Dissociative Charge Exchange Research Articles

Ion Velocity Map Imaging Study of the Reactive Collisions between Carbon Dioxide and Helium Ion.

The reactive collision between He+ and CO2 plays an important role in substance evolutions of the planetary CO2-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He+ + CO2 → He + CO2+/He + CO+ + O/He + CO + O+. The velocity images of the products CO+ and O+ of dissociative charge-exchange reactions are distinctly different from those of charge-exchange product CO2+. The remarkable features of stereodynamics are observed in the dissociative charge-exchange reaction and are attributed to the spatial alignment of the initially random target CO2 during the He+ approach. Branching ratios of different channels of dissociative charge exchange are further obtained with the Doppler kinematics model, indicating a high preference for the energy-resonant channel.

Ion-velocity imaging study of dissociative charge exchange reactions between Ar+ and trans- / cis- dichloroethylene

Dissociative charge exchange reactions between Ar+ ion and trans- /cis- dichloroethylene (trans-/cis-C2H2Cl2) are investigated with the ion-velocity imaging technique. The dechlorinated product C2H2Cl+ is the predominant, and most of this product show the spatial distribution around the target, implying that the dissociation occurs in the large impact-parameter collision and via the energy resonant charge transfer. Meanwhile, a few C2H2Cl+ locate around the center-of-mass, which is attributed to the fragmentation of intimate association between C2H2Cl2 and Ar+ or in the small impact-parameter collision. The product C2HCl+ exhibits the velocity distribution features similar to those of C2H2Cl+. The rarest product C2HCl2+ shows the distributions around the molecular target, due to the quick dehydrogenation after the energy-resonant charge transfer in the large impact-parameter collision.

Collision-Energy Dependent Stereodynamics of Dissociative Charge Exchange Reaction between Ar+ and CO.

Long-distance charge-dipole attraction between atomic ion and randomly oriented polar molecule potentially makes the molecular orientation, which profoundly influences the products' kinetics of collisional reaction. Using the three-dimensional ion velocity map imaging technique, here we report a collision-energy dependent stereodynamics of dissociative charge exchange reaction Ar+ + CO → Ar + O + C+ in a range of 7.46-9.97 eV. At the lowest collision energy, the most C+ products are forward-scattered and are along the collision axis and are attributed to three different dissociation channels including the predominant one experiencing the rotating intermediate ArC+. At the high collision energies, the remarkably diffusive distribution of C+ arises from the prompt dissociation of the rebounded CO+. The different dynamic processes arising from the nearly collinear collision are elaborated explicitly on the basis of the data analyses using the Doppler kinetics models.

Stereodynamics Observed in the Reactive Collisions of Low-Energy Ar+ with Randomly Oriented O2.

Stereodynamics of the collisional reaction between mutually aligned or oriented reactants has been a striking topic of chemical dynamics for decades. However, the stereodynamic aspects are scarcely revealed for the low-energy collision with a randomly oriented target. Here in the dissociative charge-exchange reaction between randomly oriented O2 and low-energy Ar+, we, using the three-dimensional ion velocity map imaging technique, clearly observe a linear alignment and a nearly isotropic distribution of the O+ yields along the collision axis. These observations are rationalized with the Doppler kinetic models in which the O2 bond is assumed to be parallel or unparallel to the collision axis of the large impact parameter collision. The linearly aligned O+, as the predominant yield, is produced in the parallel collision, while a rotating O2+, as the intermediate in the unparallel collision, leads to the isotropic distribution of O+.

