Absolute Cross Sections For Electron-impact Ionization Research Articles

On the electron impact cross-section maxima for dissociative and non-dissociative single ionization of molecules and radicals

Determining absolute electron impact ionization cross-sections for atoms and molecules, whether through experimental or theoretical means, poses a formidable challenge and is typically limited to a narrow selection of species. In this paper, by using measured cross-sections and polarizability data, it was obtained empirical scaling laws for total single ionization and non-dissociative single ionization cross-section maximum of neutral molecules by collisions with electrons. It is found that the total single ionization cross-sections scale linearly with the target electric static polarizability, while the non-dissociate cross-sections exhibit a saturation effect for large target polarizabilities. This saturation effect is interpreted as the increasing role of fragmentation with polarizability.

Electron impact ionization cross sections of highly charged open L-shell tungsten ions

L-shell electron-impact ionization (EII) cross sections for highly charged tungsten ions were measured at incident electron energies of 29.10 and 38.92 keV using the Shanghai-EBIT. Resolved x-rays from radiative recombination (RR) processes were recorded with a high-purity Ge detector in a static electron energy scanning mode. Absolute EII cross sections were obtained by normalizing to the theoretical RR cross sections. The experimental results were compared with the calculated results using the relativistic distorted-wave method implemented in the flexible atomic code and the relativistic Lotz semi-empirical equation. The measurements showed general agreement with the calculated results by two theoretical methods for Li- to N-like W ions. The experimental uncertainties are not sufficiently small to discern the two theoretical results. Furthermore, the influence of Breit interaction on the EII cross sections of open L-shell tungsten ions was studied, and the effect is small but non-negligible. The measured EII cross sections of open L-shell tungsten ions would contribute to fusion plasma studies.

I. Electron-impact ionization and dissociation of C2H 2 + and C2D 2 +

Absolute cross-sections for electron-impact ionization and dissociation of C2H2+ and C2D2+ have been measured for electron energies ranging from the corresponding thresholds up to 2.5 keV. The animated crossed beams experiment has been used. Light as well as heavy fragment ions that are produced from the ionization and the dissociation of the target have been detected for the first time. The maximum of the cross-section for single ionization is found to be (5.56 ± 0.03)× 10-17 cm2 around 140 eV. Cross-sections for dissociation of C2 H2+ (C2D2+) to ionic products are seen to decrease for two orders of magnitude, from C2D+ (12.6 ± 0.3) × 10-17 cm2 over CH+(9.55 ± 0.06) × 10-17 cm2, C+ (6.66 ± 0.05) × 10-17 cm2, C2+ (5.36 ± 0.27) × 10-17 cm2, H+ (4.73 ± 0.29) × 10-17 cm2 and CH2+ (4.56 ± 0.27) × 10-18 cm2 to H2+ (5.68 ± 0.49) × 10-19 cm2. Absolute cross-sections and threshold energies have been compared with the scarce data available in the literature.

Electron‐Impact Ionization of Be‐like Ciii, Niv, and Ov

We present recent measurements of absolute electron-impact ionization cross sections for Be-like C III, N IV, and O V forming Li-like C IV, N V, and O VI. The measurements were taken using the crossed-beams apparatus at Oak Ridge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in the ion beams. The measured ionization cross sections were compared with calculations using the R-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion.

Absolute electron-impact ionization cross sections of free radicals from threshold to 30eV: CHx=2,3, Si(CH3)x=1,2,3, and H2Si(CH3) radicals

We report absolute electron-impact ionization cross sections from threshold to 30eV for the formation of the parent ions from free radicals. A microwave discharge is used as a source to produce a large amount of these free radicals, and measurements are mainly performed by means of mass spectrometry. We detail the method used to measure electron-impact ionization cross sections of the main radicals produced. First, this method is tested on well known radicals such as CH3 and CH2, produced in a discharge sustained in an Ar–CH4 gas mixture; then it is applied to the free radicals Si(CH3)3,2,1 and H2Si(CH3), which are produced in a discharge sustained in Ar–Si(CH3)4. The experimental results are compared with the calculations performed using the binary-encounter-Bethe model and using a simplified form of this theory, the total ionization cross section model. Absolute electron-impact ionization cross sections have been measured at 30eV for Si(CH3) and CH3 by means of the selective energy transfer reaction process between Ar(P23) metastable species and the tetramethylsilane molecule [Si(CH3)4]. The value measured for Si(CH3) is used as the reference to calculate absolute values from relative ionization cross sections measured for the other radicals, Si(CH3)3,2,1 and H2Si(CH3), from threshold to 30eV. For example, we find at 30eV the absolute cross section values, 13.9×10−20±1.8×10−20, 2.0×10−20±0.3×10−20, 1.5×10−20±0.1×10−20, and 12.8×10−20±1.7×10−20m2 for m∕z=73, 58, 43, and 45, respectively.

