Date of Award

8-30-2022

Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry/Green Chemistry

First Advisor

Neil Reilly

Second Advisor

Jonathan Rochford

Third Advisor

Jason Green

Abstract

SiC2 is vastly abundant in the circumstellar envelope of carbon-rich stars, such as IRC+10216, and acts as a key molecular tracer of circumstellar "dust", aiding in the formation of new stars and planets. Recent high-angular observations of IRC+10216 reveal a plethora of unassigned (U) rovibrational lines associated with the dust formation zone, making it reasonable to posit that a portion of these U lines arises from vibrationally excited levels of SiC2. Stimulated emission pumping (SEP) is used to optically pump SiC2 via the Merrill-Sanford bands and observe dump transitions terminating in poorly characterized excited vibrational levels of the ground electronic state. For previously unobserved transitions, we find that 1ν_1 is "well-behaved", while other states experience perturbations due to Fermi-resonance (1ν_2) or Coriolis coupling (2ν_2), and the 3ν_3 level is discrepant from theory, presumably because of large amplitude motion. Fits of the observed transitions using an asymmetric top Hamiltonian with centrifugal distortion yield rotational constants for the lower rovibronic levels, which can assist in targeted searches with high-resolution techniques and, ultimately, it is hoped, observation in radio astronomical spectra.

Like SiC2 in circumstellar "dust", resonance-stabilized radicals (RSRs) are persistent in hydrocarbon flames and stellar atmospheres and postulated intermediates in the growth of carbonaceous dust, better known as soot. Cis- and trans-meta-vinylbenzyl (MVB), C9H9 RSRs generated in a supersonically cooled discharge of m-vinyltoluene, were probed by ionization potential (IP) measurements, resonant-two-color-photon ionization, optical-optical hole-burning, and laser-induced dispersed fluorescence (DF). There is a notable absence of mirror symmetry between excitation and emission spectra for several totally symmetric modes, whereby a′ fundamentals that are conspicuous in emission are nearly absent in excitation, and vice-versa. This effect is primarily ascribed to interference between Franck-Condon (FC) and Herzberg-Teller (HT) contributions to the electronic transition moment, as the former is carried with a different sign in excitation vs. emission, while the latter is not. Rampant HT activity among a′ modes is attributed to the molecule's low symmetry (Cs), which permits intensity-borrowing from several relatively bright electronic states of A′′ symmetry.

Lastly, we report para-ethynylbenzyl (PEB), a highly stable radical on the C9H7 potential energy surface, first detected amongst other highly stable C9H7 radicals. The presence of PEB is secured by highly accurate IP measurements and a convincing electronic spectrum assignment using calculated excited-state frequencies. PEB exhibits pervasive anharmonic resonances, resulting from extensive near-degeneracies of overtones of non-totally symmetric modes with Franck-Condon-active totally symmetric modes. With the identification of PEB, we find the unmistakable existence of other C9H7 isomers, phenylpropargyl, and the previously undetected meta-ethynylbenzyl, whose high concentrations in discharges cannot be accounted for by sample impurities. These findings suggest a possible rearrangement between all three isomers via an intermediate that remains unobserved.

Comments

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