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 3Σg− 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 1Δg state and the 1Σg+ state. The experimental results are in agreement with theoretical predictions and constitute the first spectroscopic observation and characterization of this elusive compound. For over a century, the mysterious compound OCCO has been pursued by chemists, but all past studies have failed to provide conclusive experimental evidence for its existence. The quasi‐bound triplet and dissociative singlet states of OCCO were now observed and spectroscopically characterized for the first time using anion photoelectron spectroscopy (eBE=electron binding energy).