Orientation effect in Ar dimer fragmentation by highly charged ion impact

The fragmentation experiment on doubly and triply ionized Ar dimers is carried out using 1.0 MeV Ne8+ impact, focusing on the angular distribution of Ar+ and Ar2+ ions. For the fragmentation of (Ar2)2+, one-site double ionization and two-site double ionization are distinguished clearly, and different angular distributions are observed for these two channels. This phenomenon is explained qualitatively in simple geometrical considerations. For the fragmentation of (Ar2)3+, we find that the Ar+ ions tend to fly along the projectile direction, while the doubly charged Ar2+ ions tend to fly in the opposite direction. This result suggests that, to form the parent ion (Ar2)3+, transfer ionization is favorable in the first collision step, while single ionization is dominant in the second collision step.