Relativistic Multiconfigurational Ab Initio Calculation of Uranyl 3d4f Resonant Inelastic X-ray Scattering

We applied relativistic multiconfigurational all-electron ab initio calculations including the spin-orbit interaction to calculate the 3d4f resonant 5f inelastic X-ray scattering (RIXS) map (3d(3/2 )-> 5f(5/2) U M-4 absorption edge and 4f(5/2) -> 3d(3/2) U M-beta emission) of uranyl (UO22+). The calculated data are in excellent agreement with experimental results and allow a detailed understanding of the observed features and an unambiguous assignment of all involved intermediate and final states. The energies corresponding to the maxima of the resonant emission and the non-resonant (normal) emission were determined with high accuracy, and the corresponding X-ray absorption near edge structure spectra extracted at these two positions were simulated and agree well with the measured data. With the high quality of our theoretical data, we show that the cause of the splitting of the three main peaks in emission is due to the fine structure splitting of the 4f orbitals induced through the trans di-oxo bonds in uranyl and that we are able to obtain direct information about the energy differences between the 5f and 4f orbitals: Delta 5f delta/phi - 4f delta/phi, Delta 5f pi* - 4f pi, and Delta 5f sigma*- 4f sigma from the 3d4f RIXS map. RIXS maps contain a wealth of information, and ab initio calculations facilitate an understanding of their complex structure in a clear and transparent way. With these calculations, we show that the multiconfigurational protocol, which is nowadays applied as a standard tool to study the X-ray spectra of transition metal complexes, can be extended to the calculation of RIXS maps of systems containing actinides.

Automated summary

Relativistic multiconfigurational all-electron ab initio calculations, including spin-orbit interaction, accurately reproduce experimental results for uranyl's 3d4f resonant 5f inelastic X-ray scattering (RIXS) map. The calculated data provide insights into the fine structure splitting of 4f orbitals induced by trans di-oxo bonds in uranyl, allowing for a clear understanding of energy differences between 5f and 4f orbitals. Ab initio calculations show the versatility of multiconfigurational protocols in studying RIXS maps of systems containing actinides.