Modelling novel resonant X-ray spectroscopy for transition metal biological complexes and nano-particles

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This is a theoretical proposal aiming at developing novel numerical approaches to describe quantum excitations and their out-of-equilbrium dynamics in materials and molecules. This project brings the complementary expertise from Dr Cedric Weber on quantum modeling approaches for molecular transition metal systems, and Dr Rafael T. M. de Rosales on functional nano-particles used in the context of medical imaging.Here, we will model the dynamics of a core-hole state induced in hard core X-ray spectroscopy, used to probe chemical and magnetic properties of molecules and nano-scopic systems.The student will develop a new modeling approach to describe typical X-ray spectroscopy methods, such as X-ray absorption spectroscopy (XAS) and extend the formalism to model resonant inelastic X- ray scattering (RIXS). A canonical system, photo-system II (PSII), will be considered throughout the project. PSII has been investigated for decades, but the mechanism underpinning the water-splitting mechanism is still lacking. The modeling difficulties stem from the non-trivial quantum physics induced by the transition metal atoms (the four Mn atoms), which require state-of-the-art approaches. The first set of theoretical techniques will be based on density functional theory (DFT) and time-dependent DFT (TD-DFT) [1], which accounts for the dynamic screening of the X-ray and the core-hole fields. In particular, a typical problem for TD-DFT is the presence of multiplets, and the student will explore extensions of DFT to obtain an accurate descriptions of multi determinantal effects in DFT [2]. The student will then extend the methodology to nano-scopic systems, and in particular focus on super pagramagnetic iron oxide nano-particles (SPIONs). SPIONs have a range of applications, in particular they are used as contrast agents in magnetic resonant imaging (MRI). In this project, the student will focus at understanding magnetic excitations in nano-particles, and how they could be resolved in the future via the RIXS technique. The relaxation of the magnetic moment of nano-particles is a complex out-of-equilibrium process, and there is a subtle interplay between the dynamics and the conformation properties. In particular, the quantum confinement stemming from their spatial reduction is expected to provide new and complex out-of-equilibrium properties, as the external magnetic field is quenched. There are many major unanswered questions, such as the role of the magnetic frustration induced by the broken bonds in the outer-layers, and the role of the ligand bonding between the core of the nano-particle and the coating. The purpose of the project is to guide the design of better contrast agents, via optimizing the magnetic relaxivities of the nano-particle. Indeed, a current limitation is that large nano-particles have optimal magnetic relaxivities, but tend to agglomerate due to surface effects, which hinders the relaxation of their magnetic moments and their application as contrast agents. We provide here a consistent theory based on quantum ab-initio simulations, including dynamical effects.This project is also connected to the construction of a new X-ray beam line at the Diamond Light Source facility, that will be key to probe excitations in these new materials, as well as in a wide range of molecules and other materials of importance in chemistry and bio-chemistry.