multiple-to-single target ionization are similar to those found for ${\mathrm{H}}^{+}+\mathrm{Ar}$ collisions. For this channel multiple ionization is well described by independent single-ionization events by a frozen projectile. For the projectile-electron-loss collision channels, on the other hand, the correlation between projectile electrons and target electrons plays an important role. Our data show that the average final charge state of the target, $⟨q⟩$, increases steeply with the final charge state of the projectile, while an independent-particle model (neglecting two-center electron-electron correlation) only accounts for small variations of $⟨q⟩$.