ABSTRACT
This chapter summarizes briefly the more significant mechanisms for ionization and deionization in a gas discharge. The processes considered include: ionization by cosmic rays, x-rays, and nuclear radiation; ion generation by electron impact, photoionization, interaction with metastable atoms, thermal ionization, and electron detachment; and deionization by recombination and by diffusion. Many of the properties of gases can be well represented by kinetic theory, which treats the gas molecules as tiny balls with no need to specify their exact size, shape, or internal construction. Ionization by electron impact is probably the most important ionization process for a gas discharge. Its effectiveness depends on the electron energy. When the electron collides with an atom or molecule, kinetic energy is exchanged. In some models, the atoms, molecules, and electrons are assumed to be rigid solid-material balls with the aim of deriving very useful expressions such as the concept of collision cross-section and free path.
