¹⁵N⁺ + CD₄ and O⁺ + ¹³CO₂ State-Selected Ion−Molecule Reactions Relevant to the Chemistry of Planetary Ionospheres

Abstract

The dissociative photoionization of N2 and O2 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+(3P) and O+(4S), or excited states, N+(1D), O+(2D), and O+(2P), is obtained by the choice of the photon energy and by further discrimination of atomic ions produced with translational recoil energy. Both reactions studied, 15N+ + CD4 and O+ + 13CO2, 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 1D 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 O2 +, 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+(2P) metastable state has a larger reaction cross section than the lower 2D 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.

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…The values of the branching ratios determined by McEwan et al (1998) are in agreement with the previous studies. The measurements by Alcaraz et al (2004), at higher collision energy (E CM = 0.36 eV) and with isotopic labeling, give similar results, except for slight differences for CH + 4 and HCN + . This study indicates that there is no major isotopic effect for this reaction, so the branching ratios are the same for the reactions with CH 4 and CD 4 .…”

“…The rate constant for the reaction with CD 4 has been measured by Alcaraz et al (2004) to be, within experimental uncertainties, equal to the Langevin rate (1.30 × 10 −9 cm 3 s −1 , calculated with the polarization of CD 4 which is lower by 0.034 than the one from CH 4 according to Bell 1942 andWong et al 1991), so it is our recommended value. Rowe et al (1985) studied this reaction at a low temperature, 8 K, and found a value of the rate constant slightly lower than the value at 300 K. Similarly, Alcaraz et al (2004) studied the reaction at collision energies higher than 0.2 eV (i.e., T > 1500 K) and found a higher value for the rate constant. These studies seem to confirm an increase of the rate constant with temperature or with collision energy.…”

“…Kinetics: Recommended value at 300 K: k = 1.15 × 10 −9 cm 3 s −1 (±20%) Tichy et al (1979) and Alcaraz et al (2004) measured the rate constant of the N + ( 1 D) + CH 4 reaction (see Table 6). Both studies showed that the rate constant values are similar to their respective values found for the reaction of the N + ( 3 P) ground state and close to the Langevin rate within experimental uncertainties.…”

An ICR study of ion-molecules reactions relevant to titan’s atmosphere: An investigation of binary hydrocarbon mixtures up to 1 Micron

“…The principal ionization sources on Titan are solar photoionization and electron impact of nitrogen particularly when Titan resides within the magnetosphere of Saturn. Some of these ions are generated with excess energy [17], and the methods we have employed here have examined the effect of additional energy present in secondary hydrocarbon ions before collisional stabilization occurs. Of the neutral reactants, methane is the most abundant component for reaction with nitrogen ions where it forms a range of hydrocarbon and nitrile ions which then react with other hydrocarbons [18].…”

Modelling the Venusian airglow

“…However if such a barrier exists, the reaction can become important if one of the reactant has kinetic energy. Indeed, many experiments show that the reaction rates are drastically modified when the reactants have even a very low kinetic energy (Alcaraz et al 2004). As N comes from the dissociation of N 2 it could easily have kinetic energy, and several processes contribute to transfer kinetic energy to the ions in the planetary ionospheres.…”