Dissociative recombination and excitation of N2+: Cross sections and product branching ratios

Modeling of collision-induced excitation and quenching of atomic nitrogen

Excited atomic nitrogen atoms play an important role in plasma formation in hypersonic shock-waves, as happens during spacecraft reentry and other high velocity vehicle applications. In this study, we have thoroughly studied collision induced excitation associated with two colliding nitrogen atoms in the N(4S), N(2D), and N(2P) states at collision energies up to 6 eV, using time-independent scattering calculations to determine cross sections and temperature-dependent rate coefficients. The calculations are based on potential curves and couplings determined in earlier multireference configuration interaction calculations with large basis sets, and the results are in good agreement with experiments where comparisons are possible. To properly consider the spin-orbit coupling matrix, we have developed a scaling method for treating transitions between different fine-structure components that only require calculations with two coupled states, and with this, we define accurate degeneracy factors for determining cross sections and rate coefficients that include all states. The results indicate that both spin-orbit and derivative coupling effects can play important roles in collisional excitation and quenching, and that although derivative coupling is always much stronger than spin-orbit, there are many transitions where only spin-orbit can contribute. As part of this, we identify two distinct pathways associated with N(2P) relaxation and one Auger-like mechanism leading to two N(2D) that could be important at high temperatures.

The associative ionization of N(2P) + O(3P)

The rate constant of the associative ionization reaction N(2P) + O(3P) → NO+ + e- was measured using a flow tube apparatus. A flowing afterglow source was used to produce an ion/electron plasma containing a mixture of ions, including N2+, N3+, and N4+. Dissociative recombination of these species produced a population of nitrogen atoms, including N(2P). Charged species were rejected from the flow tube using an electrostatic grid, subsequent to which oxygen atoms were introduced, produced either using a discharge of helium and oxygen or via the titration of nitrogen atoms with NO. Only the title reaction can produce the NO+ observed after the introduction of O atoms. The resulting rate constant (8 ± 5 ×10-11 cm3 s-1) is larger than previously reported N(2P) + O disappearance rate constants (∼2 × 10-11 cm3 s-1). The possible errors in this or previous experiments are discussed. It is concluded that the N(2P) + O(3P) reaction proceeds almost entirely by associative ionization, with quenching to the 2D or 4S states as only minor processes.

Recombination of vibrationally cold N2+ ions with electrons

Recombination of vibrationally cold N2+ ions with electrons was studied in the temperature range of 140-250 K. A cryogenic stationary afterglow apparatus equipped with cavity ring-down spectrometer and microwave diagnostics was utilized to probe in situ the time evolutions of number densities of particular rotational and vibrational states of N2+ ions and of electrons. The obtained value of the recombination rate coefficient for the recombination of the vibrational ground state of N2+ with electrons is αv=0 = (2.95 ± 0.50) × 10-7(300/T)(0.28±0.07) cm3 s-1, while that for the first vibrationally excited state was inferred as αv=1 = (4 ± 4) × 10-8 cm3 s-1 at 250 K.

Shock-Tube Measurements of Atomic Nitrogen Collisional Excitation in 8000-12000 K Partially Ionized Nitrogen-Argon Mixtures

We report on shock-tube experiments measuring the collisional excitation of atomic nitrogen using tunable diode laser absorption spectroscopy (TDLAS). Conditions behind the reflected shocks ranged from 8000 to 12000 K and 0.1 to 1.1 atm in mixtures of 1 or 2% molecular nitrogen (N₂) in argon (Ar). Absorption from the transition between atomic nitrogen quantum states 4P to 4D at 868 nm was used to monitor the formation of electronically excited nitrogen. Population measurements of the 4P state were made at a rate of 50 kHz. In connection with these measurements, a multitemperature kinetic mechanism is proposed for nitrogen excitation. Measurements suggest a multistage process. In early test times, a period of induction due to N₂ dissociation is followed by a rise via heavy particle excitation. The dominant channel causing this excitation is believed to be N + N ↔ N(4P) + N with a measured forward rate constant of 3.65 × 10^-18 exp(-119892/T) [m³/s]. As test time evolves, the excitation of 4P, referred to as N*, is subsequently interrupted and then resumes, as the kinetic environment becomes increasingly electron-dominated. The most impactful reactions of the mechanism are optimized to reduce the residual between simulations and the measurements. The measured N* populations indicate strong, though indirect, sensitivity to adjacent processes, including the excitation of metastable nitrogen, and ionization channels.

