Doubly ionized oxygen
Oxygen ion in astronomy and atomic physics
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In astronomy and atomic physics, doubly ionized oxygen is the ion O2+ (O III in spectroscopic notation).
Ion
Its emission of forbidden lines in the visible spectrum fall primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm. Concentrated levels of O III are found in emission and planetary nebulae. Consequently, narrow band-pass filters that isolate the 500.7 nm and 495.9 nm wavelengths of light, that correspond to green-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-polluted terrestrial atmosphere) where the frequencies of [O III] are much less pronounced.
These emission lines were first discovered in the spectra of planetary nebulae in the 1860s. At that time, they were thought to be due to a new chemical element which was named nebulium. In 1927, Ira Sprague Bowen published the current explanation identifying their source as doubly ionized oxygen.[1]
Other transitions include the forbidden 88.4 μm and 51.8 μm transitions in the far infrared region.[2]
Permitted lines of O III lie in the middle ultraviolet band and are hence inaccessible to terrestrial astronomy.