Applications of Multi-Resonance Broadband Rotational Spectroscopy to Interstellar and Combustion Chemistry

2020-04-27T16:50:07Z (GMT) by Sean M Fritz
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.

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 o-guaiacol as precursor.

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.

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, 0a1 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.

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/O2/Ar, dimethoxy methane/O2/Ar, and ethylene/O2/O3/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.

Lastly, preliminary data on important combustion precursors is studied including pentanal, trans-2-pentenal and o-,m- and p-vinylanisole. The rotational spectrum of these five molecules is recorded from the 8-18 GHz region under jet-cooled conditions. For pentanal and trans-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.