Doubly ionized oxygen

In astronomy and atomic physics, doubly ionized oxygen is the ion O2+ (O III in spectroscopic notation).

Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new chemical element.

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.

In 1927, Ira Sprague Bowen published the current explanation identifying their source as doubly ionized oxygen.

[2] Permitted lines of O III lie in the middle ultraviolet band and are hence inaccessible to terrestrial astronomy.