Dissociative ionisation of methane by electron impact (original) (raw)
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The electron impact partial ionization cross-sections of molecules such as methane, water and nitromethane are computed using a modified form of the binary encounter Bethe (BEB) formula. The modified form of the BEB model works on rescaling the molecular binding energies of the orbitals and the scaling of cross-sections using the electron ionization mass spectrometry data. The computed partial ionization cross-sections are consistent with the recommended data and are better than several experimental and theoretical results. The summed partial ionization cross-sections of different fragments also agree with the total ionization cross-sections obtained from BEB and the experimental data. This work highlights the utility of mass spectrometry in the modeling and interpretation of the ionization cross-section data. The limitations and the advantages of the modified form of the BEB model are also discussed.
Cross sections and ion kinetic energies for electron impact ionization of CH4
International Journal of Mass Spectrometry, 2003
Total and partial cross sections for electron impact ionization of CH 4 were measured from threshold to 1000 eV. Ion kinetic energy distributions were measured applying a deflection and retarding field method. The extraction of ions from the ion source was simulated fully three-dimensionally including the presence of a magnetic field for electron guidance. Thereby, discrimination factors were determined as a function of the initial ion kinetic energy. Multiplication of these factors with the ion signal leads to relative partial cross sections. By normalizing the sum of these partial cross sections to an absolute value taken from the literature, absolute partial cross sections were obtained that agree well with previous measurements where a complete collection of the product ions has been demonstrated. Moreover, with the present method, it is possible to determine cross sections that are differential with respect to the initial kinetic energy of the ion.
Photoionization and fragmentation of gaseous methane induced by tunable synchrotron radiation were investigated in a wide energy range, from 40 eV up to 480 eV. We report electron-ion coincidence experiments by measuring the relative partial-ion yields and precursor-specific relative yields for individual fragment ions and for ion fragment pairs as a function of photon energy. The fragmentation patterns are discussed with emphasis on the transition behavior of the bond breaking reactions and of the hydrogen rearrangements from valence to core electron ionization. Below the C 1s threshold, a comparison between photon induced dissociation and electron impact data showed that the ionic fragments formation depends for both projectiles on the same final electronic state reached upon ionization.
Ionization of methane and its electronic energy levels
Canadian Journal of Chemistry, 1967
The retarding potential difference (r.p.d.) efficiency curves of methane obtained with two different types of mass spectrometers are presented and discussed. It is shown that a Jahn–Teller splitting could explain some of the experimentally observed energy levels. Pre-ionization is shown to occur at 14.4, 16, and 19.5 eV. In particular, the latter value is rediscussed in terms of the second ionization potential of methane. It is concluded that the recent explanation of Ehrhardt and Linder, according to which the 19.5 eV level is a forbidden pre-ionized state, is compatible with our results and that the second ionization potential of methane should be around 24 eV. Ion–molecule reactions are shown to be of small importance around 19.5 eV, contrary to a hypothesis suggested by Sjögren.
CH4 ionization and dissociation by proton and electron impact
Journal of Physics B: …, 2003
Absolute dissociative and non-dissociative ionization cross sections of CH + n (n = 0-4) molecules by protons in the 0.5-3.5 MeV impact energy range have been measured. The present results are in good agreement with the previously published data, where overlap occurs. A decay-route model for CH 4 dissociation induced by proton impact is proposed and a method to obtain the corresponding branching ratios is introduced. The consistency and extent of the proposed model and methodology is verified through its application in the case of electron-impact dissociation.
Neutral dissociation of methane by electron impact and a complete and consistent cross section set
Plasma Sources Science and Technology, 2021
We present cross sections for the neutral dissociation of methane, in a large part obtained through analytical approximations. With these cross sections the work of Song et al (2015 J. Phys. Chem. Ref. Data 44 023101) can be extended, which results in a complete and consistent set of cross sections for the collision of electrons with up to 100 eV energy with methane molecules. Notably, the resulting cross section set does not require any data fitting to produce bulk swarm parameters that match with experiments. Therefore consistency can be considered an inherent trait of the set, since swarm parameters are used exclusively for validation of the cross sections. Neutral dissociation of methane is essential to include (1) because it is a crucial electron energy sink in methane plasma, and (2) because it largely contributes to the production of hydrogen radicals that can be vital for plasma-chemical processes. Finally, we compare the production rates of hydrogen species for a swarm-fitt...
III. Electron-impact dissociative ionization of C2H 2 + and C2D 2 +
The European Physical Journal D, 2010
Absolute cross-sections for electron-impact dissociative ionization of C2H + 2 and C2D + 2 to CH + , C + , C + 2 , H + , CH + 2 and C2D + fragments are determined for electron energies ranging from the corresponding threshold to 2.5 keV. Results obtained in a crossed beams experiment are analyzed to estimate the contribution of dissociative ionization to each fragment formation. The dissociative ionization cross sections are seen to decrease for more than an order of magnitude, from CH + (5.37 ± 0.10) × 10 −17 cm 2 over C + (4.19 ± 0.16) × 10 −17 cm 2 , C2D + (3.94 ± 0.38) × 10 −17 cm 2 , C + 2 (3.82 ± 0.15) × 10 −17 cm 2 and H + (3.37 ± 0.21) × 10 −17 cm 2 to CH + 2 (2.66 ± 0.14) × 10 −18 cm 2 . Kinetic energy release distributions of fragment ions are also determined from the analysis of the product velocity distribution. Cross section values, threshold energies and kinetic energies are compared with the data available from the literature. Conforming to the scheme used in the study of the dissociative excitation of C2H + 2 C2D + 2 , the cross-sections are presented in a format suitable for their implementation in plasma simulation codes.
Single electron ionization of NH 3 and CH 4 by swift proton impact
Journal of Physics: Conference Series, 2015
Single ionization from ammonia and methane molecules by impact of protons is analysed. Double differential cross sections are calculated, within the post-and prior-versions of the continuum distorted wave-eikonal initial state (CDW-EIS) model, considering two different representations of the initial bound state of the active electron. A comparison between both set of results shows a very little sensitivity of the double differential cross sections to the initial state representation in the case of ammonia molecules. On the other hand, discrepancies are found when CH4 molecular target is considered, showing that in this case the description of the corresponding molecular orbitals influences the cross section calculations.
Newly calculated absolute cross-section for the electron-impact ionization of C2H2+
The European Physical Journal D, 2006
New measurements of the cross-section for electron impact ionization of the molecular ion C2H + 2 have been carried out recently. These data differ significantly from earlier data, because cross-sections corresponding to all the possible dissociative ionization processes were determined. The new data in conjunction with the significant discrepancies between the earlier data and the results of various calculations, which disagreed among themselves by a factor of 3, motivated a renewed attempt to apply the semi-classical Deutsch-Märk (DM) formalism to the calculation of the absolute electron-impact ionization cross-section of this molecular ion. A quantum chemical molecular orbital population analysis for both the neutral molecule and the ion revealed that in the case of C2H + 2 the singly occupied molecular orbital (i.e. the "missing" electron) is highly localized near the site of a C atom in the molecule. This information is explicitly incorporated in our formalism. The results obtained by taking the ionic character directly into account are in excellent agreement with the recent experimental data.