Emission cross sections for energetic O+(4S,2D,2P)–N2 collisions

Measurements of emission cross sections for the [Formula: see text] collision system with the incident beam of 1–10 keV [Formula: see text] in the ground [Formula: see text]) and metastable [Formula: see text] and [Formula: see text] states are reported. The emission cross section induced by incident ions in the metastable state [Formula: see text] is much larger than that for the ground [Formula: see text] state. The emission cross section of [Formula: see text] ion for [Formula: see text], [Formula: see text], and [Formula: see text] bands system is measured and the ratio of intensities for these bands is established as [Formula: see text] It is shown that the cross sections for the [Formula: see text] ions emissions in the dissociative charge exchange processes increase with the increase of the incident ion energy. The energy dependence of the emission cross section of the band [Formula: see text] [Formula: see text] nm of the first-negative band system of the [Formula: see text] and degree of linear polarization of emission in [Formula: see text] collision are measured for the first time. An influence of an admixture of the ion metastable state on a degree of linear polarization is revealed. The mechanism of the processes realized during collisions of ground and metastable oxygen ions on molecular nitrogen have been established. It is demonstrated that for [Formula: see text] collision system the degree of linear polarization by metastable [Formula: see text] ions is less compared to those that are in the ground [Formula: see text] state and the sign of emission of degree of linear polarization of excited molecular ions does not change.

Destruction of dimethyl ether and methyl formate by collisions with He+

Context. To correctly model the abundances of interstellar complex organic molecules (iCOMs) in different environments, both formation and destruction routes should be appropriately accounted for. While several scenarios have been explored for the formation of iCOMs via grain and gas-phase processes, much less work has been devoted to understanding the relevant destruction pathways, with special reference to (dissociative) charge exchange or proton transfer reactions with abundant atomic and molecular ions such as He+, H3+and HCO+.Aims. By using a combined experimental and theoretical methodology we provide new values for the rate coefficients and branching ratios (BRs) of the reactions of He+ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate (MF). We also review the destruction routes of DME and MF by other two abundant ions, namely H3+and HCO+.Methods. Based on our recent laboratory measurements of cross sections and BRs for the DME/MF + He+reactions over a wide collision energy, we extended our theoretical insights on the selectivity of the microscopic dynamics to calculate the rate coefficientsk(T) in the temperature range from 10 to 298 K. We implemented these new and revised kinetic data in a general model of cold and warm gas, simulating environments where DME and MF have been detected.Results. Due to stereodynamical effects present at low collision energies, the rate coefficients, BRs and temperature dependences here proposed differ substantially from those reported in KIDA and UDfA, two of the most widely used astrochemical databases. These revised rates impact the predicted abundances of DME and MF, with variations up to 40% in cold gases and physical conditions similar to those present in prestellar cores.Conclusions. This work demonstrates that the accuracy of astrochemical models can be improved by a thorough characterisation of the destruction routes of iCOMs. The details of the chemical systems can, indeed, strongly affect their efficiency and significant deviations with respect to the commonly used Langevin model estimates are possible.

MAVEN Observations of the Response of Martian Ionosphere to the Interplanetary Coronal Mass Ejections of March 2015

AbstractThe Mars Atmosphere and Volatile EvolutioN spacecraft provides an opportunity to observe the response of the Martian upper atmosphere to the passage of interplanetary coronal mass ejections. We study the response of Martian ionosphere to two successive solar disturbances during 3 and 4 March and 8 and 9 March 2015 using the observations from Neutral Gas and Ion Mass Spectrometer and Langmuir Probe and Waves aboard Mars Atmosphere and Volatile EvolutioN. During these events, the ionospheric boundary was significantly lowered. During 3 March the ionopause is seen to be at a lower altitude, compared to that during 8 March. The higher ionopause seen on 8 March could be due to the sustained effect of heating and inflation by the 3 March interplanetary coronal mass ejection event or due to the heating by the enhanced particle precipitation. The comparison of the O+/O ratio indicates that there is a preferential decrease of O and a relative enhancement in O+, probably due to increased dissociative recombination and charge exchange as a result of the higher solar wind densities during the 3 March event.

Experimental investigation of the reaction of helium ions with dimethyl ether: stereodynamics of the dissociative charge exchange process.

