Strong crystal field effect in Np4+:ThCl4 - optical absorption study

Results of optical absorption measurements in polarized light on tetravalent neptunium diluted in a ThCl4 single crystal are reported. The recorded spectra are complex, pointing to the presence of an Np3+ impurity. The electronic transitions assigned to the Np4+ ion are interpreted in terms of the usual model, following the actual understanding of the neptunium electronic structure and independent theoretical predictions. R.m.s. deviations of the order of 36 cm-1 have been obtained for 42 levels fitted with 11 free parameters. The crystal field effect resulting from the fitting is considerably larger than that observed for the uranium ion in the same host. (author)