= Energy Levels of Neutral Helium by National Institute of Standards and Technology
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By <National Institute of Standards and Technology>. Handbook of Basic Atomic Spectroscopic Data, neutral <helium> He I table.
https://physics.nist.gov/PhysRefData/Handbook/Tables/heliumtable5.htm
The table provides ground, singlet and triplet excitation energies as spectroscopic wavenumbers, and the He II ground-state ionization limit. Multiply by $hc$ with consistent inverse-length units to obtain excitation <energy>.
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