61st Annual Report on Research 2016

The chemical complexity of hydrocarbon fuels and the fast-expanding list of oxygenated fuels used as additives present a challenge to the scientific community seeking to provide a molecular-scale understanding of their combustion. The development of accurate combustion models stands on a foundation of experimental data on the kinetics and product branching ratios of individual reaction steps. This grant has as its new direction the marriage of a hyperthermal reactor as flash pyrolysis source with broadband microwave detection to carry out isomer-specific spectroscopy on the reactive intermediates formed during their pyrolysis and combustion. Chirped-pulse Fourier Transform microwave spectroscopy (CP-FTMW) is being used to provide high resolution microwave data spanning the 7.5-18.5 GHz range to structurally characterize key fuel components and pyrolysis intermediates.

During the first year of the grant, we have focused attention on an important aspect of the long-term success of this endeavor; namely, the development of methods for selectively extracting the microwave spectral transitions due to a single component out of an otherwise complicated multicomponent mixture. The method operates the broadband chirped-pulse used to excite the sample in the strong-field limit through a combination of high power and control of the frequency sweep rate. We have introduced a procedure by which a set of three microwave transition frequencies are selected that can be incorporated as a set of resonant single-frequency microwave pulses that follow broadband chirped-pulse excitation, resulting in a reduction in the coherent signal from a set of transitions ascribable to a single component of the mixture. The difference in the CP-FTMW spectrum with and without this set of multi-resonant single-frequency pulses, produces a set of transitions that can confidently be assigned to a single component of the mixture, aiding the analysis of its spectrum. To date, the scheme has been applied to a series of model compounds, including methyl butyrate, a model biofuel, for which we have obtained conformer-specific microwave spectra of its two populated conformations. We have also refined our high-temperature pyrolysis source to improve its stability and its high temperature operation. We have also completed the construction of a 10-meter long room-temperature microwave cell that we can use to record spectra at ambient temperatures, for comparison of the jet-cooled spectra. This combination of expansion-cooled and ambient measruements can add new insight to the large-amplitude motions present in many of these flexible biofuels.

Alicia Hernandez-Castillo has been supported on a graduate research assistantship by this grant. She is currently in her third year of graduate school. During the period of her support from PRF, she has gained extensive experience in broadband microwave spectroscopy and data analysis. She has developed a suite of MatLab programs that she uses to process and analyze the broadband microwave data. She has also gained experience in the design and construction of scientific equipment through her design improvements to our pyrolysis source.

As principal investigator, this New Directions grant has provided critical resources to develop these microwave-based methods for detecting single components of mixtures, which expand the conformer-specific laser-based IR/UV double resonance tools that were used previously by my group to study aromatic-containing flexible molecules.