15N++CD4 and O++13CO2 state-selected ion-molecule reactions relevant to the chemistry of planetary ionospheres

The dissociative photoionization of N-2 and O-2 by synchrotron radiation in coincidence with threshold photoelectrons is used to produce state-selected N+ and O+ atomic ions to study their reactivity. A pure selection of their ground state, N+(P-3) and O+(S-4), or excited states, N+(D-1), O+(D-2), and O+(P-2), is obtained by the choice of the photon energy and by further discrimination of atomic ions produced with translational recoil energy. Both reactions studied, N-15(+) + CD4 and O+ + (CO2)-C-13, are of major importance for the chemistry of Titan, Mars, and Venus' ionospheres and are strongly affected by excitation of the parent atomic ion. For the reaction of N+ with methane, DCN+ and DCND+ products coming from the decomposition of a long-lived complex are surprisingly not much sensitive to the N+ excitation, whereas the branching ratio between the dissociative charge-transfer channel, leading to CD3+, which is the main product for the ground-state reaction, and the nondissociative charge-transfer channel, leading to CD4+, is completely inverted in favor of the latter when N+ is excited into the D-1 state. This unanticipated result can be well understood by the spin-orbit selection rule in the N+ recombination. For the reaction of O+ with carbon dioxide, the reactive channel producing O-2(+), which dominates for the ground-state reaction for thermal collision energies, is completely displaced in favor of the endothermic charge-transfer channel leading to CO2+ if either collision energy or O+ internal energy is brought to the system. The O+(P-2) metastable state has a larger reaction cross section than the lower D-2 metastable state. Owing to the long lifetime of the N+ and O+ metastable states studied here and to their very specific reactivity, they should be individually considered in the models describing the planetary ionospheric chemistry.