PHYSICS PHD DISSERTATION DEFENSE: Kathryn Ledbetter

Ph.D. Candidate:  Kathryn Ledbetter
Research Advisor:  Kelly Gaffney

Date: Friday, June 25, 2021
Time: 2:00 pm (PDT)
Zoom Link: https://stanford.zoom.us/j/99013128864

Zoom Password: email mariaf67 [at] stanford.edu (mariaf67[at]stanford[dot]edu) for password

Title: Resolving Structural Changes in Photoinduced Reactions in Solution with X-rays and Electrons

Abstract:

The need for efficient, low-cost solar energy capture has driven interest in mechanistic understanding of light-induced chemical reactions in solution, for instance, natural and artificial photosynthesis. Resolving the structural changes of a molecule during a photoinduced reaction in solution can be challenging, however, with many interdependent intra- and inter-molecular degrees of freedom. I will discuss three techniques that can be used to disentangle structural changes during photoinduced reactions: x-ray absorption spectroscopy, valence-to-core x-ray emission spectroscopy, and ultrafast electron diffraction. First, I will discuss an example of a catalyst for the hydrogen evolution reaction (HER) half of water-splitting artificial photosynthesis. Through the use of element-specific x-ray absorption probes, I discover a discrepancy between the behavior of the catalyst and the proposed HER mechanism from literature. The catalyst decomposes in the presence of a proton source, yet retains a high activity for proton reduction into hydrogen gas. This surprising result points to the need for tools that characterize the structural configuration of molecules at all stages of a reaction. Addressing this need, I will then discuss method development of two time-resolved probes of solution-phase reactions (femtosecond valence-to-core x-ray emission spectroscopy and ultrafast electron diffraction) and their potential applications for creating "molecular movies" of interesting solution-phase systems such as HER catalysts.