Hot H Atoms in Gas Phase Photochemistry

By photolyzing hydrogenated molecules, H or D atoms with kinetic energies in the range of about 0.5 to 5 eV are produced. The special features of their chemical properties are reviewed and compared with recoil tritium atom reactions on the one hand and with systems in thermal equilibrium on the other. Although the mechanism of reactive and non‐reactive collisions are by no means well known, hot hydrogen atom chemistry is, in first approximation, rather similar to high temperature chemistry. However, energy losses by energy transfer to molecules and even to rare gases are much smaller than expected from classical hard‐sphere models. For the sake of clearness, this article is rather arbitrarily, divided into three parts: physical aspects, various modes of formation and chemical properties.