Absolute Total Electron Impact Ionization Cross-Sections for Many-Atom Organic and Halocarbon Species

Abstract

The experimental determination of absolute total electron impact ionization cross-sections for polyatomic molecules has traditionally been a difficult task and restricted to a small range of species. This article reviews the performance of three models to estimate the maximum ionization cross-sections of some 65 polyatomic organic and halocarbon species. Cross-sections for all of the species studied have been measured experimentally using the same instrument, providing a complete data set for comparison with the model predictions. The three models studied are the empirical correlation between maximum ionization cross-section and molecular polarizability, the well-known binary encounter Bethe (BEB) model, and the functional group additivity model. The excellent agreement with experiment found for all three models, provided that calculated electronic structure parameters of suitably high quality are used for the first two, allows the prediction of total electron-impact ionization cross-sections to at least 7% precision for similar molecules that have not been experimentally characterized.

Electron-impact-ionization dynamics of five C2to C4perfluorocarbons

“…The BEB model usually is able to reproduce the ionization efficiency function in very good agreement with experiment, although a recent study considering some 65 polyatomic species has shown that the model systematically overestimates the cross section by ∼7% for nonfluorinated second-row organic species and ∼40%-45% for fluorinated species [51]. The major source of this discrepancy is the inclusion of neutral dissociation processes occurring at energies above the first ionization threshold [51,60]. This will be discussed in some detail in a later section.…”

“…Comparison of the ground and excited state B parameters employed in this paper with vertical ionization potentials from the photoelectron spectra of Jarvis et al [16,17] indicates excellent numerical agreement. The BEB model usually is able to reproduce the ionization efficiency function in very good agreement with experiment, although a recent study considering some 65 polyatomic species has shown that the model systematically overestimates the cross section by ∼7% for nonfluorinated second-row organic species and ∼40%-45% for fluorinated species [51]. The major source of this discrepancy is the inclusion of neutral dissociation processes occurring at energies above the first ionization threshold [51,60].…”

“…Collection of all ions is important in order to avoid discrimination against ions formed with large kinetic energies [49,50]. The instrument used to record TICSs has been used in the past to determine absolute TICSs in excellent agreement with benchmark experiments and has systematically performed well in measurements on a wide variety of species, ranging from diatomics to C 6 organic species [51]. The maximum instrumental uncertainty in the TICSs is ∼4%, yielding an overall final uncertainty in the measured PICSs of ∼8%.…”

“…A series of papers in which BEB theory was applied to selected molecules found that a value of Q = 1 gave universal consistency for polyatomic species, implying that, electronically, molecular bonding effects tend to dominate this parameter-see Ref. [51] for more details. Values of parameters B and U are taken from Bull et al [51].…”

“…[51] for more details. Values of parameters B and U are taken from Bull et al [51]. Comparison of the ground and excited state B parameters employed in this paper with vertical ionization potentials from the photoelectron spectra of Jarvis et al [16,17] indicates excellent numerical agreement.…”