Single Electron Impact Research Articles

A quantum approach to calculate differential and total cross sections for electron impact ionization of molecular targets

SynopsisWe present a theoretical approach to calculate the cross sections for the ionization of molecules by single electron impact. It is based on the First Born approximation (FBA), describing the ejected electron by a distorted wave (DW). The cross sections are calculated for an average molecular orientation with the proper average method. Single-center molecular wave functions are generated using Gaussian making this method easily applicable to any molecule. We show the calculated cross sections of some molecular targets of varying complexity.

Ionization of phenol by single electron impact: triple differential cross sections

Triple differential cross sections (TDCS) for the ionization by single electron impact of the highest and next-highest occupied molecular orbitals of phenol are provided in this work. A first Born approach is used, describing the ejected electron by a distorted wave. The molecular wave functions are developed in a single-center form using the computational chemistry program Gaussian. The TDCSs are calculated for an unknown molecular orientation with the proper average (PA) approach and using four different models. In the distorted wave model (DW) the atoms positions do not intervene in the interaction potential between the incoming electron and the molecular target, while in the distorted wave multi-center model (DW-MC) they are taken into account. In the DW-Gamow and the DW-MC-Gamow models, the repulsion between the outgoing electrons is described by adding the Gamow factor to the DW and DW-MC models. In general, all results are in good agreement with the experimental data. A small difference is observed between the DW and DW-MC results in the recoil region. Adding the Gamow factor improves the angular position and the relative amplitude of the two peaks observed in the binary region. When the TDCSs are compared to other theoretical results relying on the orientation averaged molecular orbital (OAMO) approximation, the improved agreement with the experimental data reinforces the importance of the PA approach.

Triple differential cross sections for the ionization of pyrimidine by electron impact

We provide a theoretical approach to determine the triple differential cross sections (TDCSs) for the ionization of molecules by single electron impact at low computational cost. This approach is based on the distorted wave (DW) method used within the first Born approximation (FBA). The short-range interaction between the ejected electron and the ionized target is explicitely taken into account. An important feature of this methodology is that it is applicable to any molecular target of known geometry. In this work, we determine the TDCSs of pyrimidine; the building block of some DNA and RNA nucleobases. The DW TDCSs are calculated for the ionization of five occupied orbitals and are compared with the available experimental data. In general, the DW TDCSs are in good agreement with the experimental data. This paper shows that the developed model can be easily applied to any molecule.

Triple differential cross sections for the ionization of tetrahydrofuran by electron impact

We provide triple differential cross sections (TDCS) for the ionization of tetrahydrofuran (THF) by single electron impact in coplanar asymmetric kinematics in the intermediate energy regime. For the Cs and C2 THF isomers, the analysis is carried out using (i) the highest occupied molecular orbitals energies available from ab initio calculations, and (ii) the TDCSs computed from a Coulomb wave (CW) and a distorted wave (DW) to describe the ejected electron. The calculated TDCSs using both the CW and DW models are in better agreement with experimental data than those obtained using the molecular three-body distorted wave model. Hence, we show that the developed framework can be applied to a relatively complex molecule and that the proposed models successfully describe the ionization of both THF isomers.

Triply Differential Ionization Cross Sections of atomic and molecular targets by single electron impact

We present numerically calculated triply differential cross sections for the simple ionization of two targets, atomic hydrogen and ammonia, in the First Born Approximation framework with one Coulomb wave describing the ejected electron. The partial wave series of the wavefunctions centered around the same origin were used and the intial state molecular wavefunction was derived using information provided by Gaussian03 software for ammonia. The theoretical results are calculated in the geometrical settings and kinematical conditions of previous experiments and are compared to published experimental and theoretical data. The results show good agreement with previous studies based on the same model for hydrogen. The general shape of the triply differential cross sections obtained for ammonia are in acceptable agreement with experimental data in the binary region but not in the recoil region where our simple framework fails to reconstruct the recoil peaks.

Dielectric barrier discharge plasma reactor

A dielectric barrier discharge plasma reactor has been designed and constructed. The space discharge regime has been implemented. The reactor operating conditions for the desired reaction have been calculated using the dissociation of water molecules as a model reaction. The operating conditions have been selected for the dissociation of water vapor via excitation of vibrational levels of the molecule by a single electron impact.

