Ionization is the removal of charge—usually electrons—from neutral atoms to produce positive ions or alternatively, the deposition of extra charge giving negative ions. The simplest examples of ionization occur in gases due to high voltage breakdown, e.g., lightning or exposure to ultraviolet or Ionizing Radiation. Similar processes ar also found in liquids and solids.
In the electronic system of an atom there are many possible energy levels or states that electrons may occupy. No two electrons may have exactly the same state and, allowing for the quantum parameter called spin for which there are two possibilities (up and down), this usually means that two electrons may occupy each possible energy level. The most strongly-bound states, corresponding to electrons in "orbits" closer to the nucleus, are filled first. An atom in its ground state has all of its electrons placed in the lowest possible available energy levels. In fact, the energy of an electron in a particular level is quoted as the binding energy or the work that would have to be done to remove it completely from the atom. The outer electrons of an atom are most easily removed and have smaller binding or ionization energies. A general discussion of this area may be found in the work of Hecht (1994).
Any process that is able to transfer a large enough amount of energy, particularly to the outer electrons of an atom, may produce ionization. Possibilities include: strong electric fields, Electromagnetic Radiation, Ionizing (nuclear) Radiation, etc.
In gases a large enough electric field strips electrons away from atoms. The electrons are then accelerated through the medium. If there is a sufficient path length, the electrons may gain enough kinetic energy to produce secondary electrons by impact ionization, leading to an avalanche type breakdown. A very similar process may occur in solids, particularly semiconductor devices.
In certain molten materials and aqueous solutions the unit chemical cell is already dissociated into ions, e.g., sodium chloride into Na+ and Cl−. The ions in these materials will support conduction, but these ions already existed before the application of an electric field.
Electrons may be excited by the absorption of photons (quanta of electromagnetic energy). Photons of visible and ultraviolet light have energies similar to those required to liberate electrons from many materials.
Various forms of nuclear and atomic radiation can be detected by their ability to ionize matter. X-rays and γ rays are high energy electromagnetic photons capable of penetrating some distance through matter and producing a trail of ionization α-particles, β-particles and other high energy particles associated with radioactive decay are stopped from absorbing materials leaving a track of ionization.
Hecht, E. (1994) Physics. Brooks Cole.