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Adding local short description: "Oxygen ion in astronomy and atomic physics", overriding Wikidata description "oxygen dication" |
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{{Short description|Oxygen ion in astronomy and atomic physics}}
[[File:Oiii-linesp.svg|thumb|right|A [[Grotrian diagram]] of doubly ionized oxygen: forbidden transitions in the visible spectrum are shown in green.]]
In [[astronomy]] and [[atomic physics]], '''doubly ionized oxygen''' is the [[ion]] [[oxygen|O]]<sup>2+</sup> ('''O III''' in [[spectroscopic notation]]). Its emission [[forbidden line]]s in the [[visible spectrum]] fall primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm. Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new [[chemical element]]. Concentrated levels of O III are found in [[diffuse nebula|diffuse]] and [[planetary nebula|planetary nebulae]]. Consequently, narrow [[Dichroic filter|band-pass filter]]s that isolate the 500.7 nm and 495.9 nm wavelengths of light, that correspond to [[green]]-[[turquoise (color)|turquoise]]-[[cyan]] [[spectral colors]], are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly [[light pollution|light-polluted]] terrestrial atmosphere) where the frequencies of [O III] are much less pronounced.
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