Recombination of simple molecular ions studied in storage ring: dissociative recombination of H2O+

Reactivity of quasi-free electrons toward N3- and its impact on H2 formation mechanism in water radiolysis

Picosecond pulse radiolysis measurements were employed to assess the effectiveness of N3- in scavenging quasi-free electrons in aqueous solutions. The absorption spectra of hydrated electrons were recorded within a 100 ps timeframe across four distinct solutions with N3- concentrations of 0.5, 1, 2, and 5 M in water. The results revealed a concentration-dependent shift in the maximum absorption spectra of fully solvated electrons. Notably, at 5 M concentration, the maximum absorption occurred at 670 nm, in contrast to 715 nm observed for water. Intriguingly, the formation yield of hydrated electrons within the initial 5 ps electron pulse remained unaffected, showing that, even at a concentration of 5 M, N3- does not effectively scavenge quasi-free electrons. This is in disagreement with conclusions from stochastic models found in the literature. This observation has an important impact on understanding the mechanism of H2 formation in water radiolysis, which we discuss briefly here.

Resonant Fragmentation of the Water Cation by Electron Impact: a Wave-Packet Study

We have investigated the dissociation of a resonant state that can be formed in low energy electron scattering from H2 O+ . We have chosen the second triplet resonance above theB ˜ 2 A ' ${{{\tilde{\rm {B}}}}\;^2 {\rm{A{^\prime}}}}$ ( B ˜ 2 B 2 ) ${{\rm{(\tilde{B}}}\;^2 {\rm{B}}_2 )}$ state of H2 O+ whose autoionization mainly produces H2 O+ (X ˜ 2 A ' ' ${{{\tilde{\rm {X}}}}\;^2 {\rm{A{^\prime}{^\prime}}}}$ ). We have considered both dissociation of the resonant state itself, dissociative recombination (DR), or the dissociation of the H2 O+ cation after autodetachment, dissociative excitation (DE). The time-evolution of a wave packet on the potential energy surfaces of the resonance and cationic states shows, for the initial conditions studied, that the probability for DR is about 38 % while the probability for DE is negligible.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Dissociative recombination of HCl+, H2Cl+, DCl+, and D2Cl+ in a flowing afterglow

Dissociative recombination of electrons with HCl⁺, H₂Cl⁺, DCl⁺, and D₂Cl⁺ has been measured under thermal conditions at 300, 400, and 500 K using a flowing afterglow-Langmuir probe apparatus. Measurements for HCl⁺ and DCl⁺ employed the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, while those for H₂Cl⁺ and D₂Cl⁺ employed both VENDAMS and the more traditional technique of monitoring electron density as a function of reaction time. At 300 K, HCl⁺ and H₂Cl⁺ recombine with k_DR = 7.7±_2.1^4.5 × 10^-8 cm³ s^-1 and 2.6 ± 0.8 × 10^-7 cm³ s^-1, respectively, whereas D₂Cl⁺ is roughly half as fast as H₂Cl⁺ with k_DR = 1.1 ± 0.3 × 10^-7 cm³ s^-1 (2σ confidence intervals). DCl⁺ recombines with a rate coefficient below the approximate detection limit of the method (≲5 × 10^-8 cm³ s^-1) at all temperatures. Relatively slow dissociative recombination rates have been speculated to be responsible for the large HCl⁺ and H₂Cl⁺ abundances in interstellar clouds compared to current astrochemical models, but our results imply that the discrepancy must originate elsewhere.

Chemistry in glow discharges of H2 / O2 mixtures. Diagnostics and modelling

The chemistry of low pressure H2 + O2 discharges with different mixture ratios has been studied in a hollow cathode DC reactor. Neutral and positive ion distributions have been measured by mass spectrometry, and Langmuir probes have been used to provide charge densities and electron temperatures. A simple zero order kinetic model including neutral species and positive and negative ions, which takes into account gas-phase and heterogeneous chemistry, has been used to reproduce the global composition of the plasmas over the whole range of mixtures experimentally studied, and allows for the identification of the main physicochemical mechanisms that may explain the experimental results. To our knowledge, no combined experimental and modelling studies of the heavy species kinetics of low pressure H2 + O2 plasmas including ions has been reported before. As expected, apart from the precursors, H2O is detected in considerable amounts. The model also predicts appreciable concentrations of H and O atoms and the OH radical. The relevance of the metastable species O(1D) and O2(a1Δg) is analysed. Concerning the charged species, positive ion distributions are dominated by H3O+ for a wide range of intermediate mixtures, while H3+ and O2+ are the major ions for the higher and lower H2/O2 ratios, respectively. The mixed ions OH+, H2O+ and HO2+ are also observed in small amounts. Negative ions are shown to have a limited relevance in the global chemistry; their main contribution is the reduction of the electron density available for electron impact processes.

When electrons meet molecular ions and what happens next: dissociative recombination from interstellar molecular clouds to internal combustion engines

The interaction of matter with its environment is the driving force behind the evolution of 99% of the observed matter in the universe. The majority of the visible universe exists in a state of weak ionization, the so called fourth state of matter: plasma. Plasmas are ubiquitous, from those occurring naturally; interstellar molecular clouds, cometary comae, circumstellar shells, to those which are anthropic in origin; flames, combustion engines and fusion reactors. The evolution of these plasmas is driven by the interaction of the plasma constituents, the ions, and the electrons. One of the most important subsets of these reactions is electron-molecular ion recombination. This process is significant for two very important reasons. It is an ionization reducing reaction, removing two ionised species and producing neutral products. Furthermore, these products may themselves be reactive radical species which can then further drive the evolution of the plasma. The rate at which the electron reacts with the ion depends on many parameters, for examples the collision energy, the internal energy of the ion, and the structure of the ion itself. Measuring these properties together with the manner in which the system breaks up is therefore critical if the evolution of the environment is to be understood at all. Several techniques have been developed to study just such reactions to obtain the necessary information on the parameters. In this paper the focus will be on one the most recently developed of these, the Ion Storage Ring, together with the detection tools and techniques used to extract the necessary information from the reaction.

