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