Knowledge Bank

The $X~1A\prime$ ground electronic potential surface of the transient disulfur monoxide $(S2O)$ molecule has been examined in regions corresponding to roughly 1 eV of vibrational excitation. $S2O$ molecules were prepared in situ under cold $(Trot=1K)$ supersonic free-jet expansion conditions and interrogated via a nonlinear variant of Stimulated Emission Pumping (SEP) based upon Degenerate Four-Wave Mixing (DFWM) spectroscopy. while the background-free, "absorption-like" response of the sub-Doppler SEP-DFWM technique offers unique advantages over more conventional fluorescence-dip methods, the quadratic dependence of DFWM signal amplitude upon number density would seem to limit this approach to the study of stable molecules maintained in high-pressure, static bulb environments. Nevertheless, by utilizing the intense $S2OC~1A\prime -X~1A\prime (\pi \ast \leftarrow \pi )$ transition at $\sim 30900cm-1$ as the first step in an optical-optical double resonance scheme, ongoing SEP-DFWM experiments have demonstrated excellent signal-to-noise ratios for target species entrained in seeded molecular beams. The substantial elongation of the S-S bond upon $C~\leftarrow X~$ electron promotion provides Franck-Condon access to ground state levels of $S2O$ having substantial amplitude in the $\nu 2S-S$ stretching mode. Analysis of vibronic features observed subsequent to $202$ and $203$ excitation yields values of $\omega 2=680cm-1$ and $x22=-2.4cm-1$ for the frequency and anharmonicity of $\nu 2$ in $X~1A\prime S2O$. These results will be discussed in conjunction with various experimental configurations for SEP detection by means of resonant four - wave mixing.