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The Dieke diagrams like the following could be easily found on the internet:
But it is impossible to determine where they get the values initially from; everyone is making cross-references. I'm especially interested in the $\rm Tb^{3+}$ ion.
Please help me to find the original works, I think from very old times, so I will take their experimental values to compare with my experimental data.
$\begingroup$Please keep in mind that the precise values depend on the host material (often a crystal). There are specific articles for some materials as pointed out below.$\endgroup$
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$\begingroup$@Numrok as the question specifically asks for a reference, and this provides a clear reference (sure there's a link, but the plain text details are also there), this looks like valid answer to me. There's even a DOI, which is about as link-rot resistant as you can get.$\endgroup$