High resolution spectroscopy of sulfur trioxide and carbon suboxide

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 forms of SO₃ (32S18O ₃ and 34S18O₃) have been investigated using both infrared spectroscopy and coherent anti-Stokes Raman scattering spectroscopy (CARS). The Raman-active symmetric stretching mode, ν ₁, shows complex Q-branch patterns due to indirect Coriolis couplings, l-resonances, and Fermi resonances with infrared inaccessible ν₂, ν₄ combination/overtone levels. 18O isotopic substitution changes the character of these interactions in such a way that their effect on the ν₁ 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 ν₁ band origin of 1004.661(24) and 1004.693(23) for 34S18O ₃ and 32S18O₃ respectively. The strong Coriolis coupling is very noticeable in these species due to the close proximity of the ν₂ and ν₄ fundamental vibrations. The effect that this and other interaction terms have on the ν₁ 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 ν ₁ 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 ν₇ 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 ν₁ Σ_g+ ← Σ _g+ transition from the ground state is confirmed by the absence of J_odd Q-branch lines in the resolved CARS spectrum. Analysis of this band leads to vibration-rotation parameters (in cm−1) of ν₁ = 2199.9721(6), (B′–B ″) = 2.029(6) × 10−4. Other transitions originating from higher ν₇ 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.