A dynamical effect that arises for a process $$\ce{A + B-C -> A-B + C}$$ when the species $\ce{A}$ is light (e.g. a hydrogen atom) compared to $\ce{B}$ and $\ce{C}$. The vibrational excitation of the product $\ce{A–B}$ is low, since the light atom $\ce{A}$ approaches to within the bonding distance of $\ce{BC}$ before the $\ce{C}$ atom retreats. The energy of reaction is therefore released as repulsion between $\ce{A–B}$ and $\ce{C}$, with the result that there is translational excitation of the products.