IR-UV DOUBLE-RESONANCE SPECTRUM OF ACETYLENE BELOW AND ABOVE THE PREDISSOCIATION THRESHOLD

The A1Au ← X1∑g+ electronic transition of the vibrationally excited acetylene molecule was studied by IR-UV double resonance spectroscopy in gas an in a supersonic jet. The C-H antisymmetric stretching vibration vVH ant in the A states was clearly observed when the molecule was excited to the cCHsym + vCHant combination vibration in the X state by the IR laser. When the vCHant fundamental vibration was excited, the C-H in-plane cis-bending vibration vcis(in) in the A state was observed strongly, while vCHant almost disappeared. The difference was interpreted in terms of Fermi resonance of the vCHant 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 vCHant were assigned. It was suggested that the nonradiative relaxation accelerated in the region above 51,744 cm-1.