Electric field response in molecules and materials [electronic resource] : computational studies of extreme conditions and strong fields

With advances in hardware and theory continually expanding the domain of numerical methods, many engineering problems are finding answers with the help of first principles computations. A growing body of research successfully employs electronic structure methods to study complex phenomena from nanometer-scale devices to single molecules. In particular, the scalability and simplicity of density-functional theory (DFT) based methods has led to their widespread application in studying materials and molecules. Time-dependent DFT (TD-DFT) is the extension of DFT to time-dependent systems, employed for studying dynamical processes like electric field response while retaining the favorable scaling of DFT methods. The electric field response of a system bears information about electronic properties, linear and nonlinear optical spectra, electron and ion dynamics, and electron-electron interaction. By applying simulated electric fields with a combination of DFT, molecular dynamics (MD) and time-dependent DFT (TD-DFT), we present studies of optical signatures in condensed matter at extreme conditions and strong field ionic response in molecules. We first present a method for identifying optical/UV signatures of chemical intermediates in reacting systems, and its application to shock-induced chemistry in the energetic material nitromethane. We find a connection between transient reaction byproducts and enhanced optical response across the entire visible spectrum, with the inclusion of a TD-DFT kernel affecting the amplitudes of the computed spectra by up to 30%. We then explore the pyrolysis of a phenolic resin similar to that used in atmospheric entry heat shields (famous for landing NASA's Mars Science Laboratory) and test the robustness of linear regression-based models for predicting features of composition from computed vibrational and electronic spectra. Lastly, by turning up the field amplitude well beyond the linear response regime, we present studies of the coupled electronic and ionic response of a molecule to a strong terahertz (THz) pulse. A lofty engineer's dream is to coherently rearrange atoms and chemical bonds at will using an electric field, but a back-of-the-envelope estimate suggests that a field strong enough to coherently rearrange bonds would push electrons through diamond, and attempts to coherently control bonds are known to suffer from intramolecular vibrational redistribution (IVR). Motivated by recent advances in strong terahertz (THz) pulse generation, we have investigated the potential of THz pulses to circumvent these obstacles. Employing TDDFT-Ehrenfest simulations, I discover that strong THz pulses can drive isomerization of the LiNC molecule over barriers greater than 0.2 eV with very low ionization rates and, in the best case, less than 10 meV of residual excess energy. This work points to new possibilities in predictively manipulating chemical bonds in molecules and materials.

Keywords: Materials Science and Engineering.
Advisor: Reed Evan J. primary advisor. Thesis advisor
Lindenberg Aaron Michael advisor. Thesis advisor
Martinez Todd J. (Todd Joseph)