THE DIRECT OBSERVATION, ASSIGNMENT, AND PARTIAL DEPERTURBATION OF THE NU-4 AND NU-6 VIBRATIONAL FUNDAMENTALS IN (A)OVER-TILDE (1)A(U) ACETYLENE (C2H2)

A pulsed-laser double resonance technique provides previously unavailable spectroscopic data on the rovibrational structure of A 1A(u) acetylene (C2H2) . Our assignment and analysis of transitions to the A state nu4' (torsion) and nu6' (antisymmetric in-plane bend) vibrational fundamentals uncovers a strong Coriolis interaction between these two nearly degenerate modes and weaker Coriolis interactions between the nu4'/nu6' pair and remote A state rovibrational levels. We deperturb the direct Coriolis interaction between nu4' and nu6' 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 A <-- X band permit an unambiguous assignment of the vibrations (nu4' = 764.9 +/- 0.1 cm-1 and nu6' = 768.3 +/- 0.2 cm-1). We use these new experimental values to reassign several A state vibrations and to assign previously unidentified A state levels. We also identify two vibrational resonances that seem to be important in determining the rovibrational structure of A 1A(u) C2H2.