Cross Sections For Electron-impact Dissociation Research Articles

Electron-impact dissociation of N2D+ cations to D+ fragments

Absolute cross sections for electron-impact dissociation of N2D+ cations to the light D+ fragments are reported for the first time. The animated electron-ion crossed-beam method is applied in the energy range from thresholds up to 2 keV. The heavy N2+ fragments are also detected and their kinetic energy distribution is analyzed at a selected electron energy. The measured cross-sections are corrected for incomplete collection of the light D+ fragments by performing careful analyser magnetic field scan together with a Monte-Carlo procedure. Inclusive absolute cross sections are determined. Dissociative excitation (DE) and dissociative ionization (DI) contributions are separated and the corresponding partial cross sections for D+ fragment production are determined at selected electron energies. The inclusive cross section for D+ fragment production has a maximum of (87 ± 7)×10−18 cm2, at 95.1 eV, with the dominant contribution from the DI process.

Self-consistent electron energy distribution functions, vibrational distributions, electronic excited state kinetics in reacting microwave CO2 plasma: An advanced model

An advanced model for the calculation of electron energy distribution functions (eedfs), vibrational distributions, and electronic excited state densities of reacting CO2 in microwave (MW) discharges has been developed for clarifying: (1) the role of electronic states of the relevant neutral species in affecting the eedf and (2) the contribution to the CO2 dissociation of the electron impact and heavy particle dissociation mechanisms. To model the discharge, the power density typical of MW discharges is used as a parameter. Different case studies including optically thick and thin plasmas and the dependence of the CO2 dissociation rates on the gas temperature are investigated. The results show that at a low gas temperature, i.e., 300 K, the heavy-particle dissociation mechanism, also called the pure vibrational mechanism, prevails on the electron impact dissociation one, while at a high gas temperature, i.e., 2000 K, the two mechanisms become competitive and the global behavior strongly depends on the choice of electron impact dissociation cross sections. Large differences appear in the eedf, especially in the post-discharge regime, when considering thick and thin plasmas. In the thick case, a well-structured eedf appears as a result of superelastic collisions mainly involving the electronic states of the relevant neutral species. In the thin plasma, many peaks disappear because the concentration of the excited states strongly decreases. Finally, our model gives the results of conversion and energy efficiency as well as vibrational distributions in satisfactory agreement with the corresponding results calculated by the Antwerp group.

Electron impact dissociation of CO2

Several CO2 electron impact dissociation cross sections are available in the literature, different in magnitude and threshold, hindering the understanding of CO2 dissociation mechanisms under gas discharges. This work reports the experimental validation of the electron impact CO2 dissociation cross section using two complementary methods: through the comparison of the measured rate coefficients with those derived from cross sections available in literature; and through the comparison of the experimental time evolution of the dissociation fraction with the simulations of a 0D model. A careful experimental approach was designed to avoid any influence from other dissociation mechanisms or chemical reactions. The experimental results match remarkably well the theoretical predictions from Polak and Slovetsky and establish the validity of the dissociation rate coefficients derived from their cross section. This validation supports the use of Polak and Slovetsky’s cross section in any theoretical or modelling approach involving CO2 molecules under electrical discharges.

Calculated electron impact dissociation cross sections for molecular chlorine (Cl2)

Electron impact dissociation of Cl2 is a key process for the formation of Cl atoms in low-temperature plasmas used for industrial etching processes. Despite this, relatively little cross section data exist for this process. In this work, electron impact dissociation cross sections were calculated for Cl2 molecules using the UK molecular R-matrix code in the low electron energy range and extended to high energies using a scaling depending on the specific nature of each transition. Our results are compared with both previous calculations and with experimental measurements, and the similarities and differences are discussed. In addition, the rate coefficients for electron impact dissociation of Cl2 are calculated by integrating the cross sections derived in this (and previous) work, with electron energy distribution functions representative of those normally found in low-temperature plasmas used in industry. Depending on the shape and effective temperature of the distribution function, significant differences arise between the rate coefficients calculated from our cross sections and those calculated using previous data. Deviations between the two sets of rate coefficients are particularly pronounced at the low electron temperatures typical of electron beam and remote plasma sources of interest for atomic layer etching and deposition. These differences are principally caused by the higher energy resolution in the near-threshold region in this work, emphasising the importance of accurate, high-resolution cross sections in this energy range.

