Products Of Dissociative Recombination Research Articles

Dissociative Recombination in the Afterglow of Low-pressure Barrier Discharge. Population of Ne(2p-=SUP=-5-=/SUP=-3d) Atoms

The decaying neon plasma produced by the dielectric barrier discharge (DBD) in a cylindrical tube at a pressure of 0.1-40 Torr has been spectroscopically investigated to analyze the dissociative recombination (DR) of molecular ions with electrons as a mechanism for the formation of excited atoms. It is shown that, at the electron density in the afterglow less than 5·1010 cm-3 the DR is the dominant source of population of 3d levels at pressures PNe≥ 0.6 Torr. At lower pressures, the optical properties of the decaying plasma are formed to a greater extent by the collisional-radiative recombination of Ne+ ions. A significant variation of the relative intensities of the 3d->3p transition lines in the afterglow with a change in gas pressure was found, reflecting the effect of inelastic collisions on the formation of the spectrum of decaying plasma in the near infrared region. From measurements carried out at a pressure of 0.6 Torr, the relative values of the partial DR coefficients for the 3dj levels of the neon atom were found. Comparison of these data with measurements in the near ultraviolet region, containing the lines of 4p->3s transitions, indicates the need to take into account the cascade 4p->3d transitions to correctly solve the problem of the final products of dissociative recombination. Keywords: dielectric-barrier discharge, low-pressure plasma, optical emission spectroscopy, dissociative recombination, molecular ions, collisional-radiative recombination, cascade transitions.

Диссоциативная рекомбинация в послесвечении барьерного разряда в неоне низкого давления. Заселение атомов конфигурации 2p-=SUP=-5-=/SUP=-3d

The decaying neon plasma produced by the dielectric barrier discharge (DBD) in a cylindrical tube at a pressure of 0.1–40 Torr has been spectroscopically investigated to analyze the dissociative recombination (DR) of molecular ions with electrons as a mechanism for the formation of excited atoms. It is shown that, at the electron density in the afterglow less than 5•1010 cm-3 the DR is the dominant source of population of 3d levels at pressures PNe ≥ 0.6 Torr. At lower pressures, the optical properties of the decaying plasma are formed to a greater extent by the collisional-radiative recombination of Ne+ ions. A significant variation of the relative intensities of the 3d -> 3p transition lines in the afterglow with a change in gas pressure was found, reflecting the effect of inelastic collisions on the formation of the spectrum of decaying plasma in the near infrared region. From measurements carried out at a pressure of 0.6 Torr, the relative values of the partial DR coefficients for the 3dj levels of the neon atom were found. Comparison of these data with measurements in the near ultraviolet region, containing the lines of 4p -> 3s transitions, indicates the need to take into account the cascade 4p -> 3d transitions to correctly solve the problem of the final products of dissociative recombination.

Knowledge-based probabilistic representations of branching ratios in chemical networks: The case of dissociative recombinations

Experimental data about branching ratios for the products of dissociative recombination of polyatomic ions are presently the unique information source available to modelers of natural or laboratory chemical plasmas. Yet, because of limitations in the measurement techniques, data for many ions are incomplete. In particular, the repartition of hydrogen atoms among the fragments of hydrocarbons ions is often not available. A consequence is that proper implementation of dissociative recombination processes in chemical models is difficult, and many models ignore invaluable data. We propose a novel probabilistic approach based on Dirichlet-type distributions, enabling modelers to fully account for the available information. As an application, we consider the production rate of radicals through dissociative recombination in an ionospheric chemistry model of Titan, the largest moon of Saturn. We show how the complete scheme of dissociative recombination products derived with our method dramatically affects these rates in comparison with the simplistic H-loss mechanism implemented by default in all recent models.