Electronic State Population Dynamics upon Ultrafast Strong Field Ionization and Fragmentation of Molecular Nitrogen

Air lasing from single ionized N_{2}^{+} molecules induced by laser filamentation in air has been intensively investigated and the mechanisms responsible for lasing are currently highly debated. We use ultrafast nitrogen K-edge spectroscopy to follow the strong field ionization and fragmentation dynamics of N_{2} upon interaction with an ultrashort 800 nm laser pulse. Using probe pulses generated by extreme high-order harmonic generation, we observe transitions indicative of the formation of the electronic ground X^{2}Σ_{g}^{+}, first excited A^{2}Π_{u}, and second excited B^{2}Σ_{u}^{+} states of N_{2}^{+} on femtosecond timescales, from which we can quantitatively determine the time-dependent electronic state population distribution dynamics of N_{2}^{+}. Our results show a remarkably low population of the A^{2}Π_{u} state, and nearly equal populations of the X^{2}Σ_{g}^{+} and B^{2}Σ_{u}^{+} states. In addition, we observe fragmentation of N_{2}^{+} into N and N^{+} on a timescale of several tens of picoseconds that we assign to significant collisional dynamics in the plasma, resulting in dissociative excitation of N_{2}^{+}.

Mutual neutralisation of O+ with O-: investigation of the role of metastable ions in a combined experimental and theoretical study

The mutual neutralisation of O⁺ with O^- has been studied in a double ion-beam storage ring with combined merged-beams, imaging and timing techniques. Branching ratios were measured at the collision energies of 55, 75 and 170 (± 15) meV, and found to be in good agreement with previous single-pass merged-beams experimental results at 7 meV collision energy. Several previously unidentified spectral features were found to correspond to mutual neutralisation channels of the first metastable state of the cation (O⁺(²D^o), τ ≈ 3.6 hours), while no contributions from the second metastable state (O⁺(²P^o), τ ≈ 5 seconds) were observed. Theoretical calculations were performed using the multi-channel Landau-Zener model combined with the anion centered asymptotic method, and gave good agreement with several experimentally observed channels, but could not describe well observed contributions from the O⁺(²D^o) metastable state as well as channels involving the O(3s ⁵S^o) state.

Significant Enhancement of N_{2}^{+} Lasing by Polarization-Modulated Ultrashort Laser Pulses

We show that the intensity of self-seeded N_{2}^{+} lasing at 391 nm, assigned to the B^{2}Σ_{u}^{+}(v^{'}=0)→X^{2}Σ_{g}^{+}(v^{''}=0) emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which N_{2} is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the X^{2}Σ_{g}^{+} state of N_{2}^{+} to the first electronically excited A^{2}Π_{u} state, leads to the depletion of the population in the X^{2}Σ_{g}^{+} state, and consequently, to the population inversion between the X^{2}Σ_{g}^{+} state and the B^{2}Σ_{u}^{+} state.

Radiation effects in solid nitrogen and nitrogen-containing matrices: fingerprints of N₄⁺ species

The radiation effects and relaxation processes in solid N2 and N2-doped Ne matrices, preirradiated by an electron beam, have been studied in the temperature range of 5-40 and 5-15 K, respectively. The study was performed using luminescence methods: cathodoluminescence CL and developed by our group nonstationary luminescence NsL, as well as optical and current activation spectroscopy methods: spectrally resolved thermally stimulated luminescence TSL and exoelectron emission TSEE. An appreciable accumulation of N radicals, N(+), N2(+) ions, and trapped electrons is found in nitrogen-containing Ne matrices. Neutralization reactions were shown to dominate relaxation scenario in the low-temperature range, while at higher temperatures diffusion-controlled reactions of neutral species contribute. It was conceived that in α-phase of solid N2, the dimerization reaction (N2(+) + N2 → N4(+)) proceeds: "hole self-trapping". Tetranitrogen cation N4(+) manifests itself by the dissociative recombination reaction with electron: N4(+) + e(-) → N2*(a'(1)Σ(u)(-)) + N2 → N2 + N2 + hν. In line with this assumption, we observed a growth of the a'(1)Σ(u)(-) → X(1)Σ(g)(+) transition intensity with an exposure time in CL spectra and the emergence of this emission in the course of electron detrapping on sample heating in the TSL and NsL experiments.

The vibrational dependence of dissociative recombination: rate constants for N2 (+)

Dissociative recombination rate constants are reported with electron temperature dependent uncertainties for the lowest 5 vibrational levels of the N2 (+) ground state. The rate constants are determined from ab initio calculations of potential curves, electronic widths, quantum defects, and cross sections. At 100 K electron temperature, the rate constants overlap with the exception of the third vibrational level. At and above 300 K, the rate constants for excited vibrational levels are significantly smaller than that for the ground level. It is shown that any experimentally determined total rate constant at 300 K electron temperature that is smaller than 2.0 × 10(-7) cm(3)/s is likely to be for ions that have a substantially excited vibrational population. Using the vibrational level specific rate constants, the total rate constant is in very good agreement with that for an excited vibrational distribution found in a storage ring experiment. It is also shown that a prior analysis of a laser induced fluorescence experiment is quantitatively flawed due to the need to account for reactions with unknown rate constants. Two prior calculations of the dissociative recombination rate constant are shown to be inconsistent with the cross sections upon which they are based. The rate constants calculated here contribute to the resolution of a 30 year old disagreement between modeled and observed N2 (+) ionospheric densities.

