Jan Zabka

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

This paper is a detailed critical review of the production processes and reactions of N, N+, N+ 2, N++, and N++ 2 of relevance to Titans atmosphere. The review includes neutral, ion-molecule, and recombination reactions. The review covers all possible active nitrogen species under Titans atmospheric conditions, specifically N2 (A3Σ+ u), N (4 S), N (2 D), N (2 P), N+ 2, N+ (3 P), N+ (1 D), N++ 2, and N++ species, and includes a critical survey of the reactions of N, N+, N+ 2, N++, and N++ 2 with N2, H2, D2, CH4, C2H2, C2H4, C2H6, C3H8 and the deuterated hydrocarbon analogs, as well as the recombination reactions of N+ 2, N+, N++ 2, and N++. Production processes, lifetimes, and quenching by collisions with N2 of all reactant species are reviewed. The N (4 S) state is reactive with radicals and its reactions with CH2, CH3, C2H3, and C2H5 are reviewed. Metastable states N2(A3Σ+u), N (2 D), and N (2 P) are either reactive or quenched by collisions with the target molecules reviewed. The reactions of N+ (1 D) have similar rate constants as N+ (3 P), but the product branching ratios differ significantly. Temperature effects and the role of the kinetic energy content of reactants are investigated. In all cases, experimental uncertainties of laboratory data are reported or estimated. Recommended values with uncertainties, or estimated values when no data are available, are given for rate constants and product branching ratios at 300 K and at the atmospheric temperature range of Titan (150-200 K for neutral reactions and 150 K for ion reactions).

Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

Competition of electron transfer, dissociation, and bond-forming processes in the reaction of the CO22+ dication with neutral CO2

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --> 2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.

Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

Abstract The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion–molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O2, N2, NO, CO, Ar and CO2. This study shows the strong influence of electronic as well as vibrational internal energy on most ion–molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O2+ and NO+ ions in their a4Πu and a3Σ+ metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O2+ with NO as well as the reactions of CO2+ with NO, O2, CO and to a less extent N2 are driven by vibrational excitation. N2+ and CO+ reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO++M reactions as we expect that a significant proportion of these CO+ could be produced with translational energy by dissociation of doubly charged CO22+, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.

Selected ion flow tube study of ion-molecule reactions of N(+)(3P) and Kr+ with C3 hydrocarbons propane, propene, and propyne.

Reactions of (14)N(+)((3)P), (15)N(+)((3)P), and Kr(+) with propane, propene, and propyne were studied using the selected ion flow tube, SIFT, technique. Thermal rate constants in all N(+)/C(3) systems were k = (2 ± 0.4) × 10(-9) cm(3) molecule(-1) s(-1), close to the collisional rate constants. With propane and propene, only hydrocarbon ions were found among the products of reactions with N(+); in propyne about 15% of the products were N-containing ions (C(3)H(2)N(+), C(2)H(4)N(+), C(2)H(3)N(+), C(2)H(2)N(+)), and the rest were hydrocarbon ions. A comparison with product ions from electron transfer between Kr(+) (of recombination energy similar to that for N(+)((3)P)) and the C(3) hydrocarbons and further analysis of the results led to an estimation of an approximate ratio of electron transfer vs hydride-ion transfer reactions leading to the hydrocarbon product ions: in propane the ratio was 2:1, in propene 3:1, and in propyne 5:1. A fraction of product ions resulted from reactions leading to the excited neutral product N*.

Crossed-Beam Scattering Studies of Electron-Transfer Processes between the Dication CO22+ and Neutral CO2: Electronic States of Reactants and Products Involved

Crossed-beam scattering experiments were carried out at collision energies of 4.51 and 2.71 eV to elucidate the electronic states involved in the nondissociative and dissociative electron-transfer reactions observed following CO(2)(2+)/CO(2) collisions. Specifically, we focus on the observation that, in the dissociative electron-transfer reaction, forming CO(+), the majority of the CO(+) product ions are formed via electron capture by the CO(2)(2+) rather than via ejection of an electron from the neutral CO(2) reaction partner. The main channels resulting in nondissociative electron transfer are reactions of the ground (X(3)Sigma(g)(-)) and excited states of CO(2)(2+) to give different combinations of the ground and excited states of the product pair of CO(2)(+) ions in which the combination AA appears to be significant. The CO(+) ions appear mainly to arise from slow dissociation of CO(2)(+)(b(4)Pi(u)) formed following electron capture by the ground state of the dication reactant (X(3)Sigma(g)(-)), with possible contributions from electron capture by higher triplet excited states of the dication.

Correlations between Survival Probabilities and Ionization Energies of Slow Ions Colliding with Room-Temperature and Heated Surfaces of Carbon, Tungsten, and Beryllium†

Survival probabilities, S(a) (%), of hydrocarbon ions C1, C2, and C3 and several nonhydrocarbon ions (Ar(+), N(2)(+), CO(2)(+)) on room-temperature (hydrocarbon-covered) and heated (600 degrees C) surfaces of carbon (HOPG), tungsten, and beryllium were experimentally determined using the ion-surface scattering method for several incident energies from a few electronvolts up to about 50 eV and for the incident angle of 30 degrees (with respect to the surface). A simple correlation between S(a) and the ionization energy (IE) of the incident ions was found in the semilogarithmic plot of S(a) versus IE. The plots of the data at 31 eV were linear for all studied surfaces and could be fitted by an empirical equation log S(a) = a - b(IE). The values of the parameters a and b were determined for all investigated room-temperature and heated surfaces and can be used to estimate unknown survival probabilities of ions on these surfaces from their ionization energies.

Dynamics of formation of products D2CN+, DCN+, and CD3+ in the reaction of N+ with CD4: a crossed-beam and theoretical study.

The formation of D(2)CN(+) in the reaction of N(+) ((3)P) with CD(4) was studied using the crossed beam technique at collision energies of 3.66 and 4.86 eV. The experiments were complemented by calculations of stationary points on the triplet hypersurface of the system. The scattering data showed that the reaction proceeds by the formation of two intermediate complexes having different lifetimes: a long-lived statistical intermediate and a short-lived complex (mean lifetime about one period of an average rotation) with more energy in translation than the statistical complex. Comparison with theoretical calculations suggests that the long-lived complex leads the CDND(+) isomer of the product ion, whereas the short-lived complex leads prevailingly to the CD(2)N(+) isomer. The product DCN(+) results from further decomposition of the primary product D(2)CN(+), whereas CD(3)(+) is formed both by a hydride-ion transfer and a long-lived complex decomposition.

FT-ICR studies of anionic reactions for the chemistry of planetary ionospheres

The rate constants of several ion-molecule reactions involving CN− and C3N− anions and HC3N molecules have been measured in a FT-ICR spectrometer. These measurements are of particular importance for the chemistry of Titans ionosphere and interstellar medium.

Effect of the Vibrational Excitation of CH3+ cations on their reactivity with CH4

The VUV photoionisation of CH3 radicals formed in a beam by pyrolysis of CH3NO2 precursors is used to produce CH+3 cations and control their vibrational excitation. The reactivity of CH+3 cations with methane is then studied on a Guided Ion Beam setup as a function of both collision energy and CH+3 internal energy.