The energy level structure, relative line strengths, and Landé <i>g</i> factors of two-electron configurations are discussed for four important types of pure coupling: <i>LS</i>, <i>LK</i>(<i>Ls</i>), <i>jK</i> (<i>jl</i>), and <i>jj</i>. Transitions from one type of coupling to another are discussed in detail, the configuration <i>pf</i> being used as an example. The appropriateness of <i>LS</i>- and <i>jj</i>-coupling notation in two-electron spectra is quite limited for atoms of medium atomic weight, where nearly all excited configurations show a strong tendency toward pair (<i>LK</i> to <i>jK</i>) coupling. For other atoms, pair coupling occurs mainly for high values of orbital angular momentum of the excited electron: the coupling may then be near <i>LK</i> for small values of the principal quantum number of this electron, approaching pure <i>jK</i> as this quantum number increases. Either <i>LK</i> or <i>jK</i> notation can serve unambiguously to identify levels throughout the range of intermediate pair couplings, but neither will correctly designate the nature of the quantum states in all cases because of exchanges of the <i>L</i> (and of the <i>j</i>) compositions of certain states which occur as the coupling conditions change from pure <i>LK</i> to pure <i>jK</i>. Examples are discussed from the spectra of N, P, Ge, and the rare gases.
ROBERT D. COWAN and KENNETH L. ANDREW, "Coupling Considerations in Two-Electron Spectra," J. Opt. Soc. Am. 55, 502-516 (1965)