Dissociative double photoionization of CO2 molecules in the 36–49 eV energy range: angular and energy distribution of ion products

Abstract

Dissociative double photoionization of CO₂, producing CO⁺ and O⁺ ions, has been studied in the 36–49 eV energy range using synchrotron radiation and ion–ion coincidence imaging detection. At low energy, the reaction appears to occur by an indirect mechanism through the formation of CO⁺ and an autoionizing state of the oxygen atom. In this energy range the reaction leads to an isotropic distribution of products with respect to the polarization vector of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO₂²⁺ dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO⁺ and O⁺ product ions separate predominantly with a total kinetic energy between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a kinetic energy between 5 and 6 eV is dominant.