Absorption and emission spectra of U4+ diluted in the incommensurate structure of ThCl4

The absorption and fluorescence spectra of U4+ diluted in single crystals of ThCl4 have been measured at temperatures ranging from 4.2 K to room temperature. β-ThCl4 exhibits on incommensurate structure below 70 K with a loss of periodicity along the c axis. This results in a variation of the distance between the metal and the halogen from one cell to another. The site symmetry of the actinide ions is then reduced. The lines corresponding to the sites of the resulting symmetries S4 AND D2 are identified spectroscopically. The S4 symmetry is approximated by the D2d one and a parametric analysis of the energy levels of U4+ in the D2d and D2 symmetries is reported. For 25 levels in the D2d site the root mean square deviation σ is 46 cm-1 and for 34 levels in D2, σ=56 cm-1. The parameters which occur in both symmetries are only slightly changed. 19 refs, 8 tables