multiphoton ionization

in mass spectrometry
https://doi.org/10.1351/goldbook.M04053
Occurs when an atom or molecule and their concomitant ions have energy states whereby the energy in two or more photons is absorbed.
Source:
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 1548 [Terms] [Paper]