Spectroscopists customarily refer to the spectrum arising from a given ionization state of a given element by the element's symbol followed by a Roman numeral. The numeral I is used for spectral lines associated with the neutral element, II for those from the first ionization state, III for those from the second ionization state, and so on. For example, "He I" denotes lines of neutral helium, and "C IV" denotes lines arising from the third ionization state, C, of carbon. This notation is used for example to retrieve data from the [http://physics.nist.gov/PhysRefData/ASD/levels_form.html NIST Atomic Spectrum Database].