CARS OBSERVATION AND ANALYSIS OF THE $\nu_{1}$ BAND OF $^{32}S^{16}O_{3}$

Author Institution: Oregon State University; Department of Chemistry, Oregon State University; Department of Chemistry, National Science Foundation; Department of Chemistry, Pacific Northwest National LaboratorySulfur trioxide is an important participant in reactions in the upper atmosphere and also in a number of industrial processes. It is a $D_{3h}$ 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 $^{32}S^{16}O_{3}$ as well as of the $^{34}S^{16}O_{3}$, $^{32}S^{18}O_{3}$ and $^{34}S^{18}O_{3}$ isotopic forms. High resolution $(0.001 cm^{-1})$ 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 $^{32}S^{16}O_{3}$ and its various isotopomers. The $\nu_{1}$ spectrum of $^{32}S^{16}O_{3}$ 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 $^{32}S^{16}O_{3}$