Chirped Pulse Microwave and Single-Shot Terahertz Spectroscopy Studies of Intermolecular Interactions

Citation

Mead, Griffin John (2021) Chirped Pulse Microwave and Single-Shot Terahertz Spectroscopy Studies of Intermolecular Interactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7wzt-ak58. https://resolver.caltech.edu/CaltechTHESIS:12072020-235133438

Abstract

While the glow of a sodium vapor lamp or the crisp reds in autumn leaves are eye-catching examples of transitions between atomic and molecular energy levels (hv ~2-3 eV), it is arguably the much lower energy, thermally populated intermolecular "bath" states (hv ~10⁻⁵-10⁻² eV) that contribute most directly to the physical properties of matter. Although invisible to the human eye, in this thesis we study fundamentals of these low-energy interactions with two complementary techniques: chirped pulse microwave spectroscopy and nonlinear single-shot terahertz (THz) Kerr effect spectroscopy.

In the first section, we apply chirped pulse-Fourier transform microwave (CP-FTMW) spectroscopy from 8-16 GHz to study fundamental hydrogen bonding motifs in gas phase alcohol water dimers. Hydrogen bonding is ubiquitous in nature and directly contributes to a range of phenomena from phase transitions in water to solvation of ions to enzymatic activity. Our focus on gas phase dimers reduces the spectral ambiguity arising in condensed phase samples, where inhomogeneous and homogeneous broadening can hamper observation of conserved intermolecular interaction motifs. The hydrogen bonding conformation of two alcohol-water dimers, n-propanol-water and isopropanol-water, were characterized. Both were found to adopt a shared water donor-alcohol acceptor conformation.

The following sections use nonlinear THz spectroscopy from 0.1-10 THz to investigate molecular dynamics in the condensed phase. We focus on halogenated methane liquids, whose intense intramolecular vibrational modes are commensurate in energy to the intermolecular bath states. One central goal of this section was developing a technique to more rapidly collect nonlinear, multi-dimensional data from liquid systems. To that end, we developed a single-shot measurement approach using a reflective nickel echelon mirror and a high frame rate camera. With this new device we achieved an order of magnitude reduction in experimental integration times. High resolution, nonlinear multi-dimensional THz studies of several halogenated methane liquids and materials were produced as a result. From these data, we identified important spectral contributions from the experimental instrument response function.

Thesis Files