R-matrix study of electron impact excitation and dissociation of CH+ ions

Electron impact excitation and electron impact dissociation of CH+ ions are studied in the framework of the R-matrix method using the diatomic version of the UK molecular R-matrix codes. A configuration interaction calculation is first performed to yield the potential energy curves of the lowest eight singlet and triplet states of CH+. Scattering calculations are then performed to yield vibrationally-resolved electronic excitations to the lowest three bound states, namely the a , A and the b . Electron impact dissociation cross sections are obtained from the assumption that all electronic excitations above the dissociation threshold result in dissociation. Bound states of CH and resonance positions and widths of Feshbach resonances in the e–CH+ system are also calculated at the CH+ equilibrium bond length 2.137 a0.

Electron-neutral scattering cross sections for CO2: a complete and consistent set and an assessment of dissociation

This work proposes a complete and consistent set of cross sections for electron collisions with carbon dioxide (CO2) molecules to be published in the IST-Lisbon database with LXCat. The set is validated from the comparison between swarm parameters calculated using a two-term Boltzmann solver and the available experimental data. The importance of superelastic collisions with CO2(0 1 0) molecules at low values of the reduced electric field is discussed. Due to significant uncertainties, there are ongoing debates regarding the deconvolution of cross sections that describe generic energy losses at specific energy thresholds into cross sections that describe individual processes. An important example of these uncertainties is with the dissociation of CO2, for which the total electron impact dissociation cross section has not yet been unambiguously identified. The available dissociation cross sections are evaluated and discussed, and a strategy to obtain electron-impact dissociation rate coefficients is suggested.

Modeling plasma-based CO2 conversion: crucial role of the dissociation cross section

Plasma-based CO2 conversion is gaining increasing interest worldwide. A large research effort is devoted to improving the energy efficiency. For this purpose, it is very important to understand the underlying mechanisms of the CO2 conversion. The latter can be obtained by computer modeling, describing in detail the behavior of the various plasma species and all relevant chemical processes. However, the accuracy of the modeling results critically depends on the accuracy of the assumed input data, like cross sections. This is especially true for the cross section of electron impact dissociation, as the latter process is believed to proceed through electron impact excitation, but it is not clear from the literature which excitation channels effectively lead to dissociation. Therefore, the present paper discusses the effect of different electron impact dissociation cross sections reported in the literature on the calculated CO2 conversion, for a dielectric barrier discharge (DBD) and a microwave (MW) plasma. Comparison is made to experimental data for the DBD case, to elucidate which cross section might be the most realistic. This comparison reveals that the cross sections proposed by Itikawa and by Polak and Slovetsky both seem to underestimate the CO2 conversion. The cross sections recommended by Phelps with thresholds of 7 eV and 10.5 eV yield a CO2 conversion only slightly lower than the experimental data, but the sum of both cross sections overestimates the values, indicating that these cross sections represent dissociation, but most probably also include other (pure excitation) channels. Our calculations indicate that the choice of the electron impact dissociation cross section is crucial for the DBD, where this process is the dominant mechanism for CO2 conversion. In the MW plasma, it is only significant at pressures up to 100 mbar, while it is of minor importance for higher pressures, when dissociation proceeds mainly through collisions of CO2 with heavy particles.

Electron-impact dissociation cross sections of vibrationally excited He molecular ion

Electron-impact cross sections for the dissociation process of vibrationally excited He molecular ion, as a function of the incident electron energy are calculated for the dissociative transition by using the R-matrix method in the adiabatic-nuclei approximation. The potential energy curves for the involved electronic states and transition dipole moment, also calculated with the R-matrix method, were found to be in good agreement with the results reported in literature. The vibrationally resolved dissociation cross sections of He exhibit a resonant structure around 7 eV. The observed strong variation of the magnitude of this structure with the vibrational level is explained in terms of the overlap of initial and final (continuum) state wave functions in the Franck–Condon region.

