ALDAIR MISAEL Wilken : Simulating resonant inelastic X-ray scattering across the whole periodic table

Thesis summary: 

Spectroscopic techniques that probe core electron excitations (the most internal) with X-rays offer important advantages over those exciting valence electrons (UV-visible, near IR) in terms of selectivity and sensitivity. Moreover, because core excitations are very energetic, small perturbations such as hydrogen bonds or coulombic interactions from molecules or ions in the near environment will result in large energy shifts in the spectra [1]. These changes allow a better understanding of the effects of the environment, and more easily characterize conceptually important quantities such as the oxidation state [2, 3], hybridization [4] or the degree of covalency of the bonds [5].

In resonant inelastic X-ray scattering (RIXS) the excitation of core orbitals to unoccupied low energy orbitals is followed by the emission of a photon, due to the transition of an electron from an occupied orbital that takes the place of the excited core electron. Therefore, RIXS allows one to find correlations between different cores in a molecule, making this technique very powerful in obtaining information about chemical bonding in the ground state than other widely used core spectroscopies, such as X-ray absorption (XAS), near-absorption front spectroscopy (XANES), or X-ray emission (XES) [6, 7].

However, it is not possible to interpret the experimental results obtained by these core spectroscopies without reliable theoretical models. The latter require the treatment of relativistic effects (due to the importance of spin-orbit coupling in the core region, even for light elements), of electronic correlation and of long range effects (from the environment).

The ambition of this project is to develop new theoretical tools that will allow us to simulate RIXS spectra for molecules containing atoms everywhere in the periodic table, with a particular attention to actinides. These tools will be based on the equation of relativistic motion method (EOMCC) [8], and will take into account environmental effects (solvent, crystal, etc.) through quantum embedding methods such as frozen density embedding (FDE) [9], which allows us to reduce the computational cost while keeping a quantum description for the whole system (CC-in-DFT embedding) [10]. These developments will be done in the DIRAC [11] and PyADF [12] codes.

This project is the framework of the ANR-DFG CompRIXS project, in collaboration with Christoph JACOB (TU Braunschweig), centered on the development of new ab initio electronic structure approaches coupling various scales of simulation. 

[1] X Kong et al., The Journal of Physical Chemistry Letters 2017, 8, 4757
[2] B Kosog et al., Inorganic Chemistry 2012, 51, 7490
[3] ES Ilton, PS Bagus, Surface and Interface Analysis 2011, 43, 1549
[4] RD dos Reis et al., Nature Communications, 2017, 8, 1203
[5] Baker et al., Coordination Chemistry Reviews 2017, 345, 182
[6] K Kunnus et al., Chemical Physics Letters 2017, 669, 196
[7] T Vitova et al., Nature Communications 2017, 8, 1
[8] A Shee, T Saue, L Visscher, ASP Gomes, The Journal of Chemical Physics 2018, 149, 174113
[9] ASP Gomes, CR Jacob, Annual Reports Section "C" (Physical Chemistry) 2012, 108, 222
[10] Y Bouchafra, A Shee, F Real, V Vallet, ASP Gomes, Physical Review Letters 2018, 121, 266001

[11] diracprogram.org

PhD student: ALDAIR MISAEL Wilken - LinkedIn

Thesis supervisor:  André Severo Pereira Gomes