Revealing Transition State Signatures in Collisional Vibrational Quenching Aided by Conical Intersections

We dissect the dynamics of vibrational quenching of HBr by collisions with atomic iodine, which are affected by conical intersections in the strongly interacting regions. Trajectory surface hopping calculations using a newly developed ab initio-based diabatic potential energy matrix reveal that the vibrational inelasticity stems largely from a “frustrated reaction” mechanism, in which the trajectories access the vicinity of a reactive transition state where intermodal coupling is strong. This is aided by nonadiabatic transitions near conical intersections. In addition, the vibration–rotation energy transfer leads to a forward scattering bias, following a hard collision glory scattering mechanism, facilitated by a delicate counterbalance between attractive and repulsive forces.