Developments in the Theory of X-ray Absorption and Compton Scattering.

Thesis (Ph.D.)--University of Washington, 2016-06

X-ray spectroscopy is a widely used technique in experimental and theoretical physics for studying electronic and chemical properties of materials. Theoretical approaches for studying x-ray absorption (XAS) and x-ray photoemision (XPS) properties are described by single particle Green’s function theory. In strongly correlated systems, such as transition metal oxides, it is necessary to go beyond single particle treatment in order to reasonably describe peaks in XAS spectra. In this thesis I present a simplified, semi-empirical model which describes charge-transfer excitations in XAS, thus extending the formulations of Lee, Gunarson, and Hedin. I model the XAS by a localized three-state system coupled to a photoelectron, and implemented using FEFF real space Green’s function ab initio code to include solid state effects and intrinsic losses. In the second part, I study valence electron ground state properties as a function of temperature density for Be, Li and Si. I study thermal disorder and thermal expansion effects on high momentum components of the Compton profile (CP). To calculate the CP, I use the real space Green’s function (RSGF) approach coupled with molecular dynamic simulations to create a representation of a disordered system. Thermal disorder has the effect of smearing the high momentum umklapp peak in the case of Be and Li. I find good agreement with experimental results and, to a good degree of precision I predict changes in the CP of polycrystalline Si, which undergoes a solid to liquid phase transition. In the last part of this work, I study valence electron ground state properties of Be as a function of temperature and density in the warm dense matter (WDM) regime. I compare my thermally disordered calculations of electron momentum density (EMD) with perfect crystal calculations, using effective temperature to quantitatively describe the differences. I conclude that solid state effects are important in the WDM regime and that simplified models of degenerate Fermi gas might overestimate the temperature of the system.