Electron-impact dissociation of ozone cations toO+fragments

Absolute cross sections for electron-impact dissociation of ${{\text{O}}_{3}}^{+}$ ions yielding an ${\mathrm{O}}^{+}$ fragment are measured. The animated electron-ion crossed-beam method is applied in the energy range from 1 eV to 2.5 keV. The maximum total cross section for ${\mathrm{O}}^{+}$ fragment production is found to be $(3.6\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 75 eV and such a large value of the cross section extends down to low energies. The threshold energy is found to be below 2 eV. Contributions of dissociative excitation and dissociative ionization to ${\mathrm{O}}^{+}$ fragment production are determined separately. The total kinetic energy release distributions are determined at select electron energies. Present data are discussed and compared with recent results [S. H. M. Deng et al., Phys. Rev. A 82, 062715 (2010)].

Electron and photon dissociation cross sections of the D2 singlet ungerade continua

The dissociation of H2 and its isotopologues through excitation to the singlet ungerade continua is one of the major channels for the production of slow hydrogen atoms at high temperature (T > 10 000 K) and electron energy >25 eV. State-specific photodissociation cross sections and oscillator strengths for molecular deuterium from the X 1Σ+g(vi, Ji) levels to the continuum levels of the B 1Σ+u, C 1Πu, B′ 1Σ+u, D 1Πu, B, D′ 1Πu and 5pσ 1Σ+u states have been calculated. The corresponding (vi, Ji) state-specific electron impact dissociation cross sections have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ⩾ 5) npσ 1Σ+u and npπ 1Πu continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B′ 1Σ+u continuum is the dominant dissociation channel at room temperature, the C 1Πu and B 1Σ+u continua become more important at high temperature (>5000 K). The large increase of the C 1Πu and B 1Σ+u cross sections with (vi, Ji) is primarily responsible for making the continuum dissociation from a minor break-up channel at room temperature into a major one at high temperature. The electron dissociation cross section of D2 via the singlet ungerade continua is smaller than its H2 counterpart, although this difference decreases with temperature. This work, along with the previous calculations of H2 by Liu et al (2009, 2012), provides the complete electron impact dissociation cross sections of H2 and D2 through the singlet ungerade continua. Thermally averaged electron dissociation cross sections are provided at various temperatures for applications in plasma physics.

Electron and photon dissociation cross sections of the H2 singlet ungerade continua

Photodissociation cross sections and oscillator strengths for H2 from the X 1Σ+g (vi, Ji) levels to the continuum levels of the B 1Σ+u, C 1Πu, B, D′ 1Πu and 5pσ 1Σ+u states have been calculated. The (vi, Ji) state-specific electron impact dissociation cross sections to the continuum levels of these states have been obtained for the first time over a wide energy range using calculated continuum oscillator strengths along with previously published excitation functions of the Lyman and Werner bands. Estimated cross sections to the higher (n ⩾ 5) npσ 1Σ+u and npπ 1Πu continua are also provided. Both photon and electron impact excitation cross sections show strong dependences on the initial (vi, Ji) quantum numbers. Thermally averaged electron impact cross sections of all singlet ungerade states increase monotonically with temperature. While excitation to the B′ 1Σ+u continuum is the dominant dissociation channel at room temperature, the C 1Πu and B 1Σ+u continua become more important at high temperature (>5000 K). This work, along with the previous calculation of the B′ 1Σ+u and D 1Πu states by Liu et al (2009a J. Phys. B: At. Mol. Opt. Phys. 42 185203), provides the complete electron impact dissociation cross section of H2 through the singlet ungerade continua. Electron dissociation cross sections of the singlet ungerade continua are provided for the purpose of modelling atmospheric heating, analysis of occultation measurements and the hot atomic hydrogen plume observed at Saturn.