Single differential electron impact ionization cross sections in the binary-encounter-Bethe approximation for the low binding energy regime

An analytical expression based on the binary-encounter-Bethe model for energy differential cross sections in the low binding energy regime is presented. Both the binary-encounter-Bethe model and its modified counterpart are extended to shells with very low binding energy by removing the constraints in the interference term of the Mott cross section, originally introduced by Kim et al. The influence of the ionic factor is also studied for such targets. All the binary-encounter-Bethe based models presented here are checked against experimental results of low binding energy targets, such as the total ionization cross sections of alkali metals. The energy differential cross sections for H and He, at several incident energies, are also compared to available experimental and theoretical values.

Statistical model of electron impact ionization of multielectron ions

The statistical method for ab initio calculations of the electron impact ionization cross sections of multielectron ions and related ionization rates is developed. It is created based on an idea of collective excitations of atomic electrons alike in condensed medium. The Thomas–Fermi model density distribution of atomic electrons is assumed and their collective oscillations are described by the local plasma frequency model. Using a proposed statistical approach the calculations of the total single electron impact ionization cross sections of multielectron ions for several chosen chemical elements from argon up to uranium, taken in the various charge states, are performed and compared with available experimental and theoretical data, demonstrating a satisfactory agreement.

Implementation of the external complex scaling method in spheroidal coordinates: Impact ionization of molecular hydrogen

We develop an ab initio procedure based on the driven Schr\odinger equation formalism and the external complex scaling method for the determination of the multifold differential cross sections of the single and double ionization of molecular hydrogen by single photon and fast electron impact. We take advantage of the separability of the two-center Schrodinger equation in prolate spheroidal coordinates in the numerical calculation of the two-electron two-center wave function of the initial and final states of the target. After having verified our procedure by reproducing existing confirmed triple differential cross sections of the $(e,2e)$ ionization of ${\text{H}}_{2}$, we have extended our calculation to the double ionization of ${\text{H}}_{2}$. Our results on double photoionization agree with existing experimental results. We observe in the mean time a small difference with respect to the absolute results obtained by similar ab initio calculations using spherical bases. For the case of the double ionization by fast electron impact for which very few experimental results exist, our results confirm the existing disagreement between the theoretical results and the unique experimental one in the case of $(e,3\text{\ensuremath{-}}1e)$. This we think makes it clear that for $(e,3e)$ the introduction of the higher terms of the Born series for mean energy electron-impact regime is necessary.

Near-threshold electron impact ionization of Ne and Xe

Doubly differential cross-sections for the single electron impact ionization of Ne and Xe have been measured at several energies below the second ionization energy. The results indicate that the ionization of Ne is strongly influenced by the polarization of the ionized 22P3/2,1/2 core, where as this influence is significantly reduced for Xe. Single differential cross-sections are derived from the doubly differential cross-sections and for Xenon these show profiles similar to Helium ("smile"), whereas for Neon they show a dissimilar profile ("frown").

Electron impact ionization of Kr XI–Kr XIX ions

Cross sections for the single ionization of Kr XI–XIX ions by electron impact arecalculated in the Coulomb–Born approximation by the code ‘ATOM’ andcompared with experimental data and other theoretical results for Kr XI, Kr XIIand Kr XIX. Single electron impact ionization is the sum of the contributionsfrom direct ionization and from excitation of the autoionizing levels. The first partincludes only ionization from the 3s, 3p and 3d shells since ionization from the 1s,2s, 2p shells results in double ionization. The autoionizing levels (1s22s2p63s23p63dm nland 1s22s22p53s23p63dm nl)decay via autoionization and finally contribute to single ionization. The results arefound to be in satisfactory agreement with the experimental results obtained in acrossed electron–ion beam experiment in the energy range from threshold toabout 6 keV. The case of ions formed in metastable states is studied as they wereobserved experimentally.

Electron impact ionization of Kr10+–Kr18+ ions

Cross-sections for single ionization of Kr10+–Kr18+ ions by electron impact are calculated in the Coulomb–Born approximation by the code “ATOM” and compared with experimental data and with other theoretical results for charge states 10 and 18. Single electron impact ionization is sum of the contributions from direct ionization and from excitation of the autoionizing levels. Direct ionization includes ionization from 3s, 3p and 3d shells only since ionization from 1s, 2s, 2p results in double ionization. The autoionizing levels 2s2p63s23p63dmnl and 2s22p53s23p63dmnl decay via autoionization and finally contribute to single ionization. The results are found to be in satisfactory agreement with experimental results obtained in a crossed electron-ion beam experiment. The case of ions formed in metastable states is studied for Kr18+ where it was observed in the experiments.

