Spectroscopy of Ethylenedione.

The long sought-after, intrinsically short-lived molecule ethylenedione (OCCO) was observed and investigated by anion photoelectron spectroscopy. The adiabatic electron affinity of its quasi-bound (3)Σ(g)(-) state is 1.936(8) eV. The vibrational progression with a 417(15) cm(-1) frequency observed within the triplet band corresponds to a trans-bending mode. Several dissociative singlet states are also observed, corresponding to two components of the (1)Δg state and the (1)Σ(g)(+) state. The experimental results are in agreement with theoretical predictions and constitute the first spectroscopic observation and characterization of this elusive compound.