Knowledge Bank

Sulfur trioxide is an important participant in reactions in the upper atmosphere and also in a number of industrial processes. It is a $D3h$ planar oblate top whose spectroscopy is incomplete, perhaps in part due to its corrosive properties. We are engaged in a comprehensive investigation of the fundamental and combination-overtone bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">32</mrow>}S16O3$ as well as of the $\mathrm{<mrow\; data-mjx-texclass="ORD">34</mrow>}S16O3$, $\mathrm{<mrow\; data-mjx-texclass="ORD">32</mrow>}S18O3$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">34</mrow>}S18O3$ isotopic forms. High resolution $(0.001cm-1)$ coherent anti-Stokes Raman scattering (CARS) was used at Oregon State University to determine for the first time the Q-branch structure of the IR-inactive $\nu 1$ symmetric stretching mode of $\mathrm{<mrow\; data-mjx-texclass="ORD">32</mrow>}S16O3$ and its various isotopomers. The $\nu 1$ spectrum of $\mathrm{<mrow\; data-mjx-texclass="ORD">32</mrow>}S16O3$ reveals two intense Q-branch regions, with surprisingly complex vibrational-rotational structure. The modeling of this has involved a subtle combination of Fermi resonance and indirect Coriolis interactions with nearby hidden states; $2\nu 4(\ell =0,\pm 2)$, $\nu 2+\nu 4(\ell =\pm 1)$, $2\nu 2(\ell =0)$. The analysis of the perturbed $\nu 1$ spectrum was made possible by locating some of these states via concurrent infrared hot-band studies at PNNL by T.A. Blake et al. The results of this combined effort will be presented for $\nu 1$ of $\mathrm{<mrow\; data-mjx-texclass="ORD">32</mrow>}S16O3$.