Division of Plasmaphysics
Institute for Atomic- and Molecularphysics, University of Hannover

Spectroscopic Notation

Spectroscopists often refer to ionization stages of elements by what is called the "spectroscopic notation": The neutral atom (of, e.g., Neon) is called "Ne I", the singly ionized atom is called "Ne II", and so on.
So the ionization stages are denoted by adding a roman number to the element's symbol. The charge (Z_out * e) of an ion can easily be determined by subtracting 1 from the spectroscopic number of the ionization stage.
Obviously, the highest ionization stage of an element that yields spectral lines (and not only a "bremsstrahlung" continuum) is given by a spectroscopic number that equals the number of protons in the atomic core of that element (e.g., H I, Ne X, or Fe XXVI).

In contrast, in Bob Kurucz' data files, element and ionization stage are denoted by a different code: Number of protons in core and unbalanced electric charge of the atom or ion (Z_core.(Z_core - n_el)). This gives, e.g., 1.00 for neutral hydrogen (H I), 6.03 for triply ionized Carbon atoms (C IV), or 26.01 for singly ionized iron atoms (Fe II).

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References for the data used.
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