Editing Ion source (section)

==Photoionization==
{{Main|Photoionization}}
[[Photoionization]] is the ionization process in which an ion is formed from the interaction of a [[photon]] with an atom or molecule.<ref>{{GoldBookRef|title=photoionization|file=P04620}}</ref>

===Multi-photon ionization===
In multi-photon ionization (MPI), several photons of energy below the ionization threshold may actually combine their energies to ionize an atom.

[[Resonance-enhanced multiphoton ionization]] (REMPI) is a form of MPI in which one or more of the photons accesses a [[bound-bound transition]] that is [[resonant]] in the atom or molecule being ionized.

===Atmospheric pressure photoionization===
{{Main|Atmospheric pressure photoionization}}
[[Atmospheric pressure photoionization]] (APPI) uses a source of photons, usually a vacuum UV (VUV) lamp, to ionize the analyte with single photon ionization process. Analogous to other atmospheric pressure ion sources, a spray of solvent is heated to relatively high temperatures (above 400 degrees Celsius) and sprayed with high flow rates of nitrogen for desolvation. The resulting [[aerosol]] is subjected to UV radiation to create ions. [[Atmospheric-pressure laser ionization]] uses UV laser light sources to ionize the analyte via MPI.