ADAM H. STEEVES, HANS A. BECHTEL, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139; NAMI YAMAKITA, Department of Chemical and Biological Sciences, Japan Women's University, Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan; ANTHONY J. MERER, 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; AND ROBERT W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.
A number of new low-lying vibrational levels of the A 1Au state of acetylene have been identified, following infrared-ultraviolet double resonance experiments via the 3 ( +u) and 3+ 4 ( u) vibrational levels of the ground state, and high sensitivity one photon laser-induced fluorescence experiments with jet-cooled samples. These new levels involve the two lowest frequency vibrations, the torsion ( 4) and the in-plane bend ( 6), which are nearly degenerate and have been shown to be strongly coupled by a- and b-axis Coriolis interactions. The most prominent bands in spectra recorded from the ground vibrational state or via \ell''=0 vibrational intermediates go to K'a = 1 levels of the upper state (following the K'a - \ell'' = \pm 1 selection rule for the transition), however data from the K'a 0 levels are affected by severe a-axis Coriolis coupling, which complicates vibrational assignment. Spectra recorded from u-symmetry vibrational intermediate states access the K'a = 0 levels that reveal the purely vibrational interactions.
The combinations of 3 and 6 are highly anharmonic. Since theoretical calculations indicate that the shape of the molecule at the cis-trans isomerization barrier will be a half-linear structure obtained by simultaneous excitation of 3 and 6, this is not unexpected. The effective 6 interval in the highest assigned combination (3561) is found to have dropped to 60% of the fundamental frequency, indicating that it must lie close to the barrier.