U4+ spectroscopic properties in Dsub(2d) with ThCl4, UCl4 and ThSiO4

This thesis is concerned with the study of the electronic structure of the tetravalent actinide ions in solid state. The technique used was high resolution optical spectroscopy. We deal with the U4+ ion (sf2) in the monocrystals ThCl4, UCl4, and ThSiO4 where the U4+ ion is substituded for the Th4+ ion by doping. Visible and infrared optical spectra were recorded between 300 and 4.2K. With these three compounds, it is possible to compare the influence of different environments of Dsub(2d) symmetry: real symmetry for U4+ in UCl4 and ThSiO4; approximate symmetry in ThCl4, whose structure is incommensurate and modulated at low temperature. The fitting of the data was carried out by diagonalizing the hamiltonian which describes the interactions of the U4+ ion in a crystal field with its environment. This fitting procedure led to a coherent set of spectroscopic parameters. The fluorescence of U4+ was observed in ThCl4 and ThSiO4 and the effect of the incommensurate structure of ThCl4 on the optical spectra was studied. The symmetry of the U4+ sites was identified by site selective excitation experiments and a relationship between the incommensurate structure and the lifetime of U4+ energy levels was found. In conclusion, the U4+ energy levels in a relatively low crystal field were determined for compounds that have a similar coordination polyhedron about the actinide ion. The values for these energy levels were then compared to those of lanthanide and 3d elements