Collisions in gases and flames involving excited particles

Whereas in the past the main source of ionization of cold and hot gases in electric fields has been ascribed to collisions between electrons and ordinary gas atoms or molecules, it has recently become apparent that the presence of excited particles cannot always be neglected. In this review experimental evidence is presented, together with elementary theoretical considerations, emphasizing the importance of collisions in which the excitation energy of the particles causes emission of light, chemical changes, or speeding up of electrons. Ionization processes in gases subject to an electric field are contrasted with ionization in combustion flames, and recent advances in the understanding of the mechanism are reported. The question of the deviation from thermodynamic equilibrium of the state of a hot gas is discussed quantitatively. The presence of a large number of excited molecules in their parent gas is shown to reduce considerably the strength of a gas. New methods of finding the spatial distribution of excited molecules, as well as their concentration and potential energy, are included.