In Spectroscopy, and especially in conjunction with astrophysics, there are "lines" of the spectrum of visible light that are called "forbidden lines". (A misnomer, to say the least. A much more accurate name would be "very improbable".)

In some of the ions, e.g. in planetary nebulae, there exist excited states which - if left undisturbed by collisions with other particles - may last for several hours, before the ion drops to a lower state, and emits a photon. However, in laboratories on earth, we are unable to produce a strong enough vacuum to observe these transitions, because the particles tend to collide with eachother or the walls of the chamber before the transition takes place. But in areas with extremely low density, such as inside a gaseous nebulae, they do occur, and in many cases will the emitted light contribute significantly to the light we can observe.

Doubly ionized oxygen (oxygen atoms missing two electrons, often called Oxygen-III, or just O-III) emits green light at wavelengths around 5007 Ångstrøm, a part of the spectrum not associated with any known element. Sir William Huggins tried in 1868 to explain the phenomenon by introducing a new element, called Nebulium.

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