Dissociative recombination of CH4(+)

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

The vibrational dependence of dissociative recombination: cross sections for N2+

Theoretical ab initio calculations are reported of the cross sections for dissociative recombination of the lowest four excited vibrational levels of N2(+) at electron energies from 0.001 to 1.0 eV. Rydberg vibrational levels contributing to the cross section structures are identified as are dissociative channels contributing more than 10(-16) cm(2) to the total cross sections. In contrast to the prior study of v = 0 (S. L. Guberman, J. Chem. Phys. 137, 074309 (2012)), which showed 2(3)Πu to be the dominant dissociative channel, 4(3)Πu is dominant for v = 1. Both 2 and 4(3)Πu are major routes for dissociative recombination from v = 2-4. Other routes including 2(3)Σu(+), 3(3)Πu, 2(1)Πu, 2(3)Πg, 2(1)Σg(+), 1(1)Δg, and b('1)Σu(+) are significant in narrow energy ranges. The results show that minor dissociative routes, included here for N2(+), must be included in theoretical studies of other molecular ions (including the simplest ions H2(+) and H3(+)) if cross section agreement is to be found with future high resolution dissociative recombination experiments. The calculated predissociation lifetimes of the Rydberg resonances are used in a detailed comparison to two prior storage ring experiments in order to determine if the prior assumption of isotropic atomic angular distributions at "zero" electron energy is justified. The prior experimental assumption of comparable cross sections for v = 0-3 is shown to be the case at "zero" but not at nonzero electron energies. Circumstances are identified in which indirect recombination may be visualized as a firefly effect.

The double electrostatic ion ring experiment: a unique cryogenic electrostatic storage ring for merged ion-beams studies

We describe the design of a novel type of storage device currently under construction at Stockholm University, Sweden, using purely electrostatic focussing and deflection elements, in which ion beams of opposite charges are confined under extreme high vacuum cryogenic conditions in separate "rings" and merged over a common straight section. The construction of this double electrostatic ion ring experiment uniquely allows for studies of interactions between cations and anions at low and well-defined internal temperatures and centre-of-mass collision energies down to about 10 K and 10 meV, respectively. Position sensitive multi-hit detector systems have been extensively tested and proven to work in cryogenic environments and these will be used to measure correlations between reaction products in, for example, electron-transfer processes. The technical advantages of using purely electrostatic ion storage devices over magnetic ones are many, but the most relevant are: electrostatic elements which are more compact and easier to construct; remanent fields, hysteresis, and eddy-currents, which are of concern in magnetic devices, are no longer relevant; and electrical fields required to control the orbit of the ions are not only much easier to create and control than the corresponding magnetic fields, they also set no upper mass limit on the ions that can be stored. These technical differences are a boon to new areas of fundamental experimental research, not only in atomic and molecular physics but also in the boundaries of these fields with chemistry and biology. For examples, studies of interactions with internally cold molecular ions will be particular useful for applications in astrophysics, while studies of solvated ionic clusters will be of relevance to aeronomy and biology.

Combined crossed molecular beam and theoretical studies of the N(2D) + CH4 reaction and implications for atmospheric models of Titan

The dynamics of the H-displacement channel in the reaction N((2)D) + CH(4) has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH(4)N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N((2)D) + CH(4) reaction, with the production of the CH(3)N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere of Titan are discussed.

Effects of chemical composition and the addition of H2 in a N2 atmospheric pressure dielectric barrier discharge on polymer surface functionalization

We examined the effect of hydrogen content in various polymers in a N2/H2 discharge for surface amine functionalization. Three polymers (polyethylene (PE), polyvinylidene fluoride (PVDF), and poly(tetrafluoroethylene) (PTFE)) containing various amounts of hydrogen and fluorine were treated with an atmospheric pressure dielectric barrier discharge (DBD). While surface modification was observed on the PE and the PVDF in a pure N2 discharge, adding H2 in a N2 discharge was necessary to observe the fluorine etching on the surface of the PVDF and PTFE polymers. The presence of a slight amount of hydrogen in the gas mixture was also a prerequisite to the formation of amino groups on the surface of all three polymers (max NH2/C approximately 5%). Aging revealed that the modified polymer surfaces treated in a N2-H2 discharge were less prone to hydrophobic recovery than were surfaces treated in pure N2, due primarily to the presence of a higher density of polar groups on the surfaces. We demonstrated that H atoms in the discharge are necessary for the surface amine functionalization of polymers in a N2 atmospheric pressure DBD, regardless of polymer chemical composition. It is therefore possible to control the plasma functionalization process and to optimize the concentration and specificity of NH2 grafted onto polymer surfaces by varying the H2 concentration in a N2 atmospheric pressure DBD.

