The fine-structure levels of helium from microwave data are coalesced into a single-term-value table by means of theories. It is found to yield transition frequencies in excellent agreement with some diode laser measurements and a consistent ionization energy of 198{}310.772{}2(5) cmsup-1. This value is 1 order of magnitude higher in precision and in accord with Martin's value [Phys. Rev. A 29, 1883 (1984)], subject to the same assumption on the 2 ¹P level. A semiempirical theory is developed and fitted to all levels (L{≥}2). The result suggests a modification in the usual usage of the polarization formula for the direct energy and extends its utility to the exchange energy.
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Edward S. Chang (1987) studied this question.