Doctor of Philosophy (PhD)
This dissertation contains two separate sections aside from an introduction (Chapter 1): theory and methods (Chapter 2), time dependent density functional theory with Ehrenfest for excited state lifetimes in materials (Chapter 3), and simulated field modulated X-ray absorption in titania (Chapter 4).
Excited state lifetime in insulators and semiconductors can be difficult to compute using quantum chemistry due to their dense excited states. Non-radiative decay in these materials requires the use of non-adiabatic effects to dissipate energy through the means of electron-nuclear coupling such as coherent phonon generation. One method of approaching this challenge in these materials is using Ehrenfest Theorem. Chapter 3 provides a method of correcting the nuclear gradient to include adiabatic effects. This provides an implementation of this correction using expressions from existing quantum chemistry packages. This method is validated using with a minimal basis set.
Chapter 4 presents a method of calculating the X-ray absorption near-edge spectra of titania using real-time time-dependent density functional theory. This is done using anatase titania bulk mimicking embedded clusters, which allow for the inclusion of hybrid-functionals without the use of pseudopotentials. These also include spin-orbit coupling in order to get accurate peak-splitting in the spectrum. These were then subjected to different static electric fields and the changes in the electronic structure are examined from changes in the spectrum. These lead to an increase in the peak splitting between t2g peaks and the eg peaks of the Ti L-edge.
Meyer, Alexander Matthew, "Non-Adiabatic Excited State Molecular Dynamics Using Ehrenfest and Modulated X-Ray Absorption in Titania" (2021). LSU Doctoral Dissertations. 5712.