Application of the Semi-Classical Approximation to the Single Electron-Impact Ionization of Helium-Like Ions

The semi-classical approximation is used to compute total cross sections for single electron-impact ionization of helium like ions in the ground state. It is shown that the results agrees, in general fairly well with the Coulomb–Born exchange approximation and with the available experimental data.

Estimation of Emission Cross Section for CH+(A1Π-X1Σ+: 419.0–441.0 nm) Produced by Electron Impact of CH4Using Spectral Simulation Technique

Emission cross sections for CH + (A 1 Π-X 1 Σ + ) produced by single electron impact of CH 4 are estimated at 100 and 400 eV by subtracting the calculated band intensities for CH(A 2 Δ-X 2 Π) from the experimentally measured band spectra. Vibrational and rotational temperatures are analyzed for CH molecule in the A 2 Δ state by fitting calculated band intensities assuming the Boltzman distributions in the upper electronic state to experimental ones at 17, 28, 100 and 400 eV. Rotational population distribution at 17 eV is quite different from others.

Electron impact ionization cross section of Ar +, Kr +, In + and Ge +

The single electron impact ionization cross sections of Ar +, Kr +, In + and Ge + were measured on the improved electron-ion impact apparatus at Fudan University. The Ar +, Kr + and In + experimental results are in good agreement with data from other laboratories. The cross section of Ge + was measured for the first time. The experimental results indicate that the indirect ionization process gives a larger contribution to the total ionization cross section of In +, as compared to the other three targets. The experimental data were compared with our Distorted-Wave-Born-Exchange-Approximation (DWBEA) calculation, the Lotz formula, and Configuration-Averaged-Distorted-Wave (CADW) calculations.

Electron impact dissociation of the NH3 molecule: Absolute and relative cross sections for NH*(c 1Π,A 3Π), NH+*(C 2Σ+, B 2Δ,A 2Σ−), and NH*2(Ã 2A1) fragment emissions

The emission spectrum between 2700 and 5000 Å following single electron impact on ammonia has been investigated. Emission bands of the electronically excited NH(c 1Π,A 3Π), NH+(C 2Σ+,B 2Δ,A 2Σ−), and NH2(Ã 2A1) fragments have been identified and absolute emission cross sections have been determined at 100 eV incident energy. Relative cross sections as a function of the impact energy reveal information about the excitation mechanisms of the parent molecule. Conclusions can be drawn on the dissociation process by a comparison of the observed appearance potentials with thermochemical thresholds and with the results of photodissociation experiments.

Absolute cross section measurements for electron impact ionization of Ar7+

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar7+ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.

Threshold Behaviour of Electron Impact lonisation Cross Sections for Rare Gas Atoms

AbstractFor electron energies from direct ionisation threshold up to several eV above cross sections for the production of He+, He2+, Ne+ up to Ne4+, Kr2+, Kr3+ and Xe2+ up to Xe4+ ions by single electron impact on atoms were measured. By a least square fit a power law was fitted to the cross sections. The exponents obtained by this procedure are compared with several models, i.e. a simple phase‐space calculation, the Wannier model considering interelectronic repulsion and a statistical approach similar to Russek's statistical energy deposition model. We find that especially ionisation of many‐electron atoms to high degrees of ionisation can not be described by the Wannier model, however, within the statistical approach.

Electron-impact dissociation of ammonia: Formation of NH+ ions in excited states

The emission spectrum following single-electron impact on NH3 has been investigated with the objective to search for emissions from excited NH+ ions produced by single-step dissociative excitation/ionization of NH3. The NH+ (C 2Σ+-X 2Π, 0-0) transition with an appearance potential of 28±1.5 eV was identified and an apparent emission cross-section of (7.0±2)×10−20 cm2 at 100 eV incident electron energy was determined for this transition.

Partial and state-selective cross sections for multiple ionisation of rare-gas atoms by electron impact

Partial ionisation cross sections for single electron impact on rare-gas atoms (Ne, Kr and Xe) have been measured for charge states up to q=8. In addition, by using the translational energy spectroscopy technique (TES) the authors have determined state-selective cross sections for the production of doubly and triply charged neon ions in specific electronic states. The contributions of direct and inner-shell processes are discussed.