Editing Ionization (section)

==Production of ions==
[[File:Electron avalanche.gif|thumb|upright=1.65|Avalanche effect in an electric field created between two electrodes. The original ionization event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision: the ionizing electron and the liberated electron.]]
Negatively charged ions<ref>{{Cite journal |last=Andersen |first=T |title=Atomic negative ions: structure, dynamics and collisions |journal=Physics Reports |date=2004 |volume=394 |issue=4–5 |pages=157–313 |bibcode=2004PhR...394..157A |doi=10.1016/j.physrep.2004.01.001 |via=157-313}}</ref> are produced when a free electron collides with an atom and is subsequently trapped inside the electric potential barrier, releasing any excess energy. The process is known as [[electron capture ionization]].

Positively charged ions are produced by transferring an amount of energy to a bound electron in a collision with charged particles (e.g. ions, electrons or positrons) or with photons. The threshold amount of the required energy is known as [[ionization energy]]. The study of such collisions is of fundamental importance with regard to the [[Few-body systems|few-body problem]], which is one of the major unsolved problems in physics. [[Kinematically complete experiment]]s,<ref name="schulz">{{cite journal |last1=Schulz |first1=Michael |title=Three-Dimensional Imaging of Atomic Four-Body Processes |journal=Nature |volume=422 |issue=6927 |date=2003 |pages=48–51 |doi=10.1038/nature01415|bibcode= 2003Natur.422...48S |pmid=12621427|url=http://edoc.mpg.de/51141 |hdl=11858/00-001M-0000-0011-8F36-A |s2cid=4422064 |hdl-access=free }}</ref> i.e. experiments in which the complete momentum vector of all collision fragments (the scattered projectile, the recoiling target-ion, and the ejected electron) are determined, have contributed to major advances in the theoretical understanding of the few-body problem in recent years.

=== Adiabatic ionization ===
Adiabatic ionization is a form of ionization in which an electron is removed from or added to an [[atom]] or [[molecule]] in its lowest [[energy state]] to form an ion in its lowest energy state.<ref>{{GoldBookRef|title=adiabatic ionization|file=A00143}}</ref>

The [[Townsend discharge]] is a good example of the creation of positive ions and free electrons due to ion impact. It is a cascade reaction involving [[electron]]s in a region with a sufficiently high [[electric field]] in a gaseous medium that can be ionized, such as [[air]]. Following an original ionization event, due to such as ionizing radiation, the positive [[ion]] drifts towards the [[cathode]], while the free electron drifts towards the [[anode]] of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionization when the next collisions occur; and so on. This is effectively a chain reaction of electron generation, and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche.<ref>Glenn F Knoll. Radiation Detection and Measurement, third edition 2000. John Wiley and sons, {{ISBN|0-471-07338-5}}</ref>

Ionization efficiency is the ratio of the number of ions formed to the number of electrons or photons used.<ref>{{cite journal |journal=Pure Appl. Chem. |date=1991 |volume=63 |pages=1541–1566 |issue=10 |title=Recommendations for Nomenclature and Symbolism for Mass Spectroscopy (including an appendix of terms used in vacuum technology)(IUPAC Recommendations 1991) |doi=10.1351/pac199163101541 |last1=Todd |first1=J. F. J.|doi-access=free }}</ref><ref>{{GoldBookRef|title=ionization efficiency|file=I03196}}</ref>