Observation of photo-double ionization of carbon monoxide below the adiabatic double-ionization potential by threshold-photoelectron-photoelectron coincidence spectroscopy

The production of C+ and O+ ion pairs in carbon monoxide at photon energies below the adiabatic double-ionization threshold of 41.25 eV has been probed in a threshold photoelectron-photoelectron coincidence (TPEPECO) experiment using tunable VUV radiation and a sensitive electron spectrometer. The TPEPECO spectra provide evidence of C+ + O+ production that does not involve creation and dissociation of a molecular dication, but instead results from complete dissociation of a molecular cation followed by autoionization of the atomic oxygen fragment. Furthermore, an electron-electron coincidence signal has been detected at photon energies as low as 36.5 eV, well below the previously measured onset for C+ + O+ production.