The analysis of the first two spectra of protactinium ²³¹Pa was undertaken by Giacchetti at Argonne National Laboratory (63.4). Electrodeless discharge tubes containing protactinium iodide were excited by means of a microwave generator and the spectra were photographed in the region 250 - 920 nm by using the 9.15 m Paschen Runge spectrograph. The Pa II lines were distinguished from Pa I lines by means of differential excitation on the electrodeless discharge tubes and 19 % of the 8801 measured lines were attributed to Pa II. The lines with large hyperfine structures led to find the first levels of Pa II and, in 1967, 25 low even levels were reported (67.1). Only 7 levels were attributed to a configuration in this first publication and the 164 high odd energy levels and 656 classified lines remained unpublished.

In 1969, the Fourier transform spectra of ²³¹Pa emitted by the same light source were recorded at Laboratoire Aimé Cotton between 3000 and 11978 cm^-1. These new data allowed to determine new levels and to improve the identifications. A second system of transitions, Pa IIB unconnected with the first system, was mentionned, of which four low levels (presumably belonging to 5f6d²7s) were listed (70.9).

This analysis progressed after 1970, but was never published so far. The number of even levels was increased from 25 to 66, and the number of odd levels from 65 to 298, both systems being unified (85.5). Only the ground configuration 5f²7s² has been studied by means of the Slater-Condon method (80.4), but the results of the survey of 5f^N7s2 configurations in singly-ionized actinides were not detailed in this publication. Except for ¹G₄, the LS label represents more than 50 % of the eigenfunction for all known levels of the ground configuration. The other designations are empirical and make use of hyperfine total widths, scarce Zeeman data (unp.5), line intensities and regularities of electron jumps within the actinide period.

Two levels at 22549 and 22918 cm^-1 are equally good candidates for being the lowest level of 5f²7s7p and the choice given in (71.2) has been deleted from the present compilation.