Absolute electron impact ionization cross-sections and polarizability volumes for the C 2 to C 6 methanoates and C 3 to C 7 ethanoates

Absolute total positive ion electron impact ionization cross-sections from 15 to 285 eV are reported for the C 2 to C 6 methanoates (formates) and the C 3 to C 7 ethanoates (acetates), including all isomers. The reproducibility of the measured cross-sections over the full energy range is better than ±4%, and absolute cross-sections measured for a series of reference species (N 2 and inert gases) are in excellent agreement with values reported by several other groups claiming accuracies of around ±5%. A monotonic relationship is shown to exist between the maximum ionization cross-sections for the esters, reported here, the C 1 to C 4 alcohols, reported previously, and the total number of carbon atoms in the skeletal chain. The relationship provides a useful means for estimating unknown cross-sections. Polarizability volumes for esters may be estimated using a previously established correlation with the maximum ionization cross-section, and for the four species for which published data are available the predictions are in good agreement with experiment. Finally, the experimental data are compared with calculations carried out using the binary encounter Bethe (BEB) method for the calculation of total ionization cross-sections.

Comment on the accuracy of absolute electron-impact ionization cross sections for molecules

Electron impact ionization of molecules has been studied extensively in many laboratories and it is perhaps surprising therefore that uncertainty in the values of many cross sections has persisted. In an effort to rectify this situation absolute partial and total electron-impact ionization cross sections have been measured in this laboratory for a range of molecular targets, including H2, N2, O2, CO, NO, CO2, CH4, and SF6. These total cross sections for these molecules are compared here to those measured by Rapp and Englander-Golden, which have long been widely adopted as the de facto standard. The excellent agreement observed between these two independent data sets would seem to warrant a high level of confidence in them, and in the corresponding partial cross sections determined in this laboratory.

Calculated absolute electron-impact ionization cross sections for AlO, Al2O, and WOx (x=1–3)

The Deutsch–Märk (DM) formalism was used to calculate absolute electron impact ionization cross sections for the metal oxide molecules AlO, Al2O, and WOx (x=1–3). These molecules are important in materials research and they are also found as impurities in the plasma edge of fusion reactors. We also calculated ionization cross sections for the atoms Al and W. In the case of the Al-containing compounds, we find an unexpected ordering of the maximum ionization cross section σmax, σmax(AlO)<σmax(Al)<σmax(Al2O). Furthermore, the maximum ionization cross section for all four W-containing compounds W, WO, WO2, and WO3 is roughly the same with σmax values in the range of 6–7×10−16 cm2. These findings can be understood by analyzing the DM calculations for these species as well as on the basis of semi-classical arguments. In addition, calculations using the semi-empirical modified additivity rule confirm the trends in the cross section ordering for, respectively, the Al-containing and W-containing compounds that was predicted by the DM formalism, at least qualitatively.

Electron impact ionization of Ar 8+

Absolute electron impact ionization cross-sections have been measured for the neonlike Ar 8+ in the energy range from below the threshold for the metastable state to 2500 eV. No contribution of metastable states is observed. The results are well reproduced by the distorted wave Born approximation.

Electron-impact ionization of multicharged metal ions: Ni3+, Cu2+, Cu3+, and Sb3+

Absolute electron-impact ionization cross sections were measured as a function of collision energy for the multicharged metallic ions ${\mathrm{Ni}}^{3+}$, ${\mathrm{Cu}}^{2+}$, ${\mathrm{Cu}}^{3+}$, and ${\mathrm{Sb}}^{3+}$. The measurements cover an energy range from below threshold to 1000 eV (1500 eV for ${\mathrm{Cu}}^{2+}$). Recent distorted-wave direct ionization calculations agree with the measurements for ${\mathrm{Ni}}^{3+}$, ${\mathrm{Cu}}^{2+}$, and ${\mathrm{Cu}}^{3+}$ within 20%. Inner-shell excitation followed by autoionization produces up to a factor-of-2 enhancement in the ${\mathrm{Sb}}^{3+}$ cross section, and a rapid decrease in this indirect contribution at higher energies indicates the dominance of dipole-forbidden excitations.

Electron-impact ionization of Mg-like ions: S4+, Cl5+, and Ar6+

Absolute electron-impact ionization cross sections were measured as a function of collision energy for ions in the Mg-isoelectronic sequence ${\mathrm{S}}^{4+}$, ${\mathrm{Cl}}^{5+}$, and ${\mathrm{Ar}}^{6+}$. The measurements cover the energy range from threshold to 1500 eV and show onsets due to the indirect ionization process of inner-shell excitation followed by autoionization. The relative magnitude of the indirect ionization process increases dramatically in comparison with the direct process along the sequence, a feature which is also emphasized by earlier data for ${\mathrm{Al}}^{+}$.

Electron impact ionization of N 4+ and N 5+

A new crossed beams apparatus for measuring absolute electron impact ionization cross-sections of multiply charged ions is described. First measurements are also reported which deal with N 4+ and N 5+. The results are in good agreements with those previously obtained by the Oak Ridge group. For N 5+ a non-negligible signal is observed below the ground state ionization threshold. It is attributed to the presence of ions in metastable states. From the semi-empirical Lotz formula, the population of ions in those states is estimated to be approximately 3%.