Trans–cis isomerization of acetylene in the à 1Au state as studied by dispersed fluorescence spectroscopy

Laser-induced fluorescence and resonance enhanced multiphoton ionization spectra were measured in the energy region of the V6K1 level of acetylene à 1Au. The dispersed fluorescence spectra of the near-lying two levels observed in the excitation spectra show significantly different spectral patterns: One is characterized by the trans-CH bending and CC stretching progressions, while the other one seems to be based on the cis-CH bending progression. We propose a plausible interaction model that the zero-order V6K1 level interacts with the dark level including the cis-CH bending mode through anharmonic resonance, since these two levels are located above the isomerization barrier.