Editing Photoionization

{{Short description|Ion formation via a photon interacting with a molecule or atom}}
{{Use American English|date = April 2019}}
[[File:Hubble green filaments 20150402.jpg|thumb|300px|Photoionization is the process that makes once-invisible filaments in deep space glow.<ref>{{cite news|title=Hubble finds ghosts of quasars past|url=http://www.spacetelescope.org/news/heic1507/|access-date=23 April 2015|work=ESA/Hubble Press Release}}</ref>]]

'''Photoionization''' is the physical 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>

==Cross section==
Not every interaction between a photon and an atom, or molecule, will result in photoionization. The probability of photoionization is related to the [[Photoionisation cross section|photoionization cross section]] of the species – the probability of an ionization event conceptualized as a hypothetical cross-sectional area. This cross section depends on the energy of the photon (proportional to its wavenumber) and the species being considered i.e. it depends on the structure of the molecular species. In the case of molecules, the photoionization cross-section can be estimated by examination of Franck-Condon factors between a ground-state molecule and the target ion. This can be initialized by computing the vibrations of a molecule and associated cation (post ionization) using quantum chemical software e.g. QChem. For photon energies below the ionization threshold, the photoionization cross-section is near zero. But with the development of pulsed lasers it has become possible to create extremely intense, coherent light where multi-photon ionization may occur via sequences of excitations and relaxations. At even higher intensities (around {{nowrap|10<sup>15</sup> – 10<sup>16</sup> W/cm<sup>2</sup>}} of infrared or visible light), [[non-perturbative]] phenomena such as ''barrier suppression ionization''<ref>{{cite journal | last1=Delone | first1=N. B. | last2=Krainov | first2=V. P. | year=1998 | title=Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field | journal=[[Physics-Uspekhi]] | volume=41 | issue=5 | pages=469–485 | doi=10.1070/PU1998v041n05ABEH000393 |bibcode = 1998PhyU...41..469D | s2cid=94362581 }}</ref> and ''rescattering ionization''<ref>{{cite conference |last1=Dichiara |first1=A. |title=2005 Quantum Electronics and Laser Science Conference |display-authors=etal |year=2005 |chapter=Cross-shell multielectron ionization of xenon by an ultrastrong laser field |book-title=Proceedings of the Quantum Electronics and Laser Science Conference |volume=3 |pages=1974–1976 |publisher=[[Optical Society of America]] |doi=10.1109/QELS.2005.1549346 |isbn=1-55752-796-2 }}</ref> are observed.

==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}}

==Tunnel ionization==
When either the laser intensity is further increased or a longer wavelength is applied as compared with the regime in which multi-photon ionization takes place, a quasi-stationary approach can be used and results in the distortion of the atomic potential in such a way that only a relatively low and narrow barrier between a [[bound state]] and the continuum states remains. Then, the electron can [[Quantum tunnelling|tunnel through]] or for larger distortions even overcome this barrier. These phenomena are called [[tunnel ionization]] and [[over-the-barrier ionization]], respectively.{{citation needed|date=November 2023}}

==See also==
* [[Ion source]]
* [[Radiolysis]]

==References==
{{reflist}}

==Further reading==

* A. Lampros  and A. Nikolopoulos  (2019) ''[https://iopscience.iop.org/book/mono/978-1-68174-712-5 Elements of Photoionization Quantum Dynamics Method]'',  IOP Conscise Physics    ''{{ISBN|978-1643276533}}''
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* {{cite book|author1=Uwe Becker|author2=David Allen Shirley|title=VUV and Soft X-Ray Photoionization|url=https://books.google.com/books?id=m9chfY8jtxYC|date=1 January 1996|publisher=Springer Science & Business Media|isbn=978-0-306-45038-9}}
* {{cite book|author=Cheuk-Yiu Ng|title=Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters|url=https://books.google.com/books?id=bbg_Xkbm9BcC|year=1991|publisher=[[World Scientific]]|isbn=978-981-02-0430-3}}
* {{cite book|author=Joseph Berkowitz|title=Photoabsorption, photoionization, and photoelectron spectroscopy|url=https://books.google.com/books?id=POeHAAAAIAAJ|year=1979|publisher=[[Academic Press]]|isbn=978-0-12-091650-4}}
* {{cite book|author=V. S. Letokhov|title=Laser photoionization spectroscopy|url=https://books.google.com/books?id=dOXvAAAAMAAJ|year=1987|publisher=Academic Press|isbn=978-0-12-444320-4}}
{{refend}}

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[[Category:Spectroscopy]]
[[Category:Ionization]]