IR-UV Double Resonance Spectrum of Acetylene Below and Above the Predissociation Threshold

The A1Au ← X ˜ 1 ∑ g + electronic transition of the vibrationally excited acetylene molecule was studied by IR–UV double resonance spectroscopy in gas and in a supersonic jet. The C–H antisymmetric stretching vibration ν CH ant in the A state was clearly observed when the molecule was excited to the ν CH sym + ν CH ant combination vibration in the X ˜ state by the IR laser. When the ν CH ant fundamental vibration was excited, the C–H in-plane cis-bending vibration νcis(in) in the A state was observed strongly, while ν CH ant almost disappeared. The difference was interpreted in terms of Fermi resonance of the ν CH ant fundamental vibration in X ˜ . The predissociation threshold was newly determined to be 46,439˼46,673 cm-1 (133.11 ± 0.33 kcal/mol). In the region above the predissociation threshold, strong vibrational mixing was found. The higher members of the progression of the trans-bending vibration starting from ν CH ant were assigned. It was suggested that the nonradiative relaxation accelerated in the region above 51,744 cm-1.