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In 1920, James Franck together with Fritz Reiche and Paul Knipping found strong experimental evidence that the lowest lying triplet state in helium is metastable—an atom in this state cannot make a spontaneous transition to the ground state. Even though their evidence was entirely experimental, they tied their results almost inextricably to Alfred Landé’s 1919 model of the helium atom, and in the process, misunderstood the new theoretical selection rules of Adalbert Rubinowicz and Niels Bohr. In an additional complication, experiments of the English physicists Frank Horton and Ann Catherine Davies contradicted Franck’s. Although Franck’s result has held up, the reasons for the discrepancies remain unclear.
© 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license.
This article was originally published as: Gearhart, C.A. (2017). James Franck, the ionization potential of helium, and the experimental discovery of metastable states. Studies in History and Philosophy of Modern Physics, 60, 95-109. DOI: https://doi.org/10.1016/j.shpsb.2017.01.002