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Title:High-resolution infrared spectroscopy in pursuit of C60 and other astrochemical targets
Author(s):Gibson, Bradley Michael
Director of Research:McCall, Benjamin J
Doctoral Committee Chair(s):McCall, Benjamin J
Doctoral Committee Member(s):Scheeline, Alexander; Eden, James G; Vura-Weis, Josh
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Buckminsterfullerene C60
Cavity ring-down spectroscopy
Tilt-tuned etalon
Supercritical fluid expansion
Quantum cascade lasers
External-cavity quantum cascade lasers
Abstract:This work is primarily concerned with the development of a mid-infrared cavity ringdown spectrometer with the intent of observing rotationally-resolved vibrational spectra of C60 and other astronomically relevant molecules, including the polyoxymethylene 1,3,5-trioxane. C60 was first discovered during experiments intended to simulate the conditions found in carbon stars, and has since been observed via emission spectroscopy in several planetary nebulae as well as the inerstellar medium. Due to its low ionization potential, much of the C60 in interstellar space is expected to be ionized to C60+, which has long been suspected as one of the carriers of the diffuse interstellar bands; recent work supports this assignment, further increasing interest in the astrochemistry of C60. While these observations through emission spectroscopy have had a significant impact, emission spectroscopy is not an effective approach in cold regions of space with low ultraviolet flux - as such, obtaining a high-resolution absorption spectrum remains an important goal. This is an extension of previous development work and attempts to observe the spectrum of C60 - as the principle shortfall of those previous attempts was the the inability to vibrationally cool C60 produced via thermal vaporization, much of this following will focus on the development and validation of an alternative vaporization method involving the expansion of supercritical fluids containing the target molecules. This should allow vapor to be produced at much lower temperatures, allowing more C60 to reach the ground state in spite of inefficient supersonic cooling. The first generation source has been constructed and tested, with the results informing the design of a second-generation source that has been constructed and is currently being evaluated. In addition to the development of an improved source for cold C60, we have also implemented a new external-cavity quantum cascade laser as the light source in our instrument. This new laser has significantly improved spectral coverage when compared to the previous light source, in addition to considerably easier and more reliable frequency tuning. However, the addition of a vibration-sensitive grating as a wavelength selective element introduced an unacceptable jitter in the lasing frequency. To address this, we developed a novel laser locking system based on side-of-fringe locking to a solid germanium etalon; in order to maintain continuous wavelength tuning, the free spectral range of the etalon is tuned by stepping its angle with respect to the incident laser. This has allowed us to stabilize the laser to acceptable levels while maintaining the ability to tune its lasing frequency very precisely. Finally, we recorded, simulated, and assigned the rotationally-resolved absorption spectrum of the v16 vibrational band of 1,3,5-trioxane. In addition to the importance of trioxane to understanding formaldehyde chemistry in comets, trioxane was chosen as a means of testing the various improvements made to the spectrometer. The spectrum described in this work was obtained using the new external-cavity quantum cascade laser and locking system; efforts to evaluate the second-generation supercritical fluid expansion source will use trioxane spectroscopy to estimate the efficiency of rotational and vibrational cooling.
Issue Date:2015-11-23
Rights Information:Copyright 2015 Bradley Michael Gibson
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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