Stereodynamics Observed in the Reactive Collisions of Low-Energy Ar⁺ with Randomly Oriented O₂

Stereodynamics of the collisional reaction between mutually aligned or oriented reactants has been a striking topic of chemical dynamics for decades. However, the stereodynamic aspects are scarcely revealed for the low-energy collision with a randomly oriented target. Here in the dissociative charge-exchange reaction between randomly oriented O₂ and low-energy Ar⁺, we, using the three-dimensional ion velocity map imaging technique, clearly observe a linear alignment and a nearly isotropic distribution of the O⁺ yields along the collision axis. These observations are rationalized with the Doppler kinetic models in which the O₂ bond is assumed to be parallel or unparallel to the collision axis of the large impact parameter collision. The linearly aligned O⁺, as the predominant yield, is produced in the parallel collision, while a rotating O₂⁺, as the intermediate in the unparallel collision, leads to the isotropic distribution of O⁺.