Editing Excimer laser (section)

== Construction and operation ==
[[Image:Nike laser amplifier.jpg|thumb|right|Final amplifier of the Nike laser where laser beam energy is increased from 150 J to ~5 kJ by passing through a krypton/fluorine/argon gas mixture excited by irradiation with two opposing 670,000 volt electron beams.]]  
An excimer laser [[laser construction|typically uses]] a combination of a [[noble gas]] ([[argon]], [[krypton]], or [[xenon]]) and a [[wikt:reactive|reactive]] gas ([[fluorine]] or [[chlorine]]). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-[[molecule]] called an [[excimer]] (or in the case of noble gas halides, [[exciplex]]) is created, which can only exist in an energized state and can give rise to [[laser]] light in the [[ultraviolet]] range.<ref>{{GoldBookRef|title=excimer laser|file=E02243}}</ref><ref>Basting, D. and Marowsky, G., Eds., Excimer Laser Technology, Springer, 2005.</ref>

Laser action in an excimer molecule occurs because it has a bound (associative) [[excited state]], but a [[repulsive state|repulsive]] (dissociative) [[ground state]]. Noble gases such as xenon and [[krypton]] are highly [[Inert gas|inert]] and do not usually form [[chemical compound]]s. However, when in an excited state (induced by electrical discharge or high-energy electron beams), they can form temporarily bound molecules with themselves (excimer) or with halogens (exciplex) such as [[fluorine]] and [[chlorine]]. The excited compound can release its excess energy by undergoing [[spontaneous emission|spontaneous]] or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly (on the order of a [[picosecond]]) dissociates back into two unbound atoms. This forms a [[population inversion]].{{citation needed|date=January 2022}}

=== Wavelength determination ===
The [[wavelength]] of an excimer laser depends on the molecules used, and is usually in the ultraviolet range of [[electromagnetic radiation]]:

{| class="wikitable"
!Excimer
!Wavelength
!Relative power
|-
|Ar<sub>2</sub>*||126&nbsp;nm||
|-
|Kr<sub>2</sub>*||146&nbsp;nm||
|-
|F<sub>2</sub>*||157&nbsp;nm||
|-
|Xe<sub>2</sub>*||172 & 175&nbsp;nm||
|-
|ArF||193&nbsp;nm||60
|-
|KrCl||222&nbsp;nm||25
|-
|KrF||248&nbsp;nm||100
|-
|XeBr||282&nbsp;nm||
|-
|[[Xenon monochloride|XeCl]]||308&nbsp;nm||50
|-
|XeF||351&nbsp;nm||45
|}

Excimer lasers, such as XeF and KrF, can also be made slightly ''tunable'' using a variety of prism and grating intracavity arrangements.<ref>[[F. J. Duarte]] (Ed.), ''Tunable Lasers Handbook'' (Academic, New York, 1995) Chapter 3.</ref>

=== Pulse repetition rate ===
[[File:Electra Laser System NRL 2013.png|thumb|alt=The electra laser at NRL is a KrF laser that demonstrated over 90,000 shots in 10 hours.|The electra laser at NRL is a KrF laser that demonstrated over 90,000 shots in 10 hours.]]
While electron-beam pumped excimer lasers can produce high single energy pulses, they are generally separated by long time periods (many minutes).&nbsp; An exception was the Electra system, designed for inertial fusion studies, which could produce a burst of 10 pulses each measuring 500 J over a span of 10 s.<ref>{{Cite journal|last1=Wolford|first1=M. F.|last2=Hegeler|first2=F.|last3=Myers|first3=M. C.|last4=Giuliani|first4=J. L.|last5=Sethian|first5=J. D.|year=2004|title=Electra: Repetitively pulsed, 500 J, 100 ns, KRF oscillator|journal=Applied Physics Letters|volume=84|issue=3|pages=326–328|bibcode=2004ApPhL..84..326W|doi=10.1063/1.1641513}}</ref> In contrast, discharge-pumped excimer lasers, also first demonstrated at the Naval Research Laboratory, are able to output a steady stream of pulses.<ref>{{cite journal | doi=10.1063/1.88934 | title=Ultraviolet-preionized discharge-pumped lasers in XeF, KRF, and ArF | date=1976 | last1=Burnham | first1=R. | last2=Djeu | first2=N. | journal=Applied Physics Letters | volume=29 | issue=11 | pages=707–709 | bibcode=1976ApPhL..29..707B }}</ref><ref>Original device acquired by the National Museum of American History's Division of Information Technology and Society for the Electricity and Modern Physics Collection (Acquisition #1996.0343).</ref> Their significantly higher pulse repetition rates (of order 100&nbsp;Hz) and smaller footprint made possible the bulk of the applications listed in the following section. A series of industrial lasers were developed at XMR, Inc<ref>Personal notes of Robert Butcher, Laser Engineer at XMR, Inc.</ref> in Santa Clara, California between 1980 and 1988. Most of the lasers produced were XeCl, and a sustained energy of 1 J per pulse at repetition rates of 300 pulses per second was the standard rating. This laser used a high power thyratron and magnetic switching with corona pre-ionization and was rated for 100 million pulses without major maintenance. The operating gas was a mixture of xenon, HCl, and Neon at approximately 5 atmospheres. Extensive use of stainless steel, nickel plating and solid nickel electrodes was incorporated to reduce corrosion due to the HCl gas. One major problem encountered was degradation of the optical windows due to carbon build-up on the surface of the CaF window. This was due to hydro-chloro-carbons formed from small amounts of carbon in O-rings reacting with the HCl gas. The hydro-chloro-carbons would slowly increase over time and absorbed the laser light, causing a slow reduction in laser energy. In addition these compounds would decompose in the intense laser beam and collect on the window, causing a further reduction in energy. Periodic replacement of laser gas and windows was required at considerable expense. This was significantly improved by use of a gas purification system consisting of a cold trap operating slightly above liquid nitrogen temperature and a metal bellows pump to recirculate the laser gas through the cold trap. The cold trap consisted of a liquid nitrogen reservoir and a heater to raise the temperature slightly, since at 77 K (liquid nitrogen boiling point) the xenon vapor pressure was lower than the required operating pressure in the laser gas mixture. HCl was frozen out in the cold trap, and additional HCl was added to maintain the proper gas ratio. An interesting side effect of this was a slow increase in laser energy over time, attributed to increase in hydrogen partial pressure in the gas mixture caused by slow reaction of chlorine with various metals. As the chlorine reacted, hydrogen was released, increasing the partial pressure. The net result was the same as adding hydrogen to the mixture to increase laser efficiency as reported by T.J. McKee et al.<ref>{{cite journal | doi=10.1063/1.91658 | title=Lifetime extension of XeCl and KRCL lasers with additives | date=1980 | last1=McKee | first1=T. J. | last2=James | first2=D. J. | last3=Nip | first3=W. S. | last4=Weeks | first4=R. W. | last5=Willis | first5=C. | journal=Applied Physics Letters | volume=36 | issue=12 | pages=943–945 | bibcode=1980ApPhL..36..943M }}</ref>