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- All Subjects: Physical Chemistry
- Creators: Sayres, Scott G.
Description
As the simplest carboxylic acid, formic acid (FA) is ubiquitous to Earth’s atmosphere, helping seed cloud nucleation and leading to acid rain. By studying the interactions between FA and high intensity light under high vacuum, conditions similar to the upper atmosphere, on other planets (either in the solar system or beyond), and even in interstellar media are emulated. These results were produced from a home built vacuum chamber system, with a Wiley-McLaren time of flight mass spectrometer and using femtosecond (fs) laser pulses. The laser characteristics were as follows: a pulse width >35 fs, center wavelength of 400 nm (probe pulse was 800 nm for the pump-probe investigation), and laser intensities at ~1015 W/cm2.At high laser intensities, the first direct experimental evidence of CO3+ was recorded from the Coulomb explosion (CE) of the formic acid dimer (FAD) from a molecular beam. Theoretical calculations provided further evidence for the formation of CO3+ from the vertical ionization of FAD. When (FA)n(H2O)mH+ clusters (n = 1-7 and m = 0-1) were exposed to similar laser intensities, the larger clusters (n = 5-7) favored complete atomization from CE, indicating that the repulsive forces within the clusters at those sizes was too great to withstand to form CO3+. The protonated nature of the clusters and the peak shapes recorded in the mass spectra suggested that neutral (FA)n+ clusters undergo a dissociation mechanism within the extraction region. A novel technique was created to calculate these dissociation times on the order of 100s of nanoseconds (ns), increasing by ~10 ns for each additional FA molecule. Using pump-probe spectroscopy, it was observed similarly that neutral (FA)n clusters with n > 1, showed evidence of ion pair formation of the form [(FA)nH+·OOCH-] on the sub-picosecond timescale, increasing by 70 fs per FA molecule. Both trends indicate that the neutral clusters prefer to form compact 3d structures, but after photoexcitation the clusters have competing pathways to ionization, either through multiphoton ionization (ns dynamics) or ion pair formation (fs dynamics) that inevitably lead to the expansion and subsequent rearrangement into linear chains for the protonated cluster.
ContributorsSutton, Shaun (Author) / Sayres, Scott G. (Thesis advisor) / Richert, Ranko (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2023
Description
One strategic objective of the National Aeronautics and Space Administration (NASA) is to find life on distant worlds. Current and future missions either space telescopes or Earth-based observatories are frequently used to collect information through the detection of photons from exoplanet atmospheres. The primary challenge is to fully understand the nature of these exo-atmospheres. To this end, atmospheric modeling and sophisticated data analysis techniques are playing a key role in understanding the emission and transmission spectra of exoplanet atmospheres. Of critical importance to the interpretation of such data are the opacities (or absorption cross-sections) of key molecules and atoms. During my Doctor of Philosophy years, the central focus of my projects was assessing and leveraging these opacity data. I executed this task with three separate projects: 1) laboratory spectroscopic measurement of the infrared spectra of CH4 in H2 perturbing gas in order to extract pressure-broadening and pressure-shifts that are required to accurately model the chemical composition of exoplanet atmospheres; 2) computing the H2O opacity data using ab initio line list for pressure and temperature ranges of 10^-6–300 bar and 400–1500 K, and then utilizing these H2O data in radiative transfer models to generate transmission and emission exoplanetary spectra; and 3) assessing the impact of line positions in different H2O opacities on the interpretation of ground-based observational exoplanetary data through the cross-correlation technique.
ContributorsGharib-Nezhad, Ehsan (Author) / Line, Michael R. (Thesis advisor) / Lyons, James R. (Thesis advisor) / Sayres, Scott G. (Committee member) / Heyden, Bjorn Matthias (Committee member) / Arizona State University (Publisher)
Created2019