Direct three-body dynamics govern ion-atom recombination and barrierless termolecular reactions

For over a century, termolecular (third-order) chemical reactions have been explained by the Lindemann-Hinshelwood mechanism, assuming sequential stabilization via bimolecular encounters. Here, we demonstrate that barrierless termolecular reactions are fundamentally governed by direct three-body dynamics. Using classical trajectory calculations in hyperspherical coordinates, we quantitatively reproduce ion-atom recombination kinetics across a wide temperature range without invoking intermediate complexes or steady-state assumptions. Our results not only resolve longstanding discrepancies between theory and experiment, but also establish a general mechanistic framework for barrierless termolecular reactions, with implications spanning atmospheric chemistry, plasma physics, and ultracold chemistry.