Potential Energy Curves of Core-Excited States and Vibrational Broadening of X-Ray Adsorption Spectra of Uranyl

It is well known that vibrational excitations lead to an observable broadening of the features in the X-Ray Photoelectron Spectroscopy‚ XPS‚ in ionic compounds. This broadening is described as a Franck-Condon‚ FC‚ broadening since it arises because there is a change in the equilibrium geometry of the ionized system from that in the initial‚ ground‚ state of the system. Studies have shown how the FC broadening is sensitive to coordination of the ionized atom [1] and to the covalent character of the cation – anion interaction [2]. For Uranyl UO22+ the different potential energy curves of the relevant core-excited states of the U M4‚5-edge manifold differ significantly and cause different broadenings for the three peaks which characterize the U M4‚5-edge X-Ray Adsorption Near Edge Spectroscopy‚ XANES. Thus‚ FC broadening effects affect the features as they do for XPS. This should be of particular importance in determining the resolution possible with High-Resolution XANES‚ HR-XANES [3-4]. However‚ to our knowledge‚ the possibility of different FC broadening in XANES or HR-XANES has not been considered previously and theoretical modeling of the spectra has used the same geometry for the initial and excited configurations [4-5]. ... mehrIn the present work‚ we examine vibrational excitations for the representative case of uranyl‚ UO22+. The U M4‚5-edge HR-XANES spectra reveal three distinct peaks which are assigned to excitations into different 5f valence orbitals. The corresponding core-excited states differ significantly depending on the 5f valence orbital occupation and so does the FC broadening. Based on rigorous ab initio calculations of the wavefunctions‚ WFs‚ for the M4 and M5-edge XANES‚ we show that there are considerable changes in the geometry and we provide reliable estimates of the FC broadening due to these geometry changes. We also explain the linear behavior of the observed peak splittings with the internuclear distance‚ but we can not confirm a relation of the peak splittings with the covalence of the Uranium-Oxygen bond lengths.

[1] C. J. Nelin et al.‚ Angew. Chem. Int. Ed.‚ 2011‚ 50‚ 10174-10177.

[2] P. S. Bagus and C. J. Nelin‚ 'Computation of Vibrational Excitations in XPS Spectroscopy'‚ in Rare Earth Elements and Actinides: Progress in Computational Science Applications‚ edited by D. A. Penchoff‚ et al. (American Chemical Society‚ 2021)‚ Vol. 1388‚ p. 181.

[3] T. Vitova et al.‚ Nat. Commun.‚ 2017‚ 8‚ 16053.

[4] R. Polly et al.‚ Inorg. Chem.‚ 2021‚ 60‚ 18764-18776.

[5] P. S. Bagus et al.‚ Inorg. Chem.‚ 2021‚ 60‚ 16090.