Electron-impact dissociative excitation and ionization of N2D+

Absolute cross sections for electron-impact dissociation of N${}_{2}$D${}^{+}$ producing N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragments were measured in the 5- to 100-eV range using a crossed electron-ion beams technique. In the 5- to 20-eV region, in which dissociative excitation (DE) is the principal contributing mechanism, N${}_{2}^{+}$ production dominates. The N${}_{2}^{+}$ + D dissociation channel shows a large resonant-like structure in the DE cross section, as observed previously in electron impact dissociation of triatomic dihydride species [M. Fogle, E. M. Bahati, M. E. Bannister, S. H. M. Deng, C. R. Vane, R. D. Thomas, and V. Zhaunerchyk, Phys. Rev. A 82, 042720 (2010)]. In the dissociative ionization (DI) region, 20- to 100-eV, N${}_{2}^{+}$, ND${}^{+}$, and N${}^{+}$ ion fragment production are comparable. The observance of the ND${}^{+}$ and N${}^{+}$ ion fragments indicate breaking of the $\mathrm{N}\phantom{\rule{-0.16em}{0ex}}\ensuremath{-}\phantom{\rule{-0.16em}{0ex}}\mathrm{N}$ bond along certain dissociation channels.

Electron-impact dissociation of ozone cationsO3+

Absolute cross sections for electron-impact dissociation of ${\mathrm{O}}_{3}{}^{+}$ ions yielding ${\mathrm{O}}^{+}$ and ${\mathrm{O}}_{2}{}^{+}$ fragment ions have been measured using a crossed electron-ion beams method for energies from about 3 eV to 100 eV. While the ${\mathrm{O}}_{2}{}^{+}$ channel dominates the dissociation cross section over the measured energy range, a strong enhancement is observed in the ${\mathrm{O}}^{+}$ channel at low energy.

Electron-impact dissociation ofXH2+(X=B, C, N, O, F): Absolute cross sections for production ofXH+andX+fragment ions

Absolute cross sections for electron-impact dissociation of $X$H${}_{2}^{+}$ ($X=$ B, C, N, O, F) producing $X$H${}^{+}$ and ${X}^{+}$ ion fragments were measured in the 3- to 100-eV range using a crossed-electron-ion beams technique. Dissociative excitation of BD${}_{2}^{+}$ and CH${}_{2}^{+}$ producing ${\mathrm{B}}^{+}$ and ${\mathrm{C}}^{+}$, respectively, show a propensity toward a two-body dissociation while the remaining species all tend to show a three-body dissociation dynamic. The BD${}^{+}$ and CH${}^{+}$ dissociative excitation channels show a large resonant-type contribution to the cross sections at $~10$ eV. For the ${X}^{+}$ fragment ion production cross sections, a clear dependence on the threshold energy, as it relates to the rate of rise in the cross section above threshold, is observed.

Electron-impact dissociation and ionization of NH+: formation of N+ and N2+

Absolute cross sections for electron-impact dissociation and ionization of NH+ leading to the formation of N+ and N2+ products have been measured by applying the animated electron–ion beam method, in the energy range from the respective thresholds up to 2.5 keV. The maximum total cross sections are observed to be (15.7 ± 0.7) × 10−17 cm2 and (11.1 ± 0.2) × 10−18 cm2 for N+ and N2+, respectively. Absolute cross sections are determined separately for dissociative excitation and for dissociative ionization processes. The measurements for slow N+ ions show a noticeable contribution in the low incident electron energy range; these data are attributed to excitation processes. Dissociative excitation is assumed to play a significant role in the collision energy region close to the vertical excitation energies for the lowest electronic transitions in the Franck–Condon region. The yields of fast N+ ions have also been measured; these energetic dissociations are generally ascribed to ionization processes. Kinetic energy release distributions are seen to extend from 0 eV to 15 eV for the N+ fragments and up to 20 eV for the N2+ ones. The present energy thresholds and kinetic energy release results are compared with available published data, allowing in some cases identification of fragmentation patterns and of molecular states contributing to observed processes. The possibility of dissociative excitation of the molecular ion which could occur via a mechanism involving autoionizing resonances is discussed.

