Zdenek Herman

Dynamics of charge transfer and chemical reactions of doubly-charged ions at low collision energies

Abstract Results of beam scattering studies on the dynamics of single charge transfer and chemical reactions of doubly-charged ions carried out in Prague are reviewed. Investigation of several atomic ion-atom charge transfer processes at collision energies 0·1-10 eV provides data on differential and relative total cross-sections of state-to-state processes. Populations of electronic and vibrational states of molecular product ions are obtained from experiments on molecular dication-atom and atomic ion (He2+)-molecule systems. Studies of chemical reactions of the dication CF2+ 2 with D2 show that Coulomb repulsion between the products (CF2D+ + D+) governs the dynamics and energy partitioning.

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).

Luminescent charge transfer in a beam of CO++ colliding with Ar, N2, H2, D2 and CO

Luminescent charge transfer of CO++(3Π,1Σ+) ions in a beam with several atomic and molecular target species in a scattering cell was observed at 60–2000 eVlab projectile energies. Emission from CO+(B 2Σ+→X 2Σ+) was spectrally analyzed with a resolution of up to 1 A full width at half maximum. In the case of CO+++CO collisions, CO+(B) emission due to electron capture by the projectile and to target ionization were distinguished using isotopic substitution. CO+(A 2Π→X 2Σ+) emission from the ionized target was also observed in this reaction. Similarly, with N2 as a target, strong N+2(B 2Σ+u→X 2Σ+g) emission was observed. The absolute cross sections for CO+(B) formation are 1–5 A2. The CO+(B) vibrational excitation at 2000 eV can be explained by Franck–Condon transitions from the CO++(3Π)/(1Σ+) beam components in the ratio 1:2. At low energy the product CO+ vibrational excitation is higher, especially from collisions with Ar. The vibrational population distribution differs from that derived from earlier trans...

Dynamics of low-energy charge-transfer processes: Ar2+ + He → Ar+ + He+ AT eV collision energies

The dynamics of the single-charge-transfer processes between Ar 2+ ( 3 P, 1 D) and He( 1 S) were studied in a crossed-beam experiment in the collision-energy range 0.5 – 1.6 eV. Scattering diagrams of the Ar + product show a significant difference in the relative differential cross section distributions for processes with Ar 2+ in the 3 P and 1 D states. This behaviour may be accounted for by assuming that the non-adiabatic transition occurs predominantly either before or after the system reaches the classical turning point. Ratios 3 P/ 1 D of the total cross sections of the two processes were estimated to be 0.32 and 0.83 at collision energies of 0.53 eV and 1.62 eV, respectively.

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.

A SELECTED-ION-FLOW-DRIFT-TUBE STUDY OF CHARGE TRANSFER PROCESSES BETWEEN ATOMIC, MOLECULAR, AND DIMER ION PROJECTILES AND POLYATOMIC MOLECULES ETHANE , PROPANE, AND N-BUTANE

Charge transfer processes of Ar+, Kr+, Xe+, N2+, CO+, Ar2+, Kr2+, and N4+ with the alkanes ethane, propane, and n-butane were investigated using the selected-ion-flow-drift-tube (SIFDT) technique. The relative abundances of molecular and fragment hydrocarbon product ions can be explained using the breakdown pattern of the hydrocarbon molecular ion in question and assuming that the recombination energy of the projectile ions is deposited in the quasi continuum of the energy levels of the polyatomic molecular ions in a resonant way. The observed increase of the fragment ion yields with increasing collision energy is due to collision induced excitation up to the dissociation limit of the hydrocarbon ions in collisions with the helium buffer gas atoms rather than to inelastic charge transfer from hyperthermal projectile ions.

Dynamics of chemical reactions of ions from beam scattering and state-selected studies

Information on the dynamics of ion–molecule reactions has been obtained mostly from beam scattering, product internal state analysis by spectroscopic methods and from state-selection of reactants. A combination of beam-scattering studies and state selection of reactants experiments aided better description of several exoergic and endoergic reactions. Results obtained by both methods on the reactions CH+4(CH4, CH3)CH+5, H+2(He, H)HeH+ and H+2(Ne, H)NeH+ are discussed.

Metastable decay of oxygen cluster cations and anions

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (up to n≈ 70) have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focusing sector field mass spectrometer (reversed geometry) at two different time windows after ion formation (in the metastable time region). Positive and negative mass spectra exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate in both cases with corresponding small and large metastable fractions (for the loss of O2) determined in the first field-free region, respectively. In accordance with the predictions of Klots, the probability of losing one monomer from (O2)+n exhibits a non-exponential dependence on time since ion formation. Metastable fractions determined for (O2)–n ions produced with ca. 0 eV electrons are generally larger than those for ions produced with ca. 7 eV electrons. This observation is due to a two-step dissociative production process of the latter ions via the 2πu state leading to less energetic ions than in case of direct attachment of zero-energy electrons. Furthermore, non-stoichiometric ions (O2)nO+ and (O2)nO– not only decay by O2 evaporation but also by O evaporation (and higher homologues in case of the positive ions). Branching ratios for the different decay channels observed may be rationalized in terms of energy considerations. Finally, strong autodetachment has been observed in case of small O2 cluster anions. Autodetachment lifetimes appear to increase with increasing cluster size.

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*.

Production and stability of oxygen cluster cations and anions, revisited

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O2)n+ and (O2)n− ions, respectively. (O2)n+ ions may also lose up top=(n−1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O2)n− ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O2)n− ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O2 clusters.