Knowledge Bank

Internally cold samples of the transient $S2O$ molecule are produced in a supersonic free-jet expansion and interrogated through the use of Degenerate Four-Wave Mixing (DFWM) spectroscopy. The enormous resonant enhancement inherent to this nonlinear optical technique provides an ``absorption-based'' detection scheme which can approach the trace species sensitivity afforded by conventional Laser-Induced Fluorescence (LIF) probes. The $230$ and $240$ vibronic bands of the intense $S2OC~1A\prime -X~1A\prime (\pi \ast \leftarrow \pi )$ transition $(\sim 30900cm-1)$ are examined with near-complete resolution of the corresponding rotational structure. The DFWM results are compared with data derived from simultaneous LIF measurements and are simulated through a perturbative treatment of the induced nonlinear polarization that incorporates a realistic model of the spectral bandwidth for the incident laser source. Particular emphasis will be placed on the identification of experimentally-controllable parameters (e.g., optical saturation effects) that govern the overall visibility and quality of individual rovibronic features. The limitations of resonant four-wave mixing spectroscopy, especially for the investigation of predissociating and/or non-fluorescing molecules, will be discussed in light of the present $S2O$ studies.