Electron-impact dissociation ofCD3+andCH3+ions producingCD2+,CH+, andC+fragment ions

Using a crossed electron-ion beams method, we measured absolute cross sections for electron-impact dissociation of the CD3+ molecular ions producing CD2+ fragment ions and CH3+ ions yielding CH+ and C+ fragment ions over a collision energy range from a few eV up to 100 eV. The total experimental uncertainties are about 12% at the maximum of the curves of cross sections (peak of the cross section, for the CH+ channel). The obtained results suggest important roles played by pre-dissociation of bound states in the production of both the CH+ and C+ fragment ions. Good agreement is found with other results reported for the CH+ fragment, but some differences are found for the CD2+ and C+.

R-matrix calculation of electron collisions with the BF+ molecular ion

Electron collisions with the BF+ molecular ion are studied using the framework of the diatomic version of the UK molecular R-matrix codes. A configuration-interaction calculation is performed for BF+ to obtain potential energy curves and target properties for 14 lowest doublet and quartet states. Scattering calculations are performed which yield resonance parameters and excitation cross sections in the energy range 0–20 eV. Cross sections for rotational excitations and an approximate calculation for the electron impact dissociation cross section for BF+ are also presented.

Electron-impact dissociation cross sections for CHF3 and C3F8

Absolute total dissociation cross sections, σt,diss, by electron-impact are reported for CHF3 and C3F8 from 10 to 300 eV using the chemical gettering technique described by Winters and Inokuti (1982 Phys. Rev. A 25 1420). Data are concentrated in the near-threshold region (10–30 eV). The thresholds for dissociation of CHF3 and C3F8 are determined to be 10.4 eV and 11.9 eV, respectively. Ionization thresholds occur at 16 eV for CHF3 and 16.2 eV for C3F8. Neutral dissociation cross sections of both CHF3 and C3F8 are obtained by subtracting the ionization cross sections, σt,ion, from the total dissociation cross sections, σt,diss.

Electron-impact dissociation ofCH2+ions: Measurement ofCH+andC+fragment ions

Absolute cross sections for electron-impact dissociation of CH2+ producing CH+ and C+ fragment ions were measured in the 3-100 eV range using a crossed electron-ion beams technique with total uncertainties of about 11% near the cross section peak. The cross sections are nearly identical for energies above 15 eV, but they are dramatically different at lower energies. The CH+ channel exhibits a strong peak rising from an observed threshold of about 6 eV; the C+ channel is relatively flat down to the lowest measured energy. Ionization cross sections for the CH2+ ion are also presented.

Decomposition of Ethane in Atmospheric-Pressure Dielectric-Barrier Discharges: Experiments

It is well known that electric fields can influence combustion processes. When the magnitude of an external applied electric field exceeds the breakdown field of the fuel gas or fuel/oxidizer mixture, plasma effects dominate. The earlier work in the field of plasma-assisted combustion has demonstrated that dielectric-barrier-discharge (DBD)-driven nonthermal plasmas (NTPs) can increase flame speed and extend the combustion of hydrocarbon fuel gases into very lean-burn regimes. In this paper, results on the decomposition of ethane (C2H6) by DBDs at atmospheric pressure will be presented. The authors have chosen ethane for this paper because its gaseous electronics properties (electron-impact dissociation cross sections, drift velocity) are available in the literature. A subsequent paper will present results on the calculated yield of DBD-driven plasma decomposition products of ethane, as predicted by plasma-chemistry modeling. In this paper, results on experiments carried out to determine the decomposition products of ethane, as measured by gas chromatography are presented. An atmospheric-pressure DBD reactor processed a flowing gas stream of chemically pure ethane in the regime of plasma specific energy ranging from 1200 to 2400 J/std lit. The major stable decomposition products were H2, CH4, C2H2, and C2H4. These results are important in assessing the possibility of using NTPs to enhance the combustion of hydrocarbons