NIST Energy Levels and Wavelengths Bibliographic Reference # 582
Calculated Ionization Potentials for Multiply Charged Ions,
T. A. Carlson, C. W. Nestor, Jr., N. Wasserman, and J. D. McDowell,
At. Data Nucl. Data Tables 2, 63–99 (1970)
DOI:

Comment: The ionization energy given in ASD has been obtained by extrapolating the difference between the values calculated by Sapirstein&Cheng 2011 (full reference: L16264) and by Carlson et al. 1970 along the isoelectronic sequence.
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