New flowing afterglow technique for determining products of dissociative recombination: CH5+ and N2H+

There are discrepancies in the literature for the product distributions of electron-ion (e-i) recombination when determined using different techniques. Because of this, a new technique has been developed. This is based on the flowing afterglow, with the product neutrals detected by electron impact ionization followed by mass spectrometric detection. However, in addition to the products of recombination, there are neutrals present from ion-molecule reactions and from the gases introduced into the flow tube to create the ion of interest, which often have much greater concentrations than the products. To distinguish these products, an electron attaching gas is pulsed into the flow to transiently attach the electrons, thus quenching e-i recombination. Then the difference between the attaching gas in and out yields the product distribution. Recombination products have been detected even when their signal is as much as ∼ 104 less than background. Here the details of the technique are described and the possible sources of error discussed. The viability of the technique is illustrated for the recombinations of CH5+ and N2H+. The latter establishes the major product as N2 + H (95 to 100%) correcting an error in the literature. In the former case, the major channel detected is CH4 + H (95%) which is in disagreement with a storage ring (SR) result which gave CH3 as the major channel (68%). Possible reasons for this are discussed.

PROSPECTS FOR THE DETECTION OF INTERSTELLAR CYANOVINYLIDENE

Prospects for the presence and detection of interstellar cyanovinylidene, CC(H)CN, a Y-shaped isomer of cyanoacetylene, are discussed. It is proposed that CC(H)CN can arise in interstellar clouds as one of the HC3NH+ + e – dissociative recombination products, by rearrangements of the neutral chain radical HC3NH into branched species HCCC(H)N, CC(H)C(H)N, and/or HCC(H)CN, and by the subsequent elimination of a hydrogen atom. It is deduced that the abundance of cyanovinylidene in molecular clouds should be confined between the abundances of its chain isomers HNCCC and HCNCC. Quantum chemical predictions regarding cyanovinylidene geometry, ground-state rotational constants, centrifugal distortion constants, spin-orbit coupling, IR absorption spectroscopy, and electric dipole moment are given. The spectroscopically observed molecules formyl cyanide, NC2(H)O, and propynal, HC3(H)O, with structures qualitatively resembling cyanovinylidene, served to prove the adequacy of the calculational procedures employed.

Role of dissociative recombination in the excitation kinetics of an argon microwave plasma at atmospheric pressure

A collisional radiative model was developed in order to investigate the influence of dissociative recombination on the Saha–Boltzmann plasma equilibrium. As the dissociative recombination products are not well known, their relative importance was tested through comparison with the distribution of line intensities obtained in a microwave argon discharge produced at atmospheric pressure by a surface wave. It was found that the main dissociation products are the ground state and the 4s levels, the 5p and upper levels playing a negligible role. Because the higher levels are only weakly affected by dissociative recombination, they remain in partial local thermodynamic equilibrium. Therefore, the excitation temperature determined from these levels adequately describes the electron temperature. The model well reproduces experimental measurements of excitation temperature, rotational temperature, electron density, and absolute populations of the excited levels.

Dissociative recombination angular distributions

The angular distributions of the products of dissociative recombination of diatomic molecular ions are derived. The distributions are determined by the dominant partial waves of the captured electrons. For a single dominant wave, the neutral product distribution is described by the same spherical harmonic that describes the electron partial wave. The derived distributions take into account both direct and indirect dissociative recombination and are essential for modeling the projected distributions measured in storage ring experiments. All prior storage ring experiments have assumed that the product angular distributions can be described by spherical harmonics with angular momenta ≤ 1 and isotropic product distributions for "zero" eV electron capture. These assumptions are tested here by deriving the projected distributions for the dissociative recombination of CH+. For CH+, it is shown that the "zero" eV storage ring experimental results cannot be explained by only isotropic dissociation. Instead, an anisotropic model product distribution is required. The anisotropic model yields a quantum yield of 1.0 for C(1D) instead of the incorrect value of 0.79 derived previously using an isotropic model. The quantum yields of all prior storage ring experiments on other molecular ions at "zero" eV electron energy must be reassessed in view of these findings.

