Core Level X-ray Spectroscopy

Light-induced processes at the nanoscale have rapidly become of fundamental importance for many technological applications in the field of photonics, opto-electronics, and materials science. In particular, the recent development in ultra-fast laser spectroscopies moves in the same direction, since it allows for a new paradigm to understand reaction mechanisms, chemical and hydrogen bonding, and to study surface catalytic reactions in real time using optical pump and X-ray probe. Understanding how electronic, optical, and vibrational excitations are triggered and coupled to each other in the ultrashort time-window following the perturbation is of paramount importance to rationalize and ultimately predict the behavior of complex systems such as disordered liquids and solutions, or hybrid interfaces. There is, for instance, a great interest in developing more efficient and stable devices, combining inorganic semiconductors as the substrate with organic molecular dopants. Countless combinations can be created in this sense, making it unfeasible for experimentalists to study all possible systems. There is a urgent need of modelling strategies able to address questions arising from the response of complex condensed matter systems to core excitations. Computer simulations have acquired a central role in the interpretation of spectroscopic data and in the understanding the electronic, magnetic, and structural properties of materials. When the interaction with light is taken into account, important changes take place, calling for modelling approaches beyond the electronic ground-state picture, i.e., addressing the role of excited states, non adiabatic dynamics, and charge transfer processes.

We combine ab initio molecular dynamics (AIMD), standard theoretical spectroscopy, linear response as well as real-time time-dependent density functional theory (TDDFT) and Ehrenfest molecular dynamics (EMD) to investigate the light-matter interactions and the related induced processes in condensed phase materials. The explicit inclusion of a laser pulse evolving in time with the electronic system enables the simulation of pump-probe experiments and the study of charge carriers on the femtosecond timescale. Even more interesting is the possibility to couple the electron and the nuclear dynamics through the Ehrenfest scheme. Some of the applications are in the following fields: characterization of the local structure of liquids modelling X-ray absorption and emission processes, light-induced dynamics in two-dimensional (2D) pristine and functionalized materials, of hybrid inorganic/organic materials for solar cells.