The fate of dimethyl ether (DME, CH3OCH3) in collisions with He+ ions is of high relevance for astrochemical models aimed at reproducing the abundances of complex organic molecules in the interstellar medium. Here we report an investigation on the reaction of He+ ions with DME carried out using a Guided Ion Beam Mass Spectrometer (GIB-MS), which allows the measurement of reactive cross-sections and branching ratios (BRs) as a function of the collision energy. We obtain insights into the dissociative charge (electron) exchange mechanism by investigating the nature of the non-adiabatic transitions between the relevant potential energy surfaces (PESs) in an improved Landau-Zener approach. We find that the large interaction anisotropy could induce a pronounced orientation of the polar DME molecule in the electric field generated by He+ so that at short distances the collision complex is confined within pendular states, a particular case of bending motion, which gives rise to intriguing stereodynamic effects. The positions of the intermolecular potential energy curve crossings indicate that He+ captures an electron from an inner valence orbital of DME, thus causing its dissociation. In addition to the crossing positions, the symmetry of the electron density distribution of the involved DME orbitals turns out to be a further major point affecting the probability of electron transfer. Thus, the anisotropy of the intermolecular interaction and the electron densities of the orbitals involved in the reaction are the key "ingredients" for describing the dynamics of this dissociative charge transfer.

Real-time measurements of nitrosamines in air

The ion-molecule chemistry of the three common reagent ions used in selected ion flow tube mass spectrometry (SIFT-MS), H3O+, NO+ and O2+, has been extended to include reactions with seven nitrosamines. Theoretical calculations enabled the dipole moments and polarizabilities of the nitrosamines studied to be found and from these, the proton transfer rate coefficients have been calculated. In all cases rapid proton transfer from H3O+ occurred, and charge exchange and dissociative charge exchange were common processes for NO+ and O2+. For nitrosamines such as N-nitrosodibutylamine, noticeable fragmentation occurred. When the helium carrier gas in the flow tube was changed to nitrogen, the technology still gave good results for monitoring nitrosamines but the product channel ratios for some nitrosamines were different for nitrogen than for helium. The new kinetic data obtained in this study was utilized using a commercial SIFT-MS instrument to monitor the production of N-nitrosodimethylamine and N-nitrosodiethylamine from heated salami.

Vibrational Excitation and Product Branching Ratios in Dissociation of the Isotopologs of H3O: Experiment and Theory

The dissociation dynamics of the Rydberg radical H3O and the deuterated isotopologs have been studied by dissociative charge exchange of H3O(+) with Cs. Center-of-mass kinetic energy release distributions were measured with a fast-beam translational spectrometer and compared with direct dynamics quasiclassical trajectory calculations with initial conditions from an ab initio potential energy surface for H3O(+). The experimental branching fractions for dissociation of each isotopolog were obtained and compared with the calculated branching fractions. The dominant dissociation channel for all species is elimination of an H/D atom, and the water product was formed with a significant vibrational inversion in stretching vibrations that varies with the mass of the leaving atom in the dissociation. Branching fractions for the mixed isotopologs show that H atom elimination is favored over D atom elimination. Given the importance of H3O(+) in plasmas, astrochemistry, and in condensed phases, the striking energy partitioning found in this neutralization process is notable.

Molecular Ion Dissociation Processes in a Partially Cooled Electrostatic Trap

An experimental capability being developed at the Oak Ridge National Laboratory Multi-Charged Ion Research Facility (ORNL MIRF) is described that enables stored cooling and state characterization of atomic and molecular ions of essentially any mass. Ions selected from a variety of available (hot and cold) sources are injected into a 1.5 meter long electrostatic mirror trap, in which excited internal states decay by radiative cooling. Ion lifetime studies or direct measurements of 'in-trap' interactions of molecular ions with electrons or atomic targets are accomplished with either a crossed electron beam or atomic beam (with recoil ion detection) located in the field-free region of the ion-trap, coupled with product velocity imaging. Similarly, extraction of trap-cooled ions can be used 'out-of-trap' for near state-specific electron capture (EC) and dissociative charge exchange (DCE) studies employing higher resolution COLTRIMS techniques coupled with fragment imaging.

