UC Berkeley

For many decades, electron dynamics and lattice dynamics have been studied by performing separate experiments, requiring different instrumentation. In general, electronic dynamics are most often studied by using visible pump probe spectroscopy, while lattice dynamics can be followed by core level spectroscopy performed at synchrotrons. With the advent of high harmonic generation (HHG), it became possible to generate femtosecond to attosecond bursts of extreme ultraviolet (XUV) radiation that are both spatially and temporally coherent. Since high harmonic generation can be performed on a table-top setup, it has become possible to follow both electronic and lattice dynamics concurrently, without the need for a synchrotron. In this dissertation, visible pump-XUV probe transient absorption spectroscopy is used to elucidate the excited state electronic and lattice dynamics of two materials relevant for solar energy conversion, namely a-Fe2O3 and single crystalline Si thin films.

In this dissertation, the first chapter deals with the pertinent information necessary to understand both the core-level spectroscopy that can be used to glean information about the electronic and lattice dynamics of solids, and the solid-state physics which govern the electronic and lattice dynamics, with special focus on the theoretical models used in these fields. The second chapter outlines the experimental apparatus that is used to perform the pump probe experiments on solid state samples, as well as several upgrades made to it during the course of this thesis. The third chapter details experiments performed on a-Fe2O3 that provide insights into its excited state dynamics following photoexcitation by multiple pump wavelengths (400 nm, 480 nm, 520 nm, 560 nm), including polaron formation and its impact on the photoconversion efficiency of photoelectrochemical cells. The fourth chapter details experiments performed on single crystalline Si thin films following photoexcitation with 800 nm pump pulses. This experiment highlights the strengths of HHG pump probe spectroscopy by giving insights into the electron-phonon scattering mechanisms that follow photoexcitation across the bandgap.