Study of the low-lying electronic states of CCO by photoelectron spectroscopy of CCO− and ab initio calculations

The low-lying electronic states of CCO have been investigated by photoelectron spectroscopy of CCO− at wavelengths of 266 and 355 nm in conjunction with ab initio calculations. Photodetachment is observed to occur to the X̃ 3Σ−, Ã 3Π, ã 1Δ, and b̃ 1Σ+ electronic states of CCO. This marks the first observation of the low-lying singlet states. A revised value for the electron affinity of CCO is found to be 2.289±0.018 eV. These results are compared with CASPT2 ab initio calculations of the energetics and structure of the ground and excited states of CCO and CCO−. Using the measured electron affinity of CCO, the heats of formation Δf H0298(CCO)=3.99±0.20 eV and ΔfH0298(CCO−)=1.67±0.20 eV are determined. In addition, the C–C bond dissociation energies in CCO and CCO− are determined, as well as the H–CCO bond energy in HCCO.