Graduate Thesis Or Dissertation

 

High resolution spectroscopy of sulfur trioxide and carbon suboxide Public Deposited

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  • High resolution spectroscopy was used to study the properties of two simple polyatomic molecules, sulfur trioxide, SO₃, and carbon suboxide, C₃O₂. The fundamental modes and several hot bands of the ¹⁸O isotopic fonns of SO₃ (³²S¹⁸O₃ and ³⁴S¹⁸O₃) have been investigated using both infrared spectroscopy and coherent anti-Stokes Raman scattering spectroscopy (CARS). The Ramanactive symmetric stretching mode, vt, shows complex Q-branch patterns due to indirect Coriolis couplings, l-resonances, and Fermi resonances with infrared inaccessible v₂, v₄ combination/overtone levels. ¹⁸O isotopic substitution changes the character of these interactions in such a way that their effect on the v1 CARS spectrum is unique among the different isotopomers studied. Accurate rovibrational constants are determined for all of the mixed states for the first time, leading to deperturbed values for the vt band origin of 1004.661(24) and 1004.693(23) for ³⁴S¹⁸O3 and ³²S¹⁸O₃ respectively. The strong Coriolis coupling is very noticeable in these species due to the close proximity of the v₂ and v₄ fundamental vibrations. The effect that this and other interaction terms have on the vt CARS spectrum of ³⁴S¹⁸O₃ is examined by selectively turning off the coupling between the hot bands. A global force field analysis was performed with the fundamental frequency values of all of the isotopomers studied that revealed a counterintuitive trend in the bond lengths between sulfur oxide species. In addition, band center frequencies for all the mixed ¹⁶O-¹⁸O isotopic species are computed. High-resolution CARS Spectroscopy was also used to study the v₁ symmetric CO stretching mode of the quasi-linear molecule carbon suboxide, C₃O₂. Q-branches are seen that originate from the ground state and from thermally populated levels of the unusually low frequency v₇ bending mode. The intensity variation of these on cooling to about 110 K in a jet expansion requires reversal of the order of assignment given in a previous Raman study at low resolution. The identification of the v₁ Σg⁺ ← Σg⁺ transition from the ground state is confirmed by the absence of Jodd Q-branch lines in the resolved CARS spectrum. Analysis of this band leads to vibration-rotation parameters (in cm⁻¹) of v₁ = 2199.9721(6), (B'-B") = 2.029(6) x10⁻⁴. Other transitions originating from higher v₇ levels occur at only slightly lower wavenumber values, indicating that the ground state barrier to linearity (22 cm⁻¹) increases only 1 to 2 cm⁻¹ when the CO symmetric stretch is excited.
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