Multiple papers describe the ytterbium Yb IV ion as having only two energy levels. Why aren't there more levels in its spectrum?
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There are more levels.
For many experiments, one can choose combinations of laser frequencies such that only a few energy levels are relevant; when reporting those experiments, one can just report a simplified energy-level diagram that only includes the states that are relevant to the experiment.
However, that doesn't mean that the other levels don't exist, and if you do other experiments with other excitation conditions then they can and will be relevant.
The NIST Atomic Spectra Database for energy levels has a good list of the full roster of states for atoms and ions. For Yb III, for example, it starts off as
answered May 13, 2017 at 14:12
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$\begingroup$ Thank you fr the response. I am working on upconversion system (NaYF4:Yb, Er). The reference energy level diagram of ytterbium has been given everywhere that Yb3+ is having only one excited state i.e. 2f7/2. I am totally confused with the electronic configuration of Yb with the energy level structure. Could you please clarify my doubt? I hope you understand my question. $\endgroup$– GayathriMay 13, 2017 at 14:47
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$\begingroup$ Energy level diagrams are often abridged to show only the levels that are relevant to the experiment under discussion, but that doesn't mean that the other levels don't exist; this is almost certainly what your source is doing (though it's hard to be fully conclusive without the full reference). Note also that the triply ionized ion of ytterbium is Yb IV, not Yb III (Yb I, the "first spectrum" of the element, is the neutral). $\endgroup$ May 13, 2017 at 18:17