Natural charge state distributions of highly charged ions or why are noble ions noble?

Noble atoms are chemically inert because their full-shell electronic configuration strongly binds all electrons to the parent atom; therefore, noble atoms have ionization energies that are substantially higher than the ionization energies of non-noble atoms. This is different for ions, for which the ionization energy increases with every charge state, although the increases are much larger whenever a new shell is opened. As long as the energy of the ionizing electron is far above the ionization energy threshold, comparisons need to focus on ionization cross sections, rather than ionization energies, because only the ionization cross section takes into consideration the number of electrons that can be ionized. As long as the K-shell is filled, cross sections for direct ionization fall off approximately geometrically, meaning that every time an electron is removed, it becomes roughly 30% more difficult to directly remove the next electron, a charge state dependence that is only minimally affected by the structure of the remaining electrons. For this reason, evolution of charge states calculated with cross sections for direct ionization predict only minimal enhancements for noble ions—ions with a full-shell configuration. [ABSTRACT FROM AUTHOR]