In considering the numerical magnitudes of the frequencies emitted in transitions between the deepest lying terms in atomic spectra, it appears (1) that for elements in the first two horizontal rows of the periodic table (a) the lowest PP′ multiplet in three-valence-electron system spectra is near in frequency to the first line in the principal singlet series of the spectrum of the once more ionized atom, (b) the lowest PP′ multiplet in two-valence-electron system spectra is near to the first line of the principal series, both of singlets in that system and of doublets in the spectrum of the once more ionized system; (2) that the lowest PP′ multiplet in the arc spectra of calcium, barium and strontium is near to the frequency difference of the lowest S and D terms, both of singlets in the arc spectrum and of doublets in the once ionized spectrum.
These relations are analyzed from the view point of atomic structure. Essentially they mean that the energy difference between the two lowest states in the above mentioned spectra is little affected by the absence of a second valence electron or by its presence in either the normal or the metastable lowest state.
By aid of these relations the following new combinations have been located: In Zn I 43P-43P′ and 43P-41D″; in Hg I, 63P-63P′; in Mg I, 33P-31D″; in Hg II, 62P-62S in Al I, 32P-x.
RALPH A. SAWYER, "ON THE DEEP LYING TERMS IN TWO- AND THREE-VALENCE ELECTRON SYSTEM SPECTRA," J. Opt. Soc. Am. 13, 431-440 (1926)