Dissociative Recombination of H3O+

Ab initio molecular orbital calculations have been performed on potential energy surfaces associated with products of dissociative recombination (DR) of H3O+ + e- experiments carried out in the ASTRID heavy-ion storage ring. Gradient geometry optimizations and frequency calculations on critical points on the H3O ground-electronic-state surface and its dissociation paths were performed at levels of theory up to and including MP2(full)/6-311G(d,p) with extra diffuse functions added to the oxygen atom; single-point calculations subsequently were performed at levels up to CCSD(T) with the same basis set. Dissociation pathways of the two lowest-energy valence-to-Rydberg H3O excited states were studied using CIS single-point calculations on SCF-level optimized geometries along ground-state H3O dissociation pathways, and no barriers to fragmentation were observed. The most exothermic ground-state dissociation pathway connects H3O to H2O (X1A1) + H; however, OH(X2Π) + H2 and OH(X2Π) + 2H also are energetically ac...

Dissociative recombination and dissociative excitation of 4HeH+: Absolute cross sections and mechanisms.

Absolute cross sections have been determined for the dissociative recombination and dissociative excitation of $^{4}\mathrm{HeH}^{+}$ for electron energies below 40 eV. The dissociative recombination cross section is in semiquantitative agreement with recent theoretical results by Sarpal, Tennyson, and Morgan [J. Phys. B 27, 5943 (1994)] and Guberman [Phys. Rev. A 49, R4277 (1994); in XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, AIP Conf. Proc. No. 360, edited by L. J. Dube, J. B. A. Mitchell, J. W. McConkey, and C. E. Brion (AIP, New York, 1995), p. 307]. The calculated resonant structure below a collision energy of 1 eV was not fully reproduced by the experiment. The quantum states of the dissociative recombination products at 0 eV collision energy have been determined; ground-state helium and excited hydrogen atoms (n=2) are dominantly formed, in agreement with recent predictions by Guberman. The dissociative excitation has an onset around 10 eV and follows the shape of the dissociative recombination cross section, illustrating that both processes start with the formation of doubly excited neutral states that lie in the ionization continuum as well as in the dissociation continuum. The dissociative excitation cross section is in quite good agreement with recent calculations by Orel and Kulander. \textcopyright{} 1996 The American Physical Society.

Dissociative recombination studies of Ar2+ by time-of-flight spectroscopy.

From time-of-flight spectra of metastable ${\mathrm{Ar}}^{\mathrm{*}}$ atoms emerging from sustained discharges in Ar, we observe discrete nonthermal peaks due to excited products of dissociative recombination (DR) of ${\mathrm{Ar}}_{2}^{+}$ formed by reactions in the discharge. These peaks are in addition to the expected Boltzmann distributions of metastable atoms from direct excitation by fast primary electrons. From the peak velocities, and thus the kinetic energies released in the dissociation, the DR product states are uniquely determined, regardless of subsequent radiation. As we have previously found from ${\mathrm{Kr}}_{2}^{+}$ [A. Barrios et al., Phys. Rev. Lett. 69, 1348 (1992)], most of the excited DR products are (n+1)s states, but ${\mathrm{Ar}}_{2}^{+}$ also yields smaller amounts of 4p and 3d states. We have also recently shown [G. B. Ramos et al., Phys. Rev. A 51, 2945 (1995)] that, surprisingly, in studies of ${\mathrm{Ne}}_{2}^{+}$ and ${\mathrm{Ar}}_{2}^{+}$ DR, a substantial but undetermined fraction of reactions go to a purely ground-state channel. In addition, we have found that under certain conditions in Ar discharges, the relative amount of the higher excited-state products, including 4p, 3d, 5s, and 5p can be dramatically influenced. The velocity peaks also become narrower than expected from the thermal distribution of the parent ${\mathrm{Ar}}_{2}^{+}$ as the energy level in the product atom increases. These studies in argon discharges have included variations in source pressure, discharge voltage, and strength of the axial magnetic field.

Observation of dissociative recombination of Ne2+ and Ar2+ directly to the ground state of the product atoms.

Using data gathered by the metastable atom time-of-flight technique, evidence is presented for the first time that some of the direct products of dissociative recombination (DR) in ${\mathrm{Ne}}_{2}^{+}$ and ${\mathrm{Ar}}_{2}^{+}$ are purely ground-state atoms. These results are very surprising because there are no evident curve crossings between the initial molecular-ion potentials and those of the repulsive ground states. Such curve crossings have been thought to be necessary for DR to occur. Using a laser-based atom beam deflection technique, it is shown that at least some of the fast ground-state atoms produced by DR are reexcited to metastable states that are easily detected.