Dissociative recombination of the deuterated acetaldehyde ion, CD3CDO+: product branching fractions, absolute cross sections and thermal rate coefficient

Dissociative recombination of the deuterated acetaldehyde ion CD3CDO(+) has been studied at the heavy-ion storage ring CRYRING, located at the Manne Siegbahn Laboratory, Stockholm, Sweden. Product branching fractions together with absolute DR cross-sections were measured. The branching fractions were determined at a relative collision energy between the ions and the electrons of approximately 0 eV. With a probability of 34% the DR events resulted in no ruptures of bonds between heavy atoms (i.e. no breakage of the C-C bond or the C[double bond, length as m-dash]O bond). In the remaining 66% of the events one of the bonds between the heavy atoms was broken. The energy-dependent cross-section for the DR reaction was measured between approximately 0 and 1 eV relative kinetic energy. In the energy region between 1 meV and 0.2 eV the absolute cross section could be fitted by the expression sigma(E) = 6.8 x 10(-16)E(-1.28) cm(2), whereas in the energy interval between 0.2 and 1 eV the data were best fitted by sigma(E) = 4.1 x 10(-16)E(-1.60) cm(2). From these cross section data the thermal rate coefficient (as a function of the electron temperature), alpha(T) = 9.2 x 10(-7) (T/300)(-0.72) cm(3) s(-1) was obtained.

Electron molecular ion recombination: product excitation and fragmentation

Electron-ion dissociative recombination is an important ionization loss process in any ionized gas containing molecular ions. This includes the interstellar medium, circumstellar shells, cometary comae, planetary ionospheres, fusion plasma boundaries, combustion flames, laser plasmas and chemical deposition and etching plasmas. In addition to controlling the ionization density, the process generates many radical species, which can contribute to a parallel neutral chemistry. Techniques used to obtain rate data and product information (flowing afterglows and storage rings) are discussed and recent data are reviewed including diatomic to polyatomic ions and cluster ions. The data are divided into rate coefficients and cross sections, including their temperature/energy dependencies, and quantitative identification of neutral reaction products. The latter involve both ground and electronically excited states and including vibrational excitation. The data from the different techniques are compared and trends in the data are examined. The reactions are considered in terms of the basic mechanisms (direct and indirect processes including tunneling) and recent theoretical developments are discussed. Finally, new techniques are mentioned (for product identification; electrostatic storage rings, including single and double rings; Coulomb explosion) and new ways forward are suggested.

Hydrated Electron Yields in the Heavy Ion Radiolysis of Water

Experimental measurements coupled with Monte Carlo track simulations have been used to examine the yields of hydrated electrons in the radiolysis of water with protons, helium ions, and carbon ions. Glycylglycine, in concentrations ranging from 10(-4) to 1 M, was employed as a scavenger and the production of the ammonium cation used as a probe of hydrated electron yields from about 2 ns to 20 mus. Monte Carlo track simulations employing diffusion-kinetic calculations of product yields are found to reproduce experimental observations satisfactorily. Model details are used to elucidate the heavy ion track physics and chemistry. Comparison of the heavy ion results with those found in gamma radiolysis shows intratrack reactions are significant on the nanosecond to microsecond time scale as the ion track relaxes, and that a constant (escape) yield is never attained on this time scale. Numerical interpolation techniques are used to obtain both track average and track segment yields for use in practical applications or comparison with other models. The model results give the first hints that initial ( approximately 5 ps) hydrated electron yields, and possibly other water decomposition products, are dependent on the type and energy of the incident radiation.

Investigating the breakup dynamics of dihydrogen sulfide ions recombining with electrons

This paper presents results concerning measurements of the dissociative recombination (DR) of dihydrogen sulfide ions. In combination with the ion storage ring CRYRING an imaging technique was used to investigate the breakup dynamics of the three-body channel in the DR of 32SD2(+). The two energetically available product channels S(3P) + 2D(2S) and S(1D) + 2D(2S) were both populated, with a branching fraction of the ground-state channel of 0.6(0.1). Information about the angle between the two deuterium atoms upon dissociation was obtained together with information about how the available kinetic energy was distributed between the two light fragments. The recombination cross sections as functions of energy in the interval of 1 meV to 0.3 eV in the center-of-mass frame are presented for 34SH2(+). The thermal rate coefficient for the DR of 34SH2(+) was determined to be (4.8+/-1.0) x 10(-7)(T/300)(-0.72+/-0.1) cm3 s(-1) over this interval.

Dynamics of three-body breakup in dissociative recombination: H(2)O(+)

To better understand the propensity for the three-body breakup in dissociative recombination (DR) of dihydrides ( H(3)(+), NH(2)(+), CH(2)(+), and H(2)O(+)), we undertook a study of the dynamics of this process. A study of DR of H(2)O(+) to give O + H + H was carried out at the CRYRING Heavy-Ion Storage Ring in Stockholm. With the stored beam energy of 4.5 MeV, we separated the O signal from the H signals with a differential absorber, thus reducing the problem to a sum of two two-body problems. Results included (1) the ratio of O((3)P) to O((1)D) product, (2) the distribution of recoil-kinetic energy between the two hydrogen atoms, (3) the angular distribution between the hydrogen atoms in the O((3)P) channel and in the O((1)D) channel.