Alignment and orientation dependence of collision induced dissociation*

A single collision between an $H_2^+$ or HeH$^+$ projectile (at $\sim$ keV/amu) and an Ar atom leads predominantly to dissociative capture (DC) and collision-induced dissociation (CID). The CID process can be driven by an electronic or vibrational excitation - the latter typically occurring in close-encounter collisions. One interesting question we focus on is the dependence of these CID mechanisms on the alignment of the molecule relative to its velocity. Our experimental evidence for $H_2^+$ on Ar collisions suggests that CID is favored for perpendicular alignment. Using hetero-nuclear molecules, namely HD$^+$ and HeH$^+$, we explore also the molecular-orientation dependence, i.e. is CID enhanced when the heavy or light nucleus is the one passing closer to the target atom?