A method of analysis of all level positions in multiple strongly perturbed series of levels is presented and illustrated by examples from the rare gas and Ba spectra. The method is suggested by Seaton's multichannel quantum defect theory but is presented here as an empirical approach. It emphasizes the dependence of a perturbation on the periodicity of the perturbing series and aims at extracting significant information from experimental data in compact form; this goal will be pursued in later works.

• Received 19 January 1970

DOI:https://doi.org/10.1103/PhysRevA.2.81