IDENTIFICATION OF THE LOWEST-LYING BENDING VIBRATIONAL LEVELS OF THE $\tilde{A}$ ${^1A_{u}}$ STATE OF ACETYLENE, C$_{2}$H$_{2}$
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Date
2007
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Publisher
Ohio State University
Abstract
Acetylene is linear in its ground state, but becomes \textit{trans}-bent in its $\tilde{A}$ ${^1A_u}$ state. The vibrational structure of the $\tilde{A}\leftarrow\tilde{X}$ system consists}, \textbf{95}, 101 (1982).} of long progressions in the $\nu_{3}'$ (\textit{trans}-bending) vibration based on the origin and the $\nu_{2}'$ (C-C stretch) fundamental. These progressions become irregular at a vibrational energy of about 5000 \wn, with many extra bands appearing}, \textbf{111}, 185 (1985).}. Spectra of jet-cooled acetylene showed recently}, \textbf{101}, 663 (2003).} that extra bands can also be found at much lower energy. Two of the extra levels were identified as the $a_{g}$ combinations 1$^{1}$3$^{1}$ and 2$^{2}$ 3$^{1}$, but the other levels are combinations involving the two low frequency bending vibrations, \nub{4} (torsion) and \nub{6} (in-plane bending). Two types of high resolution spectra have been recorded in order to assign these extra levels. Infrared-ultraviolet double resonance spectra via the $\tilde{X}$, \nub{3} level have identified large numbers of \textit{ungerade} vibrational levels at energies of 2000-7000 \wn, while high sensitivity laser-induced fluorescence spectra of jet-cooled acetylene have given the positions of many low-lying \textit{gerade} levels. The low-lying bending levels are affected by strong \textit{a}-axis Coriolis coupling, which distorts their \textit{K}-structures considerably. In addition, there is very strong Darling-Dennison resonance between \nub{4} and \nub{6}, with $k_{4466} \approx 50$ \wn. Nevertheless a good fit to the structures of the pure bending polyads can be obtained, allowing for both the Darling-Dennison resonance and the Coriolis coupling.
Description
Author Institution: Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10617, Department of Chemistry, University of British Columbia, 6174 University Boulevard, Vancouver, B.C., Canada V6T 1Z3; Department of Chemical and Biological Sciences, Japan Women's University, Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan; Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139