Electron-impact-ionization dynamics of five C2to C4perfluorocarbons

Abstract

Reprinted with permission from the American Physical Society: Phys. Rev. A 88, 062710 c (2013) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modied, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Perfluorocarbons (PFCs) are man-made compounds whose ion physics exhibit complex interplays between statistical and nonstatistical fragmentation and intramolecular rearrangement processes. One probe of such processes is the energy-dependent electron-impact-ionization cross section. Partial electron-impact-ionization cross sections are reported for the fragments arising from five C 2 to C 4 PFCs, namely, C 2 F 6 , C 3 F 8 , C 3 F 6 , CF 2 =CF-CF=CF 2 , and CF 3 -C≡C-CF 3 , over the energy range from threshold to ∼210 eV. Care was taken to maximize ion collection efficiency and to minimize discrimination against ions produced with high kinetic-energy release, and the measured cross sections have been calibrated using independent absolute total (gross) ionization efficiency curves measured previously in the same laboratory with an instrument that was designed to essentially have unit detection efficiency. Total ionization cross sections have also been modeled using the binary-encounter Bethe model, and the shortcomings of the model when applied to perfluorinated compounds are discussed. Analysis of the mass spectral fragmentation patterns in combination with ab initio energetics suggests that nonstatistical dissociative ionization processes play a significant role in the fragmentation dynamics of saturated PFCs. In contrast, unsaturated PFCs exhibit long-lived parent ions, which tend to undergo a higher degree of statistical dissociation following ionization, involving considerable intramolecular rearrangement.

Brief Review of Electron Collision Studies of Molecules Relevant to Plasma

“…However, for a few fluorocarbons, the results obtained using such orbital parameters do not agree better with experimental data compared to Hartree Fock (HF) orbitals and overestimate experimental results. For C 2 F 6 , Gupta et al [34] have shown that the results obtained using the density functional theory (DFT) (ωB97X) functional for orbital binding and kinetic energies are in reasonable agreement with the P3-EPT calculations of Bull et al [14], with slight variations at the peak of the cross section. The results obtained using the DFT (ω B97X-D) functional are slightly higher than those obtained using the DFT (ωB97X) functional, which may be owing to the inclusion of dispersion in the functional.…”

“…Orbital energies are calculated at various levels of theoretical approximation using the Gaussian 09 [139] package. Bull et al [14] have recently suggested that the orbital parameters employed in the calculation of Q ion using the BEB method should be obtained using a high level of theoretical approximation. They have utilized the partial third-order electron propagator theory (P3-EPT) level of approximation for obtaining orbital parameters for binding energies less than 20 eV.…”

Electron-impact-ionization dynamics of SF6

“…While a large number of electron ionization cross-sections have been reported for SF6, summarised in a number of reviews [2,7,8], there is varying agreement between the different determinations. This variation is due partly to the fact that conventional mass spectrometers and weak-field ion sources with small source apertures discriminate against ions formed with high kinetic energy [9,10]. It is now well-established that SF6, like other small saturated perfluorinated species, does not have a bound parent cation in the Franck-Condon window.…”

“…Gaining a detailed understanding of the microscopic dynamics of electron ionization for a 'large' molecular target like SF6 is a challenging task due to the array of possible ionized states that may be accessed [10,14]. Electron ionization is a collisional interaction, in which the electron may in principal transfer any fraction of its kinetic energy to the molecule.…”

Electron-impact ionization cross section of formic acid

“…However, as this is not the only available process during the electron impact, the BEB model usually overestimates the ionization cross section [40,41].…”