In the initial publication devoted to plutonium spectra, only wavelengths of transitions observed in arc sources were tabulated (60.10). The five lowest levels of 5f⁶ 7s^{2 7}F were found by Bovey and Gerstenkorn (61.1). These authors combined Zeeman effect data collected at Harwell (UK) (60.8) and interferometric measurements performed at Laboratoire Aimé Cotton on the hyperfine structure of ²³⁹Pu and on the isotope shift (239-240) (62.1). In 1961, three laboratories became involved in a joint project for the analysis of the Pu spectra: Argonne National Laboratory (ANL), Lawrence Livermore National Laboratory (LLNL) and Laboratoire Aimé Cotton (LAC). The first steps in the collaboration resulted in the completion of the ground term ⁷F (62.2) and in the identification of some 5f⁶6d7s levels (63.2). In this latter article was also mentionned a second system of transitions (Pu IB) whose lowest level 5f⁵6d7s² (⁶H_5/2, ²D_3/2) j = 4 was located at 6313 cm^-1 above the ground level (64.2). Infrared lines observed by Bovey and Steers (60.9) were used for finding the connection of Pu IB with the main system. In 1964, the number of known levels was about 600, these results being mostly unpublished (unp.4).

In parallel, similar investigations were running at Harwell (65.4) and in USSR (66.4; 83.8). In the early 70's, the Fourier transform spectrometry at LAC was used to record the spectra of ²⁴⁰Pu and of a mixture of 240, 242 and 244 isotopes up to 3590 nm. The connection between the lowest odd and even levels was strengthened by the classification of many infrared lines and the high resolution and wavenumber accuracy impulsed new advances in the analysis. The first comprehensive description of the Pu I spectrum is included in the 612 pages of the report ANL-83.95 (83.7) which collects wavelengths, wavenumbers, intensities, classifications and isotope shift data for 30789 lines and a collection of 606 even- and 589 odd-parity levels. About half of the Pu I levels have been attributed to one of the 8 even and 6 odd configurations identified so far. The details of experimental procedures and the complete level list are reported in (86.1).

The Slater-Condon method has been applied to the ground configuration 5f⁶7s² and 24 levels were attributed to this configuration (80.4). One of these attributions has now been rejected: (the level at 27637 cm^-1, is better interpreted as 5f⁵6d7s7p ⁹G) but the calculated energies of (80.4) have been used to identify ten more levels. The huge size of energy matrices for excited configurations and the need for multiconfiguration basis states (because configurations overlap and mix) hamper the application of the Slater-Condon method to this very large collection of experimental data. Nevertheless truncated energy matrices were used in the early steps of the analysis for identifying the low levels of 5f⁵6d7s² (63.2) and also more recently at ANL for 5f⁶6d7s and 5f⁵6d7s² (83.1).

Isotope shift (70.2, 85.3, 87.6) and hyperfine structure of Pu (73.2) have been studied and described several times. The last three articles include references which will not be detailed here. The hfs widths of the ²³⁹Pu I levels are either reprinted from (62.1, 62.2, 66.4), or derived from hfs line measurements given in these articles.