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

Results of optical absorption measurements in polarized light on tetravalen t neptunium diluted in a ThCl4 single crystal are reported. The recorded sp ectra are complex, pointing to the presence of an Np3+ impurity. The electr onic transitions assigned to the Np4+ ion are interpreted in terms of the u sual 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 pa rameters. The crystal field effect resulting from the fitting is considerab ly larger than that observed for the uranium ion in the same host.