The analysis of the first two spectra of protactinium ²³¹Pa was undertaken by Giacchetti at Argonne National Laboratory (63.4). The major part of the 9000 measured lines could be assigned either to Pa I or to Pa II by varying the excitation conditions of the electrodeless discharge tube. Later, Giacchetti published a list of 32 even and 252 odd levels, some of them being characterized by the g-factor or by the total width of the hyperfine structure relative to that of the ground level 5f²6d7s^{2 4}K_11/2 (66.1). Parallel ta this analysis, Richards et al. (68.2) published a level list which supplemented Giacchetti's data with 516 levels, but only five of them have been confirmed later.

In 1969, the Fourier transform spectrum of protactinium emitted by an electrodeless discharge tube was recorded at the Laboratoire Aimé Cotton between 3000 and 11978 cm^-1. These new data allowed to determine several new levels (70.9). After the lowest odd level 5f 6d²7s^{2 4}H_7/2 was found at 1978.220 cm^-1, the analysis was interrupted during a decade. The Pa I analysis was resumed for the present publication and aided by the recording of the Pa I and Pa II spectra in the range 15000 - 19500 cm^-1 by Fourier Transform Spectrometry (unp.10). The hyperfine widths W of the levels have been partly revised from these new data. Below 20000 cm^-1, 43 out of the 95 known levels have been found very recently with the help of hfs measurements (unp.5; unp.10). The main part of these levels are interpreted by the parametric study of the mixed configurations 5f²6d7s²+5f²6d²7s (85.4).

The known odd levels below 15000 cm^-1 belong to 5 configurations 5f6d²7s², 5f6d³7s, 5f²6d7s7p, 5f²7s²7p and 5f³7s², which hampers empirical designation and simple parametric interpretations.