The initial description of the plutonium spectra emitted in an arc included only wavelengths of unidentified transitions. Seven levels of the lowest terms 5f⁶7s ⁸F and ⁶F were determined by MacNally and Griffin (59.1) from Zeeman effect observations. The same experimental data were used by Bauche et al. (63.1) to complete these lowest multiplets and to increase the odd-level list. Another step was performed in the same year and led to the three lowest levels of 5f⁶6d and to 74 odd levels. The analysis of Pu II progressed simultaneously with that of Pu I in a joint project between three laboratories: Laboratoire Aimé Cotton, Argonne National Laboratory and Lawrence Livermore National Laboratory. In 1966, the connection between two systems of levels, the first one with 5f⁶7s and 5f⁶6d as low configurations and the second one with 5f⁵7s² and 5f⁵6d7s and high even levels, was just mentionned (66.2), the lowest odd level being established at 8199 cm^-1 above the ground state.

At the present time, the 252 even levels belong to 5 configurations and the 746 odd levels to four others. The wavelengths, wavenumbers, intensities, classifications, g-factors, isotope shifts of both Pu II and Pu I lines emitted by electrodeless discharge tubes are collected in the 612 pages of the report ANL-83.95 (83.7), with the complete level liste. The hfs widths of the ²³⁹Pu II levels have been deduced from measurements published in (60.2; 66.4; 73.2). The results of (83.7) have been revised for inclusion in the present work. The accuracy of the higher levels (above 35000 cm^-1) is not better than 0.02 cm^-1 in both parities (unp.4). The configuration 5f⁶7s has been studied by means of the Slater-Condon method, without truncation of the energy matrices and 26 levels have been attributed to this configuration in the range 0 - 41605 cm^-1 (80.4). Other parametric studies of heavily-truncated configurations 5f⁶6d and 5f⁵6d7s have also been performed at Argonne National Laboratory to assist in level identifications and fitted energy parameters are collected in (83.1).