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High Resolution Infrared Spectroscopy of Slit-Jet Cooled Radicals and Ions

TitleHigh Resolution Infrared Spectroscopy of Slit-Jet Cooled Radicals and Ions
Publication TypeThesis
Year of Publication2012
AuthorsRoberts, MA
Academic DepartmentChemistry and Biochemistry
DegreePh.D.
Number of Pages281
Date Published2014-04
UniversityUniversity of Colorado
CityBoulder, CO
Abstract

This thesis presents high-resolution spectra of supersonically-cooled organic radicals in the mid-infrared, the details and design of the instruments necessary to obtain the spectra, and the theory to understand the spectra and the larger context of the results. Specifically, four organic radicals are studied: singly-deuterated methyl radical (CH2D), phenyl radical (C6H5), hydroxymethyl radical (CH2OH), and ethynyl radical (C2H). All of the spectroscopic studies presented use an existing mid-infrared high-resolution spectrometer with a frequency precision of better than 10 MHz. The radicals are generated using a discharge to dissociate a neutral precursor and form the radicals. The discharge is localized at the orifice of a slit supersonic expansion, which cools the radicals to around 20 K and allows for sub-Doppler spectral resolution. In addition to the description of the existing spectrometer, the design, construction, and successful testing of a new, automated mid-infrared spectrometer is presented. The new spectrometer is based upon difference frequency generation of a scanning Ti:Sapphire laser and a single-frequency Nd:YAG laser to create high-resolution mid-infrared radiation. The new system speeds up data-taking by fully automating the scanning process.

The four radicals studied in this thesis are all intermediates in combustion processes of hydrocarbon fuels. First, the out-of-phase symmetric stretch of phenyl radical is presented. As the first high-resolution infrared study of phenyl, it paves the way for future studies of this and other aromatic radicals. Second, the two fundamental CH stretches in CH2D are studied with full rotational resolution. The narrow linewidth of the transitions reveals resolved fine structure and partially resolved hyperfine structure. This resolution yields additional information regarding the distribution of electrons in the radical. With this study of CH2D, a nearly complete set of vibrational frequencies is present in the literature. This inspired us to develop a comprehensive model that is capable of simultaneously fitting the CH and CD stretches of all the hydrogenic isotopomers of methyl radical. Third, while ethynyl absorbs in the mid-infrared, the transition studied are low-lying electronic states. The combination of a cold source of C2H and high frequency precision allows us to clarify line assignments and find new transitions. Additionally, localized shifting of transition frequencies allows for identification and partial characterization of the dark perturber states. Fourth, the symmetric CH stretch of hydroxymethyl radical is studied at high-resolution. The high resolution spectra improve upon band origin and structural information in the radical as well as set the stage for further experimental studies into potential large amplitude dynamics in the radical.