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 OPCl+ and OPCl2+: pushing the upper mass limit at CRYRING

The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63 x 10(-7)(Te/300)(-0.89) and >1.2 x 10(-6)(Te/300)(-1.22) cm3s(-1), respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at approximately 0 eV interaction energy and are determined to be N(O+P+Cl)=(16+/-7)%, N(O+PCl)=(16+/-3)% and N(OP+Cl)=(68+/-5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.

Role of excited core Rydberg states in dissociative recombination

Intermediate states formed during the dissociative recombination of molecular ions with electrons can play significant roles in determining the magnitude of the total rate coefficient. These resonances are Rydberg states of two types, that is, they can have the ground or excited states of the ion as a core. Those with the excited cores have a fundamentally different excitation mechanism than those with the ground state core. The importance of excited core states in dissociative recombination has received only limited attention in the literature. Theoretical calculations on the dissociative recombination of N2+ are reported which compare the two types of resonances. Potential curves, electronic widths, cross sections, and rate coefficients are calculated for dissociative recombination along the 2(1)Sigma(g)+ state, one of several routes for the dissociative recombination of N2+. The ground core resonances, in this example, are relatively unimportant compared to those with the excited core. Inclusion of the excited core resonances increases the rate coefficient by about a factor of 4 at room temperature, but the increase is not enough to establish 2(1)Sigma(g)+ as the dominant dissociative route.

Dissociative recombination study of N3+: Cross section and branching fraction measurements

We report an investigation into the dissociative recombination of the azide radical cation, N(3) (+). The reaction rate constant has been measured to be 6.47 x 10(-7) cm(3) s(-1) at room temperature. This value is smaller than those reported earlier for the ion-electron neutralization of N(3) (+) at nitrogen atmospheric pressure. A strong propensity to dissociate through the N(2)+N channel has been observed.

Dissociative recombination of ammonia clusters studied by storage ring experiments

Dissociative recombination of ammonia cluster ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for dissociative recombination of H+(NH3)2, H+(NH3)3, D+(ND3)2, and D+(ND3)3 in the collision energy range of 0.001-27 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 1000 K are calculated from the experimental data and compared with earlier results. The fragmentation patterns for the two ions H+(NH3)2 and D+(ND3)2 show no clear isotope effect. Dissociative recombination of X+(NX3)2 (X=H or D) is dominated by the product channels 2NX3+X [0.95+/-0.02 for H+(NH3)2 and 1.00+/-0.02 for D+(ND3)2]. Dissociative recombination of D+(ND3)3 is dominated by the channels yielding three N-containing fragments (0.95+/-0.05).

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.

Reexamination of ionospheric chemistry: high temperature kinetics, internal energy dependences, unusual isomers, and corrections

A number of aspects of ionospheric chemistry are revisited. The review discusses in detail only work performed at AFRL, but other work is mentioned. A large portion of the paper discusses measurements of the kinetics of upper ionospheric reactions at very high temperatures, i.e. the upper temperature range has been extended to at least 1400 K and in some cases to 1800 K. These temperatures are high enough to excite vibrations in O2, N2, and NO and comparing them to drift tube data allows information on the rotational temperature and vibrational level dependences to be derived. Rotational and translational energy are equivalent in controlling the kinetics in most reactions. Vibrational energy in O2 and N2 is often found to promote reactivity which is shown to cause ionospheric density depletions. NO vibrations do not significantly affect the reactivity. In a number of cases, detailed calculations accompanied the experimental studies and elucidated details of the mechanisms. Kinetics of two peroxide isomers important in the lower ionospheric have been measured for the first time, i.e. NOO+ and ONOO-. Finally, two examples are shown where errors in previous data are corrected.

Dissociative recombination of the weakly bound NO-dimer cation: cross sections and three-body dynamics

Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.

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.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

Dissociative recombination cross section and branching ratios of protonated dimethyl disulfide and N-methylacetamide

Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD.

Product angular distributions in dissociative recombination

The dependence of the dissociative recombination cross section upon the angle between the incoming electron beam and the ion internuclear axis is determined for diatomic molecules. Product angular distributions are derived for the component partial waves of the Coulomb wave function. In agreement with earlier results for dissociative attachment, it is shown that in the slow rotation approximation, if electron capture is dominated by a single partial wave, the product angular distribution is given by the square of the absolute value of the partial wave spherical harmonic describing the incoming electron.

Dissociative recombination of NH4+ and ND4+ ions: Storage ring experiments andab initiomolecular dynamics

The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.