Laboratory and Astronomical Rotational Spectroscopy

Citation

Carroll, Paul Brandon (2018) Laboratory and Astronomical Rotational Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8Y1M-6C76. https://resolver.caltech.edu/CaltechTHESIS:06062018-120951718

Abstract

Rotational spectroscopy is a capable technique with a rich history in a variety of chemical physics applications that is undergoing a renaissance thanks to new approaches and powerful new experimental capabilities. This thesis demonstrates that flexibility by examining multiple uses of rotational spectroscopy from instrument development, analytical chemistry, fundamental chemical physics, and astrochemistry.

In chapter 2 we discuss the development of two novel coherent microwave spectrometers. These low cost instruments have only recently become feasible thanks to the burgeoning development of highly flexible digital electronics. The first instrument is designed for undergraduate teaching labs and can be used to demonstrate many new concepts of coherent spectroscopy that are used in modern spectroscopy. It is a rotational spectrometer and can, therefore, also be used for a variety of basic spectroscopy experiments. The second instrument uses the stability and consistency of waveguides and broadband microwave instruments to measure the rotational spectrum and abundance of isotopologues to high accuracy.

Chapter 3 describes the measurement of the rotational spectrum of the cyclopentanol--water dimer. Using microwave spectroscopy, the spectrum is measured and assigned, and the structure of the dimer is determined. The cyclopentanol--water dimer shows a structure dominated by both strong hydrogen bonding and multiple weaker hydrogen bonds from the hydrocarbon ring. The monomer spectrum is measured, though unassigned due to the strong perturbation from the motion of the ring. Dimerization with water is shown to suppress this motion. This system is shown to be an excellent example of the effect of weak hydrogen bonding on the secondary structure and dynamics of molecular systems.

Chapter 4 moves to astronomical observations of rotational transitions with the detection of a new species: propylene oxide. Measuring the inventory, abundance, and distribution of molecular species provides tests of our understanding of interstellar chemistry. Propylene oxide is an important addition to this inventory because it is the first chiral species detected beyond our solar system. Chiral species play an enormously important role in biology on Earth, and it is believed that interstellar chemistry may contribute to the early inventory of prebiotic species on newly formed planets. The detection of propylene oxide is discussed in the context of the origin and distribution of chiral molecules in the universe.

Chapter 5 discusses recent data from the Atacama Large Millimeter/Submillimeter Array. The data maps the distribution of CH₃CN isotopologues at incredibly high spatial resolution toward the Orion KL region. The measurement of isotopic ratios in CH₃CN is used to inform our understanding the formation mechanisms of cyanides in star forming regions. More broadly, the maps are used to show the extreme spatial heterogeneity of the region, with numerous dense clumps roughly the size of a solar system, each with their own unique chemical and physical structure that reflects their distinct evolutionary histories.

Finally, chapter 6 discusses the non-detection of trans ethylmethyl ether. Ethylmethyl ether is one of the largest molecules claimed to be detected in the interstellar medium. Due to its size, it is believed to be produced on grain surfaces as a secondary or tertiary product from dissociation of ice constituents. Given its complexity, its abundance may be an important metric of the accuracy of chemical models of ice chemistry. The study claiming its detection reported an unusually high abundance of the species toward W51 e1/e2. Follow up observations and analysis showed that the original detection was mistaken, likely caused by interference from other features.

Thesis Files