INFRARED-ULTRAVIOLET DOUBLE RESONANCE SPECTROSCOPY OF ACETYLENE: DYNAMICAL SYMMETRY BREAKING IN THE $4\nu_{CH}$ ROVIBRATIONAL MANIFOLD AT $12700 cm^{-1}$
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Date
1997
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Publisher
Ohio State University
Abstract
Time-resolved fluorescence-detected infrared-Ultraviolet optical double resonance (IR-UV DR) experiments have been performed with a Raman-shifted dye laser preparing gas-phase $C_{2}H_{2}$ molecules in the ``$4\nu_{CH}$” region followed by laser-induced fluorescence (LIF) probing in the $S_{1} \leftarrow S_{0}$ vibronic band system. Unusual symmetry-breaking energy transfer has been observed, induced (at least in part) by $collisions^{h}$. This takes the form of odd-numbered changes of the rotational quantum number J, despite the fact that intramolecular transfer between the {ortho} and {para} nuclear-spin modifications of such a molecule is usually forbidden. The selection rules for IR-UV DR spectra determine that the final vibronic levels should be of gerade symmetry. UV spectra are obtained by IR excitation of particular rotational levels of the $(10300)^\circ$ vibrational eigenstate that terminate in ungerade vibronic levels. Odd-numbered changes of J are also observed in these spectra. We have verified that the mechanism is an intramolecular process. One possibility is collision-induced dynamical symmetry breaking involving transitions between ``+” and ``-” local-mode states which spoil the g/u symmetry. We also consider whether a/s nuclear-spin interchange Symmetry can be involved.
Description
$^{h}$M.A. Payne, A. P. Milce, M. J. Frost and B. J. Orr. {Chem. Phys. Lett}. \textbf{265}, 244 (1997).
Author Institution: School of Chemistry and Centre for Lasers and Applications, Macquarie University
Author Institution: School of Chemistry and Centre for Lasers and Applications, Macquarie University