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.