Doctor of Philosophy (PhD)
In this dissertation, the calculations of light-matter interactions offer insight into the structure and dynamical response of electrons in molecular systems. Such information is useful for characterizing molecules, electronic structure, photochemistry, photomaterials, and a host of other applications. In the first part of this work, simulations of broadband absorption spectra are accelerated by the use of Pad´e approximanants of Fourier Transforms and dipole decomposition. Electronic absorption spectra from valence and core levels are obtained using time-dependent methods and compared to results from established perturbative techniques. In addition, core level absorption spectra are calculated for a nickel porphyrin and shown to be consistent with experimental XANES spectra. Not only is the speed up of these real-time simulations significant (at least 5x faster), but such methods offer the ability to directly calculate the dipole response from molecular orbital pairs involved in a given transition. In the second part of this dissertation, time-dependent density functional theory (TDDFT) calculations were performed to capture core-hole-initiated charge migration. By using welldefined initial states, well-known problems of resonant excitation (with TDDFT) caused by the adiabatic approximation are avoided. Using such initial states, core-hole-initiated valence charge migration is obtained in nitrosobenzene by TDDFT. These results are shown to be in good agreement with those of more accurate methods, provided the use of a hybrid functional to incorporate both electron correlation and exchange. In addition, the timedependent electron localization function (TD-ELF) can convert the density into a Lewis-dot picture of bonds and lone pairs of electrons. Building upon these charge migration studies, initial states in the valence orbitals were also constructed. In valence ionization, the electron is partially removed from multiple channels or orbitals; thus, multiple orbitals are involved in constructing the valence hole initial states. Due to the sudden ionization approximation made by construction of the vi initial states, a broadband-like response causes many dynamical modes to be excited. Here, the amplitude and phase of Fourier transforms are used to extract and classify the modes as charge migration or density-like excitation. These modes are then used to calculate metrics like migration distance and speed. Using this information in conjuction with the TD-ELF, it becomes possible to not only interpret charge migration, but to predict it by established chemical principles.
Bruner, Adam S., "Accelerated Broadband Spectra and Attosecond Charge Migration Simulations using Real-Time Time-Dependent Density Functional Theory" (2018). LSU Doctoral Dissertations. 4618.
Available for download on Monday, June 14, 2021