Feshbach states of the propadienylidene anion H2CCC

The absorption spectra of the propadienylidene anions, H2CCC− and D2CCC−, in the visible and near-UV range were recorded by photodetachment spectroscopy. The lowest excited state is the well characterized dipole bound A2A1 showing bound levels and vibrational Feshbach resonances. The higher lying electronic states are Feshbach resonances 0.5–2 eV above the electron detachment threshold and are assigned to the 2A″, 2A1 and 2B1←2B1 electronic transitions by means of ab initio calculations. The autodetachment rate increases with energy. A comparison of the laboratory data with diffuse interstellar bands shows that not only the four strong transitions of the A 2A1←2B1 system but also several of the Feshbach resonances lie close to astronomical absorptions.