Experimental probes of transient neutral species using dissociative charge exchange

In this review, recent progress on studies of the dissociation dynamics of transient neutral species produced by charge exchange (CE) of molecular cations with caesium is reviewed. CE experiments using translational spectroscopy in fast ion beams have made important contributions to understanding the spectroscopy and dynamics of high-lying excited states of transient species for some 50 years. Recent advances in time- and position-sensitive detection techniques coupled to jet-cooled supersonic expansion ion sources have allowed measurements yielding unprecedented detail for dissociation processes occurring from complex molecular species. Applications of this technique to studying the two- and three-body dissociation of H3 and sym-triazine, as well as the two-body dissociation of the HN2 radical to H + N2 are reviewed. These systems illustrate the insights gained into the dynamics of the dissociative states from examination of product kinetic energy release distributions and detailed measurements of momentum partitioning in three-body dissociation processes.

Dissociative charge exchange dynamics of HN2+ and DN2+

Translational spectroscopy coupled with coincidence detection techniques has been used to study the dissociation dynamics of ground state H/D+N(2) products resulting from charge exchange between keV beams of HN(2)(+)/DN(2)(+) and cesium. Analysis of the product kinetic energy release suggests that dissociation of HN(2) and DN(2) proceeds from initial populations in the (2)A("), 2 (2)A('), and 3s Rydberg electronic states of the neutral molecule. Although all three excited electronic states must eventually couple to the 1 (2)A(') ground state of HN(2)/DN(2), the resulting dissociation dynamics exhibit a significant dependence on the initial electronic state. Potential mechanisms are discussed in light of the observed product kinetic energy release distributions.

Dissociative charge exchange of [formula omitted] and [formula omitted] with cesium: Dissociation dynamics and two- and three-body branching fractions

The two-and three-body center-of-mass kinetic energy release distributions for H 3 and D 3 along with their branching fractions were measured using charge exchange of H 3 + and D 3 + with Cs. Vibrational and rotational excitation in the diatomic product and radiative decay to the repulsive ground state from a manifold of Rydberg states are observed in both systems. The two-body dissociation pathway is found to be dominant, however, for higher-lying Rydberg states, the three-body pathway becomes increasingly important.

Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Plasma spraying is a well developed and widely used technology, successfully applied for ceramic and metal coatings in many fields of applications such as aeronautics, gas turbine, automotive or medical. The coatings obtained are usually intentionally porous and thick (more than 100 μm). Presently, thin films (< µm) are deposited using various physical vapour deposition (PVD) or chemical vapour deposition (CVD) processes with low deposition rates. In this paper, we make use of the high enthalpy and high ionisation degree of the plasma jet of conventional plasma spraying guns operated at low pressure (mbar) to obtain dense coatings by CVD from gaseous and/or liquid precursors. The advantages of such thermal plasma CVD processes are the high deposition rates to obtain dense and thin layers, and the possibility of combining these thin films with thermally sprayed coatings using the same equipment. An efficient injection and mixing of the liquid and gaseous precursors in the plasma jet, which is especially challenging for liquids in low-pressure processes, has been obtained by extensive developments and proper equipment design. Results of several different coatings based on liquid and gaseous precursors are presented. In particular, SiO x thin films from HMDSO (Hexamethyldisiloxane, C 6H 18OSi 2) precursor and oxygen can be deposited over large areas (50 cm diameter) at typical deposition rates of 35 nm/s, with a precursor-to-film conversion efficiency exceeding 50%. For the case of amorphous carbon deposited from CH 4 or C 2H 4, deposition rates exceeding 25 nm/s are obtained. Results from mass spectrometry of the gas sampled in the plasma jet by an enthalpy probe show that the depletion of hydrocarbon precursors can reach 95% and that higher hydrocarbon species are formed by secondary reactions. In the case of carbon-containing precursors, results from mapping of the optical emission intensity throughout the plasma jet volume are presented. The formation and transport of excited precursor-based species, such as CH, C, C 2, and H are addressed. These results show, in particular, that the very high dissociation efficiency of the precursors takes place through (dissociative) charge exchange from Ar + ions and subsequent dissociative recombination with low energy electrons. The peculiarities of plasma chemistry taking place in the low-pressure plasma jet compared to conventional low-density non-equilibrium plasmas are outlined.

