Editing Photoionization (section)

==Multi-photon ionization==
Several photons of energy below the ionization threshold may actually combine their energies to ionize an atom. This probability decreases rapidly with the number of photons required, but the development of very intense, pulsed lasers still makes it possible. In the perturbative regime (below about 10<sup>14</sup> W/cm<sup>2</sup> at optical frequencies), the probability of absorbing ''N'' photons depends on the laser-light intensity ''I'' as ''I''<sup>''N'' </sup>.<ref>{{Cite journal |last1=Deng|first1=Z. |last2=Eberly|first2=J. H. |year=1985 |title=Multiphoton absorption above ionization threshold by atoms in strong laser fields |journal=[[Journal of the Optical Society of America B]] |volume=2 |issue=3 |pages=491 |doi=10.1364/JOSAB.2.000486 |bibcode = 1985JOSAB...2..486D }}</ref> For higher intensities, this dependence becomes invalid due to the then occurring AC [[Stark effect]].<ref>{{cite journal|last1=Protopapas|first1=M|last2=Keitel|first2=C H|last3=Knight|first3=P L|title=Atomic physics with super-high intensity lasers|journal=Reports on Progress in Physics|date=1 April 1997|volume=60|issue=4|pages=389–486|doi=10.1088/0034-4885/60/4/001|bibcode=1997RPPh...60..389P|s2cid=250856994}}</ref>

[[Resonance-enhanced multiphoton ionization]] (REMPI) is a technique applied to the [[spectroscopy]] of [[atom]]s and small [[molecule]]s in which a [[tunable laser]] can be used to access an [[Excited state|excited intermediate state]].{{citation needed|date=November 2023}}

[[Above-threshold ionization]] (ATI)<ref>{{Cite journal |last1=Agostini |first1=P. |display-authors=etal |year=1979 |title=Free-Free Transitions Following Six-Photon Ionization of Xenon Atoms |journal=[[Physical Review Letters]] |volume=42 |issue=17 |pages=1127–1130 |doi=10.1103/PhysRevLett.42.1127 |bibcode=1979PhRvL..42.1127A |doi-access=free }}</ref> is an extension of multi-photon ionization where even more photons are absorbed than actually would be necessary to ionize the atom. The excess energy gives the released electron higher [[kinetic energy]] than the usual case of just-above threshold ionization. More precisely, the system will have multiple peaks in its [[photoelectron spectrum]] which are separated by the photon energies, indicating that the emitted electron has more kinetic energy than in the normal (lowest possible number of photons) ionization case. The electrons released from the target will have approximately an integer number of photon-energies more kinetic energy.{{Citation needed|date=September 2011}}