Knowledge Bank

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 \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 $Ka\prime =1$ levels of the upper state (following the $Ka\prime -\ell ″=\pm 1$ selection rule for the transition), however data from the $Ka\prime \ne 0$ 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 ($3561$) is found to have dropped to 60% of the fundamental frequency, indicating that it must lie close to the barrier.