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What do roman numerals mean in spectroscopic data

  1. Mar 8, 2012 #1
    This is a stupid question, but I'm trying to understand some spectroscopic data from the NIST website.

    Can anyone tell me what the roman numerals mean? E.g. when they gives lines for Neon as: Ne I or Ne IV, what do the I and IV mean?

    Thanks for any help!!
  2. jcsd
  3. Mar 8, 2012 #2
    It usually refers to the degree of ionization, and I think that Ne I refers to the neutral atom, and Ne II for example, would be the first ionized stage.
  4. Mar 8, 2012 #3


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    Gold Member

    Quite right. When astronomers study clouds of gas (generally composed of Hydrogen), they can map the distribution of H2 (molecular Hydrogen), HI (unionized Hydrogen), and HII (ionized Hydrogen). If an atomic substance has more than one possible ionization state, those states can be noted by incrementing the Roman numerals.
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