Double Photoionization of CO2 Molecules in the 34-50 eV Energy Range

The double photoionization of CO2 molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO22+ molecular dication, (ii) the production of a metastable (CO22+)* that dissociates, with an apparent lifetime of 3.1 ?s, giving rise to CO+ and O+ ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 ?s. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO2, for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO+ together with a neutral autoionizing oxygen atom.