Observation of four-body effects in atomic spectra

Abstract
A complete orthogonal set of effective two-electron, three-electron and four-electron operators, each scalar with respect to both S and L, has been constructed for p3d configurations. It is shown that the introduction of four-electron operators is essential in order to obtain a good fit to the 2p33d configurations in the O I isoelectronic sequence for low stages of ionisation. This is because second-order perturbation theory that underlies the usual effective-operator approach is inadequate in this case. The identification of the energy levels of the 2p33d configuration in Ne III is reported, as well as a fit using the new parameters.

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