Double Rotational Rainbows in Collisions of Homonuclear Diatoms Stemming from Steric Charge Transfer

Rotational rainbows are a common feature in inelastic molecular collisions, which directly reflect the short-range anisotropic intermolecular repulsion. It is well-established that rotationally inelastic collisions of heteronuclear diatoms might exhibit double rainbows, due to different intermolecular interactions at the two ends of the diatom. This is in sharp contrast to single rainbows for homonuclear diatoms. Here, we present experimental observations of double rainbow structures in the collisional charge-transfer reaction between spin-orbit selected Ar⁺(²P_3/2) ions and a representative homonuclear diatom (N₂), using an advanced imaging-based ion-molecule crossed-beam method. Trajectory surface hopping calculations involving multiple electronic states reproduced the experimental observations qualitatively and revealed that collinear approaches of the two colliders show higher charge-transfer efficiencies than side-on approaches due to angle-dependent nonadiabatic interactions. The closest encounters before and after charge transfer experience vastly different anisotropies of the short-range intermolecular repulsion in the two charge-transfer states, thus manifesting as the twin rainbow structures. This mechanism differs fundamentally from those observed in the adiabatic scattering.