Probing the Structure of CH5+ by Dissociative Charge Exchange

Dissociative charge exchange of CH5+ with Cs, coupled with quasiclassical trajectory calculations on an ab initio PES for CH5, has been used to probe the structure of the CH5+ cation. Product kinetic energy release distributions and branching ratios for CH5 --> CH4 + H and CH5 --> CH3 + H2 have been compared. The agreement of the product branching ratios provides evidence for the fluxional nature of CH5+.

High pressure investigations of positive ion–molecule reactions in a mixture of C 3H 6 and Ne

Dissociative charge-exchange reactions of Ne + ions produced from neon with neutral propene have been studied in a quadrupole mass spectrometer with a high pressure ion source. The measurements were performed in a mixture of 1% C 3H 6+99% Ne within the pressure range 1.33–39.99 Pa. The fractional abundance I i / ∑ I i of prominent ions: C 2H 3 +, C 3H 3 +, C 3H 5 +, C 3H 7 +, C 4H 5 +, C 4H 7 + and neon ions have been determined as a function of gas pressure in the ion source and against the repeller electrode potential.

Experimentally probing the three-body predissociation dynamics of the low-lying Rydberg states of H3 and D3

Charge-exchange neutralization of H3+ with Cs allows preparation of the low-lying Rydberg states of H3. These states are predissociated by the repulsive ground state and may play roles as intermediates in the dissociative recombination of H3+ + e−. Translational spectroscopy and measurements of product momentum partitioning in three-body dissociative charge exchange of fast (12 keV) H3+ and D3+ with Cs yields insights into the nuclear motion during dissociation for the three lowest-lying 2s 2A1′, 2p 2A2′′ and 3p 2E′ bound Rydberg states of H3 and the two 2s 2A1′ and 2p 2A2′′ states for D3. This data provide an empirical benchmark for the refinement of theoretical models involving non-adiabatic interactions and dynamics for H3.

Three-Body Dissociation Dynamics of the Low-Lying Rydberg States ofH3andD3

The dynamics of the three-body dissociative charge exchange of fast (12 keV) H+3 and D+3 with Cs have been studied using multiparticle translational spectroscopy. The observed partitioning of product momenta was found to be state-specific and yields insights into the nuclear motion during dissociation for the three lowest-lying 2s 2A'1, 2p 2A''2, and 3p 2E' metastable Rydberg states of H3 and the 2s 2A'1 and 2p 2A''2 states for D3. These results provide direct empirical information on the nonadiabatic couplings that govern the three-body dissociation of the lowest-lying Rydberg states of H3 and D3.

Total cross sections and kinetic energy release for the electron impact dissociation of H+2 and D+2

Absolute total cross sections have been measured for electron impact dissociative excitation and dissociative ionization of H+2 and D+2 in the energy range 5–3000 eV. The vibrational population of the primary H+2 beam has been analysed by dissociative charge exchange on a potassium target, and is in good agreement with the measurements of von Busch and Dunn (1972 Phys. Rev. A 5 1726). Kinetic energy release (KER) distributions have been extracted from momentum analysis of the released protons and deuterons at selected impact energies. A model calculation has been performed to interpret the different spectra. Below 100 eV, the distributions exhibit a sharp peak in the range 0–1 eV that is attributed to the dissociative excitation of high vibrational levels to the 2pσu repulsive state in the vicinity of their outer turning point. This observation is consistent with the measured vibrational population extending up to v = 13, as confirmed by the appearance threshold of the dissociative ionization (DI) channel. The KER distributions exhibit a second contribution peaking between 1 and 5 eV, resulting from the admixture of the (1sσg → 2pσu), (1sσg → 2pπu) and (1sσg → 2sσg) electronic transitions. A distinctive hump is also present around 9 eV, that coincides both with the maximum of the DI contribution, and with the high-energy shoulder of the 2pπu and 2sσg contributions. The present measurements are in qualitative agreement with the previous results of Caudano and Delfosse, and are fairly well reproduced by our first-order model.