The direct observation, assignment, and partial deperturbation of the ν4 and ν6 vibrational fundamentals in A 1A u acetylene (C2H2)

A pulsed-laser double resonance technique provides previously unavailable spectroscopic data on the rovibrational structure of à 1Au acetylene (C2H2). Our assignment and analysis of transitions to the à state ν4′ (torsion) and ν6′ (antisymmetric in-plane bend) vibrational fundamentals uncovers a strong Coriolis interaction between these two nearly degenerate modes and weaker Coriolis interactions between the ν4′/ν6′ pair and remote à state rovibrational levels. We deperturb the direct Coriolis interaction between ν4′ and ν6′ to obtain vibrational frequencies, Coriolis coupling constants and partially deperturbed rotational and centrifugal distortion constants for these previously unobserved fundamentals. Parity selection rules for the Ã←X̃ band permit an unambiguous assignment of the vibrations (ν4′=764.9±0.1 cm−1 and ν6′=768.3±0.2 cm−1). We use these new experimental values to reassign several à state vibrations and to assign previously unidentified à state levels. We also identify two vibrational resonances that seem to be important in determining the rovibrational structure of à 1Au C2H2.