Editing Ultraviolet (section)

===Tunable vacuum ultraviolet (VUV)===
The vacuum ultraviolet (V‑UV) band (100–200&nbsp;nm) can be generated by [[nonlinear optics|non-linear 4&nbsp;wave mixing]] in gases by sum or difference frequency mixing of 2 or more longer wavelength lasers. The generation is generally done in gasses (e.g. krypton, hydrogen which are two-photon resonant near 193&nbsp;nm)<ref name=straussfunk/> or metal vapors (e.g. magnesium). By making one of the lasers tunable, the V‑UV can be tuned. If one of the lasers is resonant with a transition in the gas or vapor then the V‑UV production is intensified. However, resonances also generate wavelength dispersion, and thus the phase matching can limit the tunable range of the 4&nbsp;wave mixing. Difference frequency mixing (i.e., {{nowrap|{{mvar|f}}{{sub|1}} + {{mvar|f}}{{sub|2}} − {{mvar|f}}{{sub|3}}}}) has an advantage over sum frequency mixing because the phase matching can provide greater tuning.<ref name=straussfunk/>

In particular, difference frequency mixing two photons of an {{chem|[[Argon|Ar]]||[[Fluorine|F]]}} (193&nbsp;nm) excimer laser with a tunable visible or near&nbsp;IR laser in hydrogen or krypton provides resonantly enhanced tunable V‑UV covering from 100&nbsp;nm to 200&nbsp;nm.<ref name=straussfunk>{{cite journal
 |last1        = Strauss
 |first1       = C.E.M.
 |last2        = Funk
 |first2       = D.J.
 |year         = 1991
 |title        = Broadly tunable difference-frequency generation of VUV using two-photon resonances in H{{sub|2}} and Kr
 |journal      = Optics Letters
 |volume       = 16
 |issue        = 15
 |pages        = 1192–4
 |doi          = 10.1364/ol.16.001192
 |pmid         = 19776917
 |bibcode      = 1991OptL...16.1192S
 |url          = https://www.osapublishing.org/ol/fulltext.cfm?uri=ol-16-15-1192&id=10705
 |access-date  = 2021-04-11
 |archive-date = 29 May 2024
 |archive-url  = https://web.archive.org/web/20240529134804/https://opg.optica.org/captcha/(S(c0bdkdggeh50wqakxbxp1vlf))/?guid=AEC4DC11-0A8D-48DC-8B6E-84817B588FB2
 |url-status   = live
|url-access= subscription
 }}</ref> Practically, the lack of suitable gas / vapor cell window materials above the [[lithium fluoride]] cut-off wavelength limit the tuning range to longer than about 110&nbsp;nm. Tunable V‑UV wavelengths down to 75&nbsp;nm was achieved using window-free configurations.<ref name="O2Ar">
{{Cite journal
 |last1 = Xiong  |first1 = Bo
 |last2 = Chang  |first2 = Yih-Chung
 |last3 = Ng     |first3 = Cheuk-Yiu
 |year = 2017
 |title = Quantum-state-selected integral cross sections for the charge transfer collision of {{math|{{small|O{{su|b=2|p=+}} (a{{sup|4}} Π {{sub|u 5/2,3/2,1/2,−1/2}}:}}}} {{math|{{small|v{{sup|+}}{{=}}1–2; J{{sup|+}})}}}} {{math|{{small|[ O{{su|b=2|p=+}} (X{{sup|2}} Π {{sub|g 3/2,1/2}}:}}}} {{math|{{small|v{{sup|+}}{{=}}22–23; J{{sup|+}}) ] + Ar}}}} at center-of-mass collision energies of 0.05–10.00&nbsp;eV
 |journal = Phys. Chem. Chem. Phys.
 |volume=19 |issue = 43 |pages=29057–29067
 |bibcode= 2017PCCP...1929057X  |pmid = 28920600 |doi = 10.1039/C7CP04886F
 |url = http://pubs.rsc.org/-/content/articlehtml/2017/cp/c7cp04886f
 |url-status = live
 |archive-url = https://web.archive.org/web/20171115202941/http://pubs.rsc.org/-/content/articlehtml/2017/cp/c7cp04886f
 |archive-date = 15 November 2017
|url-access = subscription
 }}
</ref>