CARS studies of the quenching of excited sulfur atoms by rare gases: Fine structure selectivity in electronic-to-translational energy transfer
In the first use of the Raman effect to detect sulfur atoms, transient CARS spectroscopy has revealed novel relaxation effects in sulfur formed through multiple photon excitation of carbon disulfide vapor. Ground term $\sp3P$ sulfur was monitored through its 396 cm$\sp{-1}$ and 573 cm$\sp{-1}$ fine structure transitions ($\sp3P\sb1\leftrightarrow\ \sp3P\sb2$ and $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$) at various delays after the photolysis laser pulse. It was found that quenching of $\sp1D\sb2$ sulfur to the $\sp3P$ term by collision with the rare gases argon, krypton, and xenon gives clear kinetic and spectral evidence of a population inversion between the $\sp3P\sb0$ and $\sp3P\sb2$ fine-structure levels. Kinetic data indicate no such inversion between the $\sp3P\sb1$ and $\sp3P\sb2$ levels. Extensive modeling of the kinetic data taken at the $\sp3P\sb0\leftrightarrow\ \sp3P\sb2$ transition was performed to obtain the branching ratio into the $\sp3P\sb0$ level for the three rate gases studied. Although kinetic models including all possible processes in this system contain too many unknown parameters to be useful, a simple three parameter model gives reasonable fits to the data. This model yields branching ratios of 0.83, 0.75, and 0.73 for the fractional formation of sulfur's $\sp3P\sb0$ level through quenching from $\sp1D\sb2$ by Ar, Kr, and Xe, respectively. The results of MCSCF-CI calculations of the relevant low-lying Ar-S potential curves suggest that quenching proceeds through a single energetically accessible intersection between molecular terms, which, when correlation and coupling rules are considered, leads adiabatically to the $\sp3P\sb0$ product level that is experimentally observed to dominate. Although calculated Xe-S and Kr-S potential energy curves are not available, comparison with the similar rare gas oxide system suggests that a general explanation for the selective quenching mechanism may involve differences in the spin-orbit coupling strengths between molecular terms at the two crossings that adiabatically correlate with the $\sp3P\sb0$ and $\sp3P\sb2$ levels.
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