ROTATIONAL AND VIBRATIONAL ANALYSIS OF SOME LOW-LYING BENDING POLYADS IN THE $\tilde{A}$ $^1A_{u}$ STATE OF ACETYLENE, C$_2$H$_2$

Author Institution: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Chemical and Biological Sciences, Japan Women's University, Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan; 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 Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139A number of new low-lying vibrational levels of the $\tilde{A}$ $^1A_{u}$ state of acetylene have been identified, following infrared-ultraviolet double resonance experiments via the \nub{3} ($\Sigma^{+}_{u}$) and \nub{3}+\nub{4} ($\Pi_{u}$) 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 (\nub{4}) and the in-plane bend (\nub{6}), which are nearly degenerate and have been shown to be strongly coupled by $a$- and $b$-axis Coriolis interactions} \textbf{95}, 2742 (1993).}. 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 $K'_{a} \neq 0$ levels are affected by severe $a$-axis Coriolis coupling, which complicates vibrational assignment. Spectra recorded from $\Pi_{u}$-symmetry vibrational intermediate states access the $K'_{a} = 0 $ levels that reveal the purely vibrational interactions. The combinations of \nub{3} and \nub{6} are highly anharmonic. Since theoretical calculations} \textbf{101}, 356 (1994).} indicate that the shape of the molecule at the $cis-trans$ isomerization barrier will be a half-linear structure obtained by simultaneous excitation of \nub{3} and \nub{6}, this is not unexpected. The effective \nub{6} interval in the highest assigned combination ($3^{5}6^{1}$) is found to have dropped to 60\% of the fundamental frequency, indicating that it must lie close to the barrier