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The [CII] 158 μm line, aka ionized carbon fine structure line or ionized carbon forbidden line, is a spectral line produced by a small (fine structure), rare (forbidden) transition in singly ionized carbon. The transition producing the line is 2P3/2→2P1/2 (i.e., states of the electron before and after the transition). The line is within the far infrared.
As far as I can make out, the amount of energy carbon must absorb to ionize it is similar to that of hydrogen, and where hot, short-live, stars produce ultraviolet black-body radiation that ionizes hydrogen (forming HII regions), they also ionize the small amount of resident carbon. The [CII] emission line serves as a tracer for the star formation indicated the presence of such stars.
The wavelength is not within an atmospheric window, but from cosmological distances, can be redshifted into windows and viewed from ideal ground locations. The degree of redshift provides a proxy for the lookback time of the detected star formation. A hope is to detect activity in ancient galaxies, during the epoch of reionization.
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