Site-selective excitation ofU4+diluted in incommensurate ThBr4

Selective-excitation experiments in the visible region are reported for the ${\mathrm{U}}^{4+}$: Th${\mathrm{Br}}_4$ system. These permit the classification of absorption and emission lines to the sites found in the incommensurately modulated structure of Th${\mathrm{Br}}_4$. The lines are assigned to a $D_{2d}\mathrm{}\left(B\right)\mathrm{}$ site, a continuous distribution of $D_2\mathrm{}\left(C\right)\mathrm{}$ sites, and to the limiting $D_2\mathrm{}\left(A\right)\mathrm{}$ site. By application of the model of Delamoye and Currat [J. Phys. (Paris) Lett. 43, L655 (1982)], the energy levels have been mapped as a function of the phase angle ${\phi }_l$ of the distortion. The agreement between the predictions of the model and experiment is very good. It is shown that the incommensurate structure of Th${\mathrm{Br}}_4$ must be of higher order than $\frac{q_s}{c^{*}}=\frac{4}{13}$ (where $q_s$ is a modulation wave vector and $c^{*}$ is one of the basis vectors of the reciprocal lattice of the tetragonal unit cell).