This occurs when an internally supra-excited atom or molecular moiety loses an electron spontaneously without further interaction with an energy source. (The state of the atom or molecular moiety is known as a pre-ionization state.)
Sources:
Orange Book, 2nd ed., p. 206 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1547 (https://doi.org/10.1351/pac199163101541)