First observation of the 3³Πg state of C2: Born-Oppenheimer breakdown.

The 3³Π_gg state of the dicarbon molecule, C₂, has been identified for the first time by a combination of resonant ionization spectroscopy, mass spectrometry, and high-level ab initio quantum chemical calculations. This marks the discovery of the final valence triplet state of C₂ spectroscopically accessible from the lowest triplet state. It is found to be vibronically coupled to the recently discovered 4³Π_gg state, necessitating vibronic calculations beyond the Born-Oppenheimer approximation to reconcile calculated rotational constants with observations. The 3³Π_gg state of C₂ is observed to have a much shorter fluorescence lifetime than expected, possibly pointing to predissociation by coupling to the unbound d³Π_gg state. [ABSTRACT FROM AUTHOR]