above-threshold ionization
{{Short description|Ionization by more photons than are required}}
{{use mdy dates|date=September 2021}}
{{Use American English|date = April 2019}}
File:Hydrogen PES 95eV 1e15.png, whilst the y axis is the differential probability. The first three above-threshold ionization peaks are visible in the image. ]]
In atomic, molecular, and optical physics, above-threshold ionization (ATI) is a multi-photon effect where an atom is ionized with more than the energetically required number of photons.{{cite journal|last=Parker|first=Jonathan|author2=Clark, Charles W.|title=Study of a plane-wave final-state theory of above-threshold ionization and harmonic generation|journal=Journal of the Optical Society of America B|date=1 February 1996|volume=13|issue=2|pages=371|doi=10.1364/JOSAB.13.000371|bibcode = 1996JOSAB..13..371P }} It was first observed in 1979 by Pierre Agostini and colleagues in xenon gas.{{cite journal|last=Bashkansky|first=M.|author2=Bucksbaum, P.|author3=Schumacher, D.|title=Asymmetries in Above-Threshold Ionization|journal=Physical Review Letters|date=13 June 1988|volume=60|issue=24|pages=2458–2461|doi=10.1103/PhysRevLett.60.2458|bibcode=1988PhRvL..60.2458B|pmid=10038359}}
- {{cite journal|last=Agostini|first=P.|author2=Fabre, F.|author3=Mainfray, G.|author4=Petite, G.|author5=Rahman, N.|title=Free-Free Transitions Following Six-Photon Ionization of Xenon Atoms|journal=Physical Review Letters|date=23 April 1979|volume=42|issue=17|pages=1127–1130|doi=10.1103/PhysRevLett.42.1127|bibcode=1979PhRvL..42.1127A|doi-access=free}} The original article on the discovery
Photoelectrons
In the case of ATI the photoelectron peaks should appear at
:
where the integer n represents the minimal number of photons absorbed, and the integer s represents the number of additional photons absorbed. W is the ionization energy, and is the electron kinetic energy of the peak corresponding to s additional photons being absorbed.{{cite book|editor=Gordon W. F. Drake|title=Springer handbook of atomic, molecular, and optical physics|year=2006|publisher=Springer Science+Business Media|location=New York|isbn=0-387-20802-X|edition=Updated and expanded}}
Structure
It typically has a strong maximum at the minimal number of photons to ionize the system, with successive peaks (known as ATI peaks) separated by the photon energy and thus corresponding to higher numbers of photons being absorbed.{{cite journal|last=Cormier|first=E|author2=Lambropoulos, P|title=Optimal gauge and gauge invariance in non-perturbative time-dependent calculation of above-threshold ionization|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|date=14 May 1996|volume=29|issue=9|pages=1667–1680|doi=10.1088/0953-4075/29/9/013|bibcode = 1996JPhB...29.1667C |s2cid=250907316}}
In the non-perturbative regime the bound states are dressed with the electric field, shifting the ionization energy. If the ponderomotive energy of the field is greater than the photon energy , then the first peak disappears.
Features from ultrashort pulses
High intensity ultrashort pulse lasers can create ATI features with 20 or more peaks.{{cite journal|last=Cormier|first=E|author2=Lambropoulos, P|title=Above-threshold ionization spectrum of hydrogen using B-spline functions|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|date=14 January 1997|volume=30|issue=1|pages=77–91|doi=10.1088/0953-4075/30/1/010|bibcode = 1997JPhB...30...77C |s2cid=250843543}} The photoelectron spectrum of electron energies is continuous since actual light sources contain a spread of energies.
References
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External links
- [http://cdn.iopscience.com/images/0953-4075/42/8/085602/Full/jpb305299fig01.jpg Above threshold ionization photoelectron spectra]
{{Lasers}}