Simulating X-ray spectroscopy using methods based on density functional theory: MBXAS studies on molecules, solids and its applications to hydrogen storage systems

Abstract

Over the last few years, there has been an increase in the use and study of X-ray spectroscopy of materials in order to understand the electronic structure of materials through the interpretation of their near-edge fine structure. Computational studies can aid in these investigations. Starting from electronic structure obtained using density functional theory (DFT), our work simulates the spectrum of benzene molecules and that of solid graphene by incorporating the core-hole e↵ects with the single particle full core-hole (FCH) approach and Many-Body X-Ray Absorption Spectroscopy (MBXAS). MBXAS uses the determinant approach to calculate the core-hole spectra of systems, taking eigen-values and eigen-states from DFT calculations [31]. The determinant method is equivalent to the Mahan-Nozieres-De Dominicis (MND) model in which many electrons interact with a core hole. A comparison of the electronic band structure calculated with and without the core-hole, respectively, reveals a significant core-hole binding e↵ects, i.e., excitons, for both the molecular benzene and the extended two-dimensional system, graphene. Furthermore, this approach provides direct access to the spatial distribution of the excited state orbitals for the purpose of analysis of XAS spectral peaks. This permitted us to draw parallels between the X-ray excited states of benzene and graphene which share certain local symmetries in atomic arrangement and corresponding electronic structure. Density of states analysis reveals the energies of excitonic peaks and their relationship with the initial-state band structure. The last part of this work deals with the applications of many-body X-ray absorption spectroscopy to hydrogen storage systems. Hydrogen has proven to be an ecient alternative energy carrier, as such, the current energy infrastructure due to its cleanliness as a carbon-free fuel, higher energy content per unit mass, and unlimited supply desire our careful investigations. We obtained the theoretical results for single metal systems - lithium amide (LiNH2), magnesium amide (Mg(NH2)2), lithium imide (Li2NH), magnesium imide (MgNH), lithium nitride (Li3N), magnesium nitride (Mg3N2) - and a double metal imide (Li2Mg(NH)2). Our X-ray absorption spectra simulations for these systems were calculated at nitrogen Kedges, with the double metals imide showing less excitonic e↵ects compared to the other single metal systems.