appearance energy

The appearance energy (or appearance potential) is the lowest energy which must be imparted to the parent molecule to cause it to produce a particular specified ion. This energy, usually stated in electron volts, may be imparted by electron impact, by photon impact, or in other ways.
(Note: It is recommended that the term 'appearance energy' should replace the term 'appearance potential' and that the energy should be stated in SI units.)

Source: IUPAC