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There is considerable confusion about the difference between the two ways of referring to a spectrum or ion, for example C III or C⁺².  These have very definite different physical meanings and cannot be used interchangeably.

C⁺² is a baryon and C III is a set of photons. 

C⁺² refers to carbon with two electrons removed, so that it is doubly ionized, with a net charge of +2.    This notation is from early 20th century atomic physics, after the discovery of the electron.

C III is the spectrum produced by carbon with two electrons removed.  This notation comes from the 19th century, before the discovery of the electron.  Atomic physicists saw that matter produced a certain "principal" spectrum when in its normal state.  If they heated material up (a famous case being Fe in a furnace) the spectrum would mysteriously change into a completely different "enhanced" spectrum.  These were given the notation Fe I and Fe II.  It was only much later, after the discovery of the electron, that they realized that the enhanced spectrum was caused by the removal of an electron.

The C III spectrum will be produced by collisional excitation of C⁺² (C⁺² + e^- -> C^+2* + e^- -> C⁺² +hn) or by recombination of C⁺³ (C⁺³ + e^- -> C⁺² + hn).  So, depending on how the spectrum is formed, C III may be emitted by C⁺² or C⁺³.  The most extreme case is Lya, the strongest emission line in many sources.  It is produced by recombination from H⁺ and by collisional excitation of H⁰. 

There is no ambiguity in absorption line studies - only C⁺² can produce a C III absorption line.  This has caused many people to think that C III refers to the matter rather than the spectrum.  But this notation is ambiguous in the case of emission lines.  It is also wrong to use them interchangeably.  Many older atomic physics texts will explain the difference.

Last changed 07/23/06.
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Copyright 1978-2006 Gary J. Ferland