Vibrational excitation of products of dissociative recombination

It is shown that a considerable fraction of any molecular product of the dissociative recombination of a polyatomic ion is normally vibrationally excited. The usual main source of the excitation is the impulsive force between the products. In the case of a monohydride, the vibrational excitation due to the impulse acting on its heavy atom is inappreciable, but that due to the much weaker impulse acting on its H atom may be great. Another source is rooted in the relic of the normal vibrational modes of the ion. This causes the initial vibrational wavefunction of the product to differ from the wavefunction of its zeroth vibrational level, even in instances where the corresponding equilibrium separations and force constants of the ion and the product are virtually the same (as they commonly are)

Development of the flowing afterglow/Langmuir probe technique for studying the neutral products of dissociative recombination using spectroscopic techniques: OH production in the HCO+2+e reaction

The flowing afterglow/Langmuir probe (FALP) technique has been extended to enable the neutral products of electron–ion dissociative recombination in thermalized afterglows to be identified by spectroscopic methods. Absolute number densities of H atoms in the afterglow have been determined using vacuum ultraviolet (VUV) absorption at the Lα wavelength. By exploiting the reaction H+NO2→OH+NO, all of the H atoms can be incorporated into OH molecules and thus observation of the intensity of laser induced fluorescence (LIF) If, obtained by exciting the (1,0) band of OH(A 2Σ←X 2Π), allows a calibration to be obtained of If against the known number density of OH X 2Π(ν″=0) in the afterglow. Following this procedure, a recombining HCO+2 /electron afterglow was probed for production of ground state OH X 2Π(ν″=0) using LIF and it was established that OH(ν″=0) resulted from 17% of the recombining ground state HCO+2 ions. It was also established that a further 17% of the recombinations resulted in OH(ν″>0), i.e., that, in total, (34±6)% of the HCO+2 ions recombine to produce OH X 2Π radicals, either directly or via the electronically excited A 2Σ state. Details of the calibration procedure for H and OH number densities, of the ion chemistry involved in the production of the HCO+2 afterglow plasmas and of the checks carried out to establish that the fluorescence observed was from OH produced in the recombination reaction are presented. During these experiments, the rate coefficient at 300 K for the H+NO2 reaction was determined to be 1.3×10−10 cm3 s−1 from observations of the H-atom decay as a function of NO2 number density in the afterglow (in good agreement with previous determinations). Also the rate coefficient for the quenching reaction of OH(ν″>0) with NO to produce OH(ν″=0) was determined to be 6×10−11 cm3 s−1.

An Experimental Study of the Products of Dissociative Recombination of Molecular Ions with Electrons

A new experimental method has been developed which allows determination of the products nature in dissociative recombination (D.R.) of molecular ions. Results are presented on H2O+ D.R. which show that there is a large yield of oxygen atoms in this reaction. the measurements give a total yield for the two channel O + H + H and O + H2 of 0.45 and therefore a yield of 0.55 for OH + H. H2O+ ions are formed by charge exchange from N+ ions and are probably vibrationally excited in this experiment.

Products of dissociative recombination of polyatomic ions

view Abstract Citations (136) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Products of Dissociative Recombination of Polyatomic Ions Bates, D. R. Abstract The dissociative recombination of polyatomic ions is discussed. It is argued that the dissociative channels involving the least rearrangement of valence bonds are favored. An explanation for the high rate of cluster and heavy inert gas dimer ion recombination is proposed. Publication: The Astrophysical Journal Pub Date: July 1986 DOI: 10.1086/184702 Bibcode: 1986ApJ...306L..45B Keywords: Gas Dissociation; Interstellar Chemistry; Interstellar Matter; Ion Recombination; Polyatomic Molecules; Carbon; Dimerization; Heavy Elements; Hydrogen; Rare Gases; Valence; Atomic and Molecular Physics; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS |