Triple ionisation of OCS: orientation dependence in the case of ion impact

Abstract

The dependence of the cross-section for multiple ionisation on the orientation of free molecules with respect to the incident projectile has long been demonstrated in the case of diatomic molecules. The dependence has been studied via the coincidence momentum imaging technique, where, under the axial recoil approximation, the orientation of the molecule with respect to the incident projectile is determined from the measured momentum vectors of fragments, which, for diatomic molecules, are necessarily emitted back-to-back. The case of triatomic molecules is complicated owing to the possibility of simultaneous or step-wise breaking of the two bonds, precluding a simple mapping of the fragment momentum vectors to the initial orientation. We report a measurement of triple ionisation of an asymmetric triatomic molecule, OCS, which clearly shows, that the process indeed has an orientation dependence similar to that in diatomic molecules. As expected from the studies on diatomic molecules, the dependence is stronger for small values of q/v and negligible for large values of q/v, where q and v are the projectile charge and velocity, respectively.