Understanding the Origins of Photoexcited XUV Spectra

Citation

Klein, Isabel McMillan (2023) Understanding the Origins of Photoexcited XUV Spectra. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s8qe-4s74. https://resolver.caltech.edu/CaltechTHESIS:12132022-201432142

Abstract

A full measurement of photoexcited dynamics, from excitation to recombination, is required to understand the photochemical processes at the heart of solar energy materials and devices. Measuring these complete dynamics is often unachievable with a single experimental tool. Transient X-ray spectroscopies, however, have proven to be powerful techniques as they can separately measure electron and hole dynamics, as well as vibrational and structural modes, all with elemental specificity. The interpretation of these measurements is still challenging, as the core-hole created following a core-level transition distorts the measured spectrum. This thesis aims to develop complementary experimental and computational techniques to measure and interpret transient X-ray spectra. Initially, the measured photoexcited dynamics of ZnTe and CuFeO₂, which reveal polaron formation and lattice coupling, as well as electron and hole kinetics and band gap dynamics, are presented. Following this experimental work, we develop an ab initio computational method for modeling transient X-ray and extreme ultraviolet (XUV) spectra. The ab initio method is a Bethe-Salpeter equation (BSE) approach based on the previously developed Obtaining Core Excitations from Ab initio electronic structure and the NIST BSE solver (OCEAN) code. Building on the foundations of the OCEAN code, we incorporate photoexcited states for a range of transition metal oxides and demonstrate the method’s ability to simulate the effects of state filling, isotropic thermal expansion and polaron states on XUV absorption spectra. Importantly, our method is also able to fully decompose the calculated spectra into the constituent components of the X-ray transition Hamiltonian, providing further insight into the origins and nature of spectral features. The XUV absorption spectra for the ground, photoexcited, and polaron states of α-Fe₂O₃, as well as for the ground, photoexcited, and thermally expanded states of other first row transition metal oxides – TiO₂, α-Cr₂O₃, β-MnO₂, Co₃O₄, NiO, CuO, and ZnO – are calculated to demonstrate the accuracy of our approach. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Thesis Files