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

For over a century, termolecular, or 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.