The emission spectrum of multiply-ionized uranium has been observed several times. Morozova and Startsev (57.4) used spark spectra of compressed pellets of U₃O₈ with copper and attributed tentatively some lines to U IV. More extensive data were obtained by exciting uranium in a coaxial plasma accelerator and about 500 lines were measured in the range 250 - 580 nm (78.8). The spatial dependence of U⁺ⁿ ions in an U-Ar plasma was used for discriminating lines of U I through U IV (80.1). Wavelength data collected by Crosswhite and Crosswhite since the early 1960's are mentionned in (82.3). A narrow spectral region (379.2 - 388 nm) was investigated by means of a dc arc and a few lines of U IV are observed (82.1). As a result of (80.1), a list of 514 U IV lines in the range 254.1 - 471.7 nm has been communicated to the authors by J.G. Conway (unp.12).

In spite of all these measurements, the energy levels of U IV are still unknown. Therefore the systematic observation of triggered-spark spectra was undertaken in 1986 by using the 10 m normal incidence spectrograph at Observatoire de Meudon. Up to now, 6000 lines emitted by U⁺ through U⁺⁴ ions have been measured, their profile being obviously bound with ionic assignment (87.7).

The lowest energy levels of the first five configurations have been predicted by Brewer (71.2). The analysis of U³⁺ ions in compounds and in solutions led to determine energy parameters which are affected by ligand-field interaction. Smoothened values for these parameters in trivalent actinides were used to predict the energy levels of the ground configuration 5f³ (84.8). Although the level structure of the gaseous free ion may be different, the theoretical energies reported below are data of interest for unraveling the transition array 5f³-5f²6d, to which should belong strong lines already observed.