HIGH-RESOLUTION NONLINEAR SPECTROSCOPY OF TRANSIENT POLYATOMIC SPECIES: DEGENERATE FOUR-WAVE MIXING STUDIES OF JET-COOLED $S_{2}O$ IN THE VICINITY OF A PREDISSOCIATION LIMIT
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Date
1995
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Ohio State University
Abstract
Internally cold samples of the transient $S_{2}O$ 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 ${2^{3}}_{0}$ and ${2^{4}}_{0}$ vibronic bands of the intense $S_{2}O \tilde{C}^{1}A^{\prime} - \tilde{X}^{1}A^{\prime} (\pi^{\ast} \leftarrow \pi)$ transition $(\sim30900 cm^{-1})$ 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 $S_{2}O$ studies.
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Author Institution: CNRS, BP 166, 38042 Grenoble, Cedex 9 (France).; Yale University, New Haven, CT 06511.