Darling–Dennison resonance and Coriolis coupling in the bending overtones of the ÃAu1 state of acetylene, C2H2

Rotational analyses have been carried out for the overtones of the ν4 (torsion) and ν6 (in-plane cis-bend) vibrations of the AAu1 state of C2H2. The v4+v6=2 vibrational polyad was observed in high-sensitivity one-photon laser-induced fluorescence spectra and the v4+v6=3 polyad was observed in IR-UV double resonance spectra via the ground state ν3 (Σ+u) and ν3+ν4 (Πu) vibrational levels. The structures of these polyads are dominated by the effects of vibrational angular momentum: Vibrational levels of different symmetry interact via strong a-and b-axis Coriolis coupling, while levels of the same symmetry interact via Darling–Dennison resonance, where the interaction parameter has the exceptionally large value K4466=−51.68cm−1. The K-structures of the polyads bear almost no resemblance to the normal asymmetric top patterns, and many local avoided crossings occur between close-lying levels with nominal K-values differing by one or more units. Least squares analysis shows that the coupling parameters change ...