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It may be that the first ionization potential is lowered by conditions within the source16 such that the 2s2p2(4P)4p3D° level now falls well above the actual limit. This phenomenon of forced ionization has been observed in the absorption spectrum of shock-excited barium by Garton, Parkinson, and Reeves.17
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The lines converging to the 2s22p2P½° limit, Fig. 5, are rather broad. Part of these widths result from almost superposition of several transitions (26 lines have been observed in the peak at 248 Å). Conditions in the source might also contribute to the widths of the lines. When levels below the lowest idealized ionization potential belong to a series in which higher members are able to autoionize, higher members of series levels converging to this first limit are expected to become drowned in their own limiting continuum.16,17 Also the merging of states16 possibly contribute to the width of these lines. Additional work is necessary to determine the contributions from each of these possibilities.