The chemical complexity of the interstellar medium and combustion environments pose a
challenge to the scientific community seeking to provide a molecular understanding of their
combustion. More refined spectroscopic tools and methodologies must be developed to selectively
detect and characterize the widening array of fuel and interstellar species. The direct relationship
between molecular structure and rotational frequencies makes rotational spectroscopy highly
structural specific; therefore, it offers a powerful means of characterizing polar molecules.
However, rotational spectra usually contain transitions from multiple components with multiple
conformations as well as other dynamical properties interleaved with one another, making the
assignment of the spectra very challenging. This thesis describes experimental work using
broadband microwave spectroscopy and vacuum ultraviolet time-of-flight mass spectrometry to
address a number of challenging problems in the spectroscopy of gas complex mixtures.<div><br></div><div>In the first part of my work, we report details of the design and operation of a single apparatus
that combines Chirped-Pulse Fourier Transform Microwave spectroscopy (CP-FTMW) with VUV
photoionization Time-of-Flight Mass Spectrometry (VUV TOFMS). The supersonic expansion
used for cooling samples is interrogated first by passing through the region between two
microwave horns capable of broadband excitation and detection in the 2-18 GHz frequency region
of the microwave. After passing through this region, the expansion is skimmed to form a molecular
beam, before being probed with 118 nm (10.5 eV) single-photon VUV photoionization in a linear
time-of-flight mass spectrometer. The two detection schemes are powerfully complementary to
one another. CP-FTMW detects all components with significant permanent dipole moments.
Rotational transitions provide high-resolution structural data. VUV TOFMS provides a gentle and
general method for ionizing all components of a gas phase mixture with ionization thresholds
below 10.5 eV, providing their molecular formulae. The advantages, complementarity, and
limitations of the combined methods are illustrated through results on two gas-phase mixtures
made up of (i) three furanic compounds, two of which are structural isomers of one another, and
(ii) the effluent from a flash pyrolysis source with <i>o-</i>guaiacol as precursor. <br></div><div><br></div><div>The broadband spectrum of 3-phenylpropionitrile was recorded under jet-cooled
conditions over the 8-18 GHz region. A novel multi-resonance technique called strong field
coherence breaking (SFCB) was implemented to record conformer-specific microwave spectra. This technique involves sweeping the broadband chirp followed by selectively choosing a set of
single frequencies pulses to yield a set of rotational transitions that belong to a single entity in the
gas-phase mixture, aiding assignment greatly. Transitions belonging to anti and gauche
conformers were identified and assigned and accurate experimental rotational constants were
determined to provide insight on the molecular structure. Experimental rotational transitions
provided relative abundances in the supersonic expansion. A modified line picking scheme was
developed in the process to modulate more transitions and improve the overall efficiency of the
SFCB multiple selective excitation technique.<br></div><div><br></div><div>The rotational spectrum of 2-hexanone was recorded over the 8-18 GHz region using a CPFTMW spectrometer. SFCB was utilized to selectively modulate the intensities of rotational
transitions belonging to the two lowest energy conformers of 2-hexanone, aiding the assignment.
In addition, the SFCB method was applied for the first time to selectively identify rotational
transitions built off the two lowest energy hindered methyl rotor states of each conformer, 0a<sub>1</sub> and
1e. Since these two states have rotational energy levels with different nuclear spin symmetries,
their intensities could be selectively modulated by the resonant monochromatic pulses used in the
SFCB method. The difference spectra, final fit and structural parameters are discussed for the three
assigned conformers of 2-hexanone.<br></div><div><br></div><div>Developing new experimental techniques that allow for species identification and
quantification in the high-temperature environment of reacting flows is a continuing challenge in
combustion research. Here, we combine broadband chirped-pulse microwave (rotational)
spectroscopy with an atmospheric-pressure jet-stirred reactor as a novel method to identify key
reactive intermediates in low-temperature and ozone-assisted oxidation processes. In these
experiments, the gas sample, after being withdrawn from reactive dimethyl ether/O<sub>2</sub>/Ar,
dimethoxy methane/O<sub>2</sub>/Ar, and ethylene/O<sub>2</sub>/O<sub>3</sub>/Ar mixtures, expands via a supersonic expansion
into the high vacuum of a microwave spectrometer, where the rotationally cold ensemble of polar
molecules is excited with short MW radiation frequency ramps (chirps). The response of the
molecular ensemble is detected in the time domain and after a Fourier transformation, the spectral
composition of the transient emission is obtained in the frequency domain. The observed rotational
frequencies are uniquely correlated to molecular structures and allow for an unambiguous
identification of the sampled species. Detection and identification of intermediates such as
formaldehyde, methyl formate, formic acid, formic acid anhydride, and the primary ethylene ozonide via literature-known rotational frequencies are evidence for the superb identification
capabilities of broadband chirped-pulse microwave spectroscopy. Strong-field coherence breaking
is employed to identify and assign transitions due to a specific component. The observation of van
der Waals complexes provides an opportunity to detect combustion intermediates and products
that are impossible to detect by rotational spectroscopy as isolated molecules. <br></div><div><br></div><div>Lastly, preliminary data on important combustion precursors is studied including pentanal,
<i>trans-</i>2-pentenal and <i>o-,m- </i>and <i>p</i>-vinylanisole. The rotational spectrum of these five molecules is
recorded from the 8-18 GHz region under jet-cooled conditions. For pentanal and <i>trans-</i>2-pentenal,
SFCB was utilized to dissect the broadband spectrum, identifying the four and two lowest energy
structures, respectively. The structural parameters and finals fits are provided. <br></div>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12200057 |
| Date | 27 April 2020 |
| Creators | Sean M Fritz (8769668) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Applications_of_Multi-Resonance_Broadband_Rotational_Spectroscopy_to_Interstellar_and_Combustion_Chemistry/12200057 |
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