Non-monotonic energy dependence of vibrational quenching in I(2 <i>P</i> 3/2) + HBr( <i>X</i> 1Σ+) collisions: Roles of transition-state topology and nonadiabatic effects

Abstract

Quasi-classical trajectory and trajectory surface hopping calculations on the I(2P3/2) + HBr(X1Σ+) system reveal a pronounced non-monotonic energy dependence of the vibrational quenching cross section. This dependence is driven by a frustrated-reaction mechanism, in which trajectories access the region near the reactive transition state but retreat without reaction. On the adiabatic aX̃1 potential energy surface, the mechanism decomposes into competing trapping-, rebounding-, and glancing-type pathways, whose interplay produces distinct peaks. The resulting vibrational energy transfer is, therefore, governed by reactive-like dynamics rather than impulsive short-range collisions, representing a clear breakdown of the Landau–Teller picture. In contrast, on the aX̃2 surface, the higher effective barrier restricts access to the transition-state region, resulting in a monotonic energy dependence governed by threshold behavior. The inclusion of nonadiabatic effects mediated by a conical intersection activates highly efficient quenching for trajectories starting on the aX̃2 surface and alters the high-collisional-energy behavior for those starting on the aX̃1 surface. These results demonstrate that the interplay between transition-state topology and nonadiabatic coupling is a key factor controlling vibrational energy transfer and suggest that similar non-Landau–Teller behavior may be a general feature of heavy–light–heavy systems.