Abstract

The Ã1Au ← 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 νCHant in the à state was clearly observed when the molecule was excited to the νCHsym + νCHant combination vibration in the X˜ state by the IR laser. When the νCHant fundamental vibration was excited, the C–H in-plane cis-bending vibration νcis(in) in the à state was observed strongly, while νCHant almost disappeared. The difference was interpreted in terms of Fermi resonance of the νCHant 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 νCHant were assigned. It was suggested that the nonradiative relaxation accelerated in the region above 51,744 cm-1.