Unraveling Material Behavior with Electron and X-Ray Spectroscopies: From Transient Electronic Responses to Electronic Topological Transitions

Author: Lee, Wonseok

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Cushing, Scott K.

Committee Members: Blake, Geoffrey A.; Goddard, William A., III; Wei, Lu; Cushing, Scott K.

Option: Chemistry

Abstract

Understanding how materials respond to external stimuli requires probes that can resolve coupled electronic, structural, and chemical changes across relevant length, time, and energy scales. This thesis investigates material dynamics and electronic structure using electron and X-ray spectroscopies, with a focus on how photoexcitation and high pressure modify the spectroscopic responses of solids.

The first part of this thesis focuses on ultrafast electron energy-loss spectroscopy as a probe of carrier, thermal, and structural dynamics. This approach is applied to crystalline silicon, where dense photoexcitation produces a transient reshaping of the bulk plasmon. The plasmon redshifts, broadens beyond the zero-loss peak response, and redistributes spectral weight, indicating that dense photoexcitation modifies the full dielectric response rather than simply introducing free carriers. These results show that the transient plasmon response arises from coupled changes in screening, electronic structure, and damping.

The second part investigates pressure-induced electronic topological transitions in cadmium using experimental and theoretical X-ray absorption spectroscopy. First-principles calculations reveal pressure-driven changes in the Fermi surface topology, while core–valence exciton projections connect these transitions to spectral changes at the Cd K edge. These results demonstrate that X-ray absorption spectroscopy can serve as a sensitive probe of electronic topological transitions.

The final part investigates resonant self-diffraction of femtosecond extreme ultraviolet pulses in cobalt near the Co M_2,3 edge. The observed self-diffraction signal is attributed to resonant changes in the complex refractive index, highlighting the potential of nonlinear extreme ultraviolet spectroscopy for probing ultrafast electronic dynamics at core-level resonances.

Together, these studies demonstrate that electron and X-ray spectroscopies provide powerful and complementary approaches for uncovering material behavior across multiple regimes, from nonequilibrium electronic dynamics to pressure-driven electronic topological transitions and nonlinear light–matter interactions.