# Applications of Two-Color Resonant Four-Wave Mining to SEP Spectroscopy

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 Title: Applications of Two-Color Resonant Four-Wave Mining to SEP Spectroscopy Creators: Dunlop, James R.; Tobiason, J. D.; Rohlfing, Eric A. Issue Date: 1994 Publisher: Ohio State University Abstract: Two-color resonant four-wave mixing (RFWM) derives its resonant enhancement from two distinct molecular resonances and is thus applicable to a variety of double-resonance spectroscopies. In this talk, we focus on the use of two-color RFWM to obtain background-free stimulated emission pumping (SEP) spectra. Two schemes are utilized: $\omega_{p}-\omega_{P}+\omega_{D}=\omega_{s}$ and $\omega_{p}-\omega_{D}+\omega_{D}=\omega_{s}$, where $\omega_{p}-\omega_{D}$, and $\omega_{s}$ denote the SEP PUMP, SEP DUMP, and RFWM signal frequencies, respectively. The first scheme is commonly interpreted as a laser-induced grating in the excited state while the second scheme can be thought of as a Raman coherence between two ground-state levels separated in energy by $\omega_{p}-\omega_{D}$. Both schemes can be interpreted in the weak-field limit using diagrammatic perturbation theory. We apply these RFWM-SEP techniques to transient molecules cooled in a free jet expansion; examples include the $\widetilde{A} {^{1}B_{2}}-\widetilde{X} {^{1} A_{1}}$ system of the $SiC_{2}$, the $\widetilde{A} {^{1} \Pi_{u}} \widetilde{X} {^{1} \Sigma_{g}}{^{+}}$ system of $C_{3}$, and the $\widetilde{B}^{2} A^{\prime}- \widetilde{X}^{2} A^{\prime}$ system of the HCO radical. We shall discuss the practical advantages and disadvantages of the RFWM-SEP approaches, with respect to each other and to the conventional technique of SEP detection via fluorescence depletion, the effects of beam geometry and relative polarizations on relative rotational line intensities, and the lineshape functions for tuning the DUMP frequency, $\omega_{D}$. Description: This work was supported by the U.S. Department of Energy. Office of basic Energy Sciences, Chemical Sciences Division. Author Institution: Sandia National Laboratories, Combustion Research Facility URI: http://hdl.handle.net/1811/13103 Other Identifiers: 1994-RA'-01