The spectrum of neptunium 237Np emitted by an electrodeless discharge tube was photographed on the Paschen-Runge spectrograph at Argonne National Laboratory in the range 200 - 1200 nm and by Fourier transform spectrometry at Laboratoire Aimé Cotton in 1971.
All spectral data, including Zeeman effect, hyperfine structures and preliminary Np I analysis, were summarized in (76.14).
Extended observations by Fourier transform spectrometry in the ranges 2500 - 4000 nm and 850 - 345 nm helped in the determination of 35 Np II levels which was only briefly mentioned in (77.7). Although the number of levels has now been doubled, the Np II analysis is still preliminary. It is noteworthy that the two excited levels of opposite parities 5f47s2 5I4 and 5f57s 7H2 are higher than the ground level 5f46d7s 7L5 , by less than 100 cm-1. This character makes Np II unique among all atomic spectra and the occurence of many low levels in both parities complicates the spectral analysis. For 14 levels, g-values have been derived from measurements by Smith (unp.29) or Blaise (unp.2).
The parametric interpretation of the ground configuration 5f46d7s has been undertaken by using two different vectorial basis sets, high terms of f4ds being discarded, as well as overlapping configurations 5f47s2 and 5f46d2 (unp.16; unp.32).
Both interpretations agree on the label of the very low levels which are reported in the Table. Due to the limited experimental data used in the parametric fitting process, and to the crude theoretical assumptions, the eigenfunctions may be not significant and have been omitted.
The level pair 5f5(6H5/2) 7s J = 2 and 3 had been reported and interpreted in (80.4).