The early observations of the spectrum of neutral neptunium at low resolution were not suitable for a determination of energy levels (49.3; 59.8; 59.9) In 1959 and 1962 exposures in the phototographic region 200 - 1200 nm were made using the Argonne National Laboratory's 9.15 m Paschen-Runge spectrograph with an electrodeless discharge tube as a light source, from which the analysis was started (63.3). The present photographic line list is from these latter investigations which included hyperfine structure and Zeeman effect measurements. In 1971, the observations were extended to 2500 nm by means of the Fourier transform spectroscopy at Laboratoire Aimé Cotton. The analysis of these data resulted in a list of 6096 lines, of which 2526 have been classified as transitions between 329 odd levels and 130 even levels of Np I. A part of the lowest terms of 5f⁴6d7s², the ground configuration, and of 5f⁵7s², 5f⁴6d²7s, 5f³6d²7s², 5f⁴7s²7p and 5f⁴6d7s7p was identified. Zeeman data and detailed hyperfine structure components of the lines were listed (76.14). A compacted version of this extensive description has been published in 1977 (77.2). It included a Slater-Condon type interpretation of the 5f⁴6d7s² and 5f⁴7s²7p configurations, in which respectively 55 and 31 levels were identified.

The component percentages for the 5f⁴6d7s² levels are from Fred and Crosswhite (unp.19) and those for the 5f⁵7s² levels are from Wyart (unp.32). Term labels within parentheses indicate a leading component percentage between 25 and 50 % and a blank entry, a percentage less than 25 %. Above 15000 cm^-1 a number of identifications are empirical.

After 1977, Fourier Transform spectra were extended through 345 nm at Laboratoire Aimé Cotton (77.7), resulting in some dozens of revised J-values, 140 new odd and 72 new even levels. Thirteen levels of 5f⁵7s² were identified theoretically (80.4). These revisions ended in 1983 (unp.2) and are reported below.

Laser spectroscopy techniques by stepwise excitation and either photoionization of the atoms, or field ionization of bound Rydberg levels led to an accurate determination of the first ionization limit (50536 ± 4 cm^-1), to Rydberg series converging to the lowest levels of Np II, to radiative lifetimes for 5 levels and to 64 new levels in the range 33000 - 37000 cm^-1, for which the uncertainty is ± 1 cm^-1 (or ± 0.5 cm^-1 for 15 even levels marked with a star) (79.2).