Orientation dependence of multiple ionisation of a diatomic molecule under proton impact

Abstract

Fragmentation of multiply ionised ${\mathrm{CO}}^{n+}(n\leqslant 4)$ molecular ions is investigated using the coincidence momentum imaging technique in an experiment involving proton–CO collisions over the energy range 25–200 keV. From the measured momentum vectors of the fragment ions, the orientation angle—that is the angle between the axis of the molecule and the direction of the projectile at the instant of collision—is determined under the axial recoil approximation. The multiple ionisation yield is found to strongly depend on the orientation angle, giving rise to an anisotropy as well as a forward–backward asymmetry, reflecting the heteronuclear nature of the CO target. A simple model is used to calculate the probability of multiple ionisation, involving Rutherford-like trajectories with an orientation dependent distance of closest approach and an impact parameter dependent single ionisation probability. The computed probabilities show an orientation dependence which agrees closely with experimental observations.