Orientation Dependence Of Ionization in Ion-Molecule Collisions

Abstract

This work deals with the study of the orientation dependence of ionization in ion- molecule collisions. The orientation of the molecule is defined by the angle between the internuclear axis of the molecule and the velocity vector of the incident ion beam. The interaction potential between the incident ion and molecule is not spherically symmetric and depends on the orientation of the molecule. Thus the outcome of the collision, which can be excitation or ionization, depends on the perturbation created or the energy deposited to the molecule, will thus depend on the orientation of the molecule. This dependence of the outcome of the collision on the orientation of the molecule is called the orientation effect. 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. All the experiments were performed at the Low Energy Ion Beam Facility at Inter-University Accelerator Center (LEIBF-IUAC), New Delhi, India. The study is limited to diatomic molecule CO and triatomic molecule OCS. We show that a high degree of ionization is more likely to achieved when a molecule is oriented parallel to the incident projectile as compare to perpendicular orientation. Further we show that the orientation effect is not purely a geometric effect , i.e. it is not merely dependent on the shape of the molecule. An asymmetry in the angular distribution of the fragment ions is observed and quantified, showing that the orientation effect depends on the type of the constituent atoms. The next question examined is how does the orientation effect change with the projectile? The answer to this is found by performing experiments with different types of projectiles such as p+, He2+, C2+, Xe9+. The orientation effect is found to depend on the interaction strength of the projectile which is parameterized by the ratio of charge (q) to velocity (v), both in atomic units. In this work, we have covered all the interaction strength from the perturbative regime (q/v << 1) to the strong interaction regime (q/v >>1). For the perturbative regime, a simple model calculation is performed to calculate the probability of multiple ionization, involving hyperbolic trajectories with an orientation-dependent distance of closest approach and an impact parameter dependent single ionization probability. The calculated probabilities show an orientation dependence and match quite well with the experimental observations.