RAMAN ULTRAVIOLET DOUBLE RESONANCE SPECTROSCOPY OF THE ACETYLENE MOLECULE
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Date
1990
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Ohio State University
Abstract
We report optical double resonance (ODR) experiments in which selective preparation of gas-phase acctylene $(C_{2}H_{2})$ in discrete rotational state of the $V_{2}=1 (C=C stretch, 1974 cm^{-1})$ vibrational level is achieved by pulsed conherent Raman excitation, with laser-induced flourescence (LIF) detection.1 The quality of these LIF-detected Raman-ODR spectra is superior to that attained in our previous work on larger polyatomic $molecules^{2}$. The method depends on a careful choice of the vibronic bands used to monitor the $V_{2}$-mode Raman excitation, since the $\widetilde{A}\leftarrow \widetilde{X}$ absorption system of $C_{2}H_{2}$ involves a bent-linear geometric transition and the Franck-Condon principle favours quanta of the symmetric bending modes ($V_{3}$ in the $\widetilde{X}$ state and $V_{4}$ in the $\widetilde{X}$ state). Several previously unreported $\widetilde{A}\leftarrow \widetilde{A}$ vibronic bands have been recorded with the intensity-enhancement of $V_{2}$-mode Raman excitation: $2^{0}_{1}3^{1}{_{0}} (242nm), 2^{0}{_{1}}3^{2}{_{0}} (236nm), 2^{0}{_{1}}3^{3}{_{0}} (231nm), 2^{0}{_{1}}3^{1}{_{0}}4^{0}{_{1}} (246nm)$ and $2^{0}{_{1}}3^{2}{_{0}}4^{0}{_{1}} (240 nm)$. Our Raman-ODR spectra clearly demonstrate the effect on rovibronic intensities of $``axis-switching''.^{3}$ resolved one rotational state at a time. Variation of the delay between the Raman pump pulses and the LIF probe also provides a view of collision-induced state-to-state- rotational energy transfer. Another Raman-ODR spectroscopic approach yields highly resolved state-specific Raman-excitation frequency through O-,Q- and S-branch rovibrational transitions. Parallel infrared-ultravoiler double resonance and Raman-ODR studies of $C_{2}D_{2}$ and $C_{2}HD$ are also is progress.
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$^{1}$ B.L. Chadwick, D.A. King, L, Berzins, and B.J. Orr, J, Chem. Phys. 91.7994 (1989). $^{2}$ A.B. Duval, D.A. King, R. Haines, N.R. Lsenor, and B.J.Orr, J, Opt, Soc Amer. B 2, 1570 (1985). $^{3}$ J.T. Hougen and J.K.G. Watson, Canada, J.Phys, 43.298 (1965).
Author Institution: School of Chemistry, Macquarei University
Author Institution: School of Chemistry, Macquarei University