LIFETIME-MEDIATED POLARIZATION EFFECTS IN NONLINEAR SPECTROSCOPY: DEGENERATE FOUR-WAVE MIXING STUDIES OF PREDISSOCIATED $S_{2}O$ IN A SLIT-JET EXPANSION
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Date
1998
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Publisher
Ohio State University
Abstract
Transient $S_{2}O$ molecules were entrained in a pulsed slit-jet expansion $(T_{rot} < 10 K)$ and interrogated through use of sub-Doppler Degenerate Four-Wave Mixing (DFWM) spectroscopy. High-resolution scans have been acquired for the $2^{v}_{o} (\nu=3-10)$ vibronic bands of the intense $\tilde{C} {^{1}}A^{\prime} \leftarrow \tilde{X} {^{1}}A^{\prime}(\pi^{\ast}\leftarrow \pi)$ absorption system, where increasing excitation of the $v_{2} S-S$ stretching mode is known to promote predissociation of the $\tilde{C}$ state. For bands involving moderately-predissociated states (e.g., $2^{4}_{0}$ where $\tau_{***} 63 ps)$, the recorded pattern of rovibronic line intensities exhibits a pronounced dependence upon DFWM polarization geometry, a situation not encountered in analogous studies performed for features terminating on long-lived levels of the $\tilde{C}$ manifold (e.g., $2^{3}_{0}$ where $\tau_{***} \simeq 22 ns)$. This behavior can be reproduced quantitatively through detailed weak-field analyses of the resonant DFWM response, however, a qualitative understanding follows from the selective dissipation of optically-induced transient gratings as incurred by unimolecular relaxation pathways. In strongly-predissociated members of the $2^{v}_{0}$ progression (i.e., $v \geq 5$), additional polarization specificity is introduced by the presence of strong depopulation pumping processes which lead to the creation of net orientation and/or alignment of the molecular ensemble on a timescale commensurate with that of the pulsed four-wave mixing experiment Owing to its absorption-based response and laser-limited spectral resolution, DFWM is often applied to target species where rapid nonraditive relaxation channels preclude successful exploitation of detection techniques based upon secondary matter-field interactions (e.g., fluorescence or ionization). Therefore, a detailed understanding of the role which molecular lifetime plays as a mediator for resonant nonlinear response is of central importance for the quantitative application of this optical scheme.
Description
$^{a}$ Permanent address: High Magnetic Field Laboratory, CNRS, BP 166, 38042 Grenoble, Cedex 9 (France) $^{b}$ Present address: G.R. Harrison spectroscopy Laboratory, MIT, Cambridge, MA 02139
Author Institution: Department of Chemistry, Yale University
Author Institution: Department of Chemistry, Yale University