There is considerable confusion about the difference between these 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.   

C III is the spectrum produced by carbon with two electrons removed.  The C III spectrum will be produced by impact 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. 

There is no ambiguity in absorption line studies - only C⁺² can produce a C III absorption line.  This had 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.

Last changed 08/08/03.
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Copyright 1978-2003 Gary J. Ferland