Total Single Ionization Cross Section 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.

He2+ impact single ionization cross sections of Cu atom

Semi-classical binary encounter approximation has been used for theoretical calculations of single ionization cross sections of Cu atom at ground state by alpha particle impact in energy range varying from threshold 35 keV/amu to 425 keV/amu. An accurate expression of cross section for energy transfer ΔΕ (σΔΕ) as given by Vriens and quantum mechanical Hartree-Fock velocity distributions for target electrons have been used in the calculation. Major contribution to the total single ionization cross sections of Cu are from 4s and 3d subshells. The ionization cross sections decrease with the increase of impact energy same as experimental data reported. The theoretical and experimental results of single ionization cross sections have same trends against the increase of impact energy. The ratio factor falls within 2, varying from 1.26 to 1.86, for given energy range. Theoretical results are under valid range. About 50% of total theoretical results of single ionization cross sections have ratio factor (R) . Major contribution to total ionization cross section is from 3d and 4s subshells-electrons whose contributions varies from 62 to 71% and 41.7% to 23.8% respectively. The higher value of linear correlation coefficient (=0.9647) and lower value of standard deviation (=0.822) shows that results calculated are close to the experimental data in the intermediate and high energy range. BIBECHANA 19 (2022) 111-118

Electron-impact ionization of Ar2+

We study electron-impact single-ionization cross-sections for all levels of the ground configuration of the Ar2+ ion using the distorted wave approximation. We investigate the influences of the direct ionization and excitation-autoionization processes on the total single-ionization cross sections. We used corrected energies for the levels of the Ar3+ ion so that the ionization threshold corresponds to the value recommended by National Institute of Standards and Technology (NIST). Radiative damping is taken into account for the ionization cross-sections of the indirect process. Excitations from the 3s and 3p subshells are investigated for the indirect process. We demonstrate that the excitations from the outermost 3p subshell of the ground configuration provide an important contribution to the single ionization cross-sections at low energies. Fairly good agreement with measurements is obtained for the ground level of the Ar2+ ion. We show that the direct ionization provides ~70% at peak of the total single-ionization cross-sections.

He2+ Impact Single Ionization Cross Sections of Fe Atom

Binary encounter approximation has been used for theoretical calculations of alpha particle (He2+) impact single ionization cross sections of iron atom at ground state in the energy range of 35 to 360 keV/amu. The cross sections for energy transfer given by Vriens’ and quantum mechanical Hartree-Fock velocity distributions for target electron have been used in the calculation. The contributions in total single ionization cross sections from 4s and 3d subshells are observed to be higher than from 3p, and the contributions from 4s decreases with increase of impact energy whereas the contribution from 3d increases. The total single ionization cross sections decrease gradually with the increase of impact energy similar to experimental results which implies that our results are in satisfactory agreement with the experimental data in the given energy range.

Excitation-autoionization contribution to single ionization of Sr by electron impact

The 4p6 excitation-autoionization (EA) cross-section of Sr atoms has been measured at electron-impact energies from the threshold to 600 eV. The cross-section has a pronounced resonance character in the near-threshold region, reaching its maximum value of (2.5 ± 0.8) × 10−16 cm2 at 24.5 eV. The shape and absolute value of the cross-section are determined by the excitation dynamics and the decay mode of autoionizing states from 4p54d5s2, 4p54d2nl and 4p54d5snl configurations. A comparative analysis of the measured EA cross-section, the calculated 5s2 ionization cross-section and the available ionization data showed that (i) direct ionization of the 5s2 shell and EA of the 4p6 shell adequately describe the single ionization of Sr atoms; (ii) 4p6 EA contribution reaches 25% of the total single ionization cross-section of Sr atoms.

Electron-impact ionization of the Kr atom

Electron-impact ionization cross sections are calculated for the ground configuration of the Kr atom. Time-dependent close-coupling cross sections for the direct ionization of the 4s and 4p subshells leading to single ionization are found to be a factor of 2 smaller than distorted-wave calculations due to the presence of strong continuum correlation effects. With the inclusion of relatively small excitation-autoionization contributions, the total single ionization cross sections for Kr are found to be slightly larger than crossed-beams experimental measurements.

Electron and photon ionization of the BeH molecule

We present electron and photon ionization cross sections for the BeH molecule. A configuration-average distorted-wave method is used to compute the electron and photon ionization cross sections based on earlier molecular structure tabulations. The electron ionization cross sections are compared with previous Deutsch-Mark and binary encounter Bethe calculations. Total single ionization cross sections are found to be larger due to the inclusion of an inner subshell. The photon ionization cross sections are compared with previous molecular-adapted quantum defect orbital calculations. The total single ionization cross sections are again found to be larger due to the inclusion of an inner subshell. We also calculated electron and photon ionization cross sections for a low lying excited configuration of BeH.

Excitation-autoionization of the 5p6 subshell in Ba atoms

The excitation-autoionization cross section for the 5p6 subshell in Ba atoms was determined in an incident electron energy range from the lowest autoionization threshold at 15.61 eV up to 600 eV. The data were obtained by measuring the total intensity of ejected-electron spectra arising from the decay of the 5p5n1l1n2l2n3l3 autoionizing states. The energy behavior of the cross section is characterized by the presence of a strong resonance structure in the near-threshold energy region of 15.6–22.7 eV as well as a broad maximum around 80 eV. The cross section reaches its maximum value of (3.2 ± 1.0)×10−16 cm2 at 17.4 eV, which makes out a contribution of up to 25% to the total single-ionization cross section of Ba atoms. The role of particular configurations in formation of the cross section is considered on the basis of the spectroscopic classification and excitation dynamics of the 5p5n1l1n2l2n3l3 autoionizing states.

Excitation and ionization of outer shells in Rb by electron impact

The relativistic distorted-wave and binary-encounter-dipole approximations were employed for calculating the electron-impact single ionization cross sections of the 5s, 4p6, 4s2, 3d10 shells and 4p6 excitation cross section for Rb atom taking into account both configuration interaction and relativistic effects. The capabilities of the most used theoretical approaches in describing the single ionization of Rb atom were considered by comparing the present and other available calculated data with the experimental total ionization and total direct single ionization cross sections over the electron-impact energy range from the 5s threshold to 600 eV. The best agreement within experimental uncertainty was obtained by using the non-relativistic binary-encounter-dipole approximation included in the LANL Atomic Physics Codes package. At present none of the used approximations (plane-wave Born or relativistic distorted wave) can satisfactorily describe the experimental excitation-autoionization cross section in rubidium because the efficient formation of the 4p6 core-excited negative-ion rubidium states at near-threshold impact energies is ignored in calculations.

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.

Resonance excitation of autoionizing states in alkali atoms by electron impact

In the work, the main results on excitation and decay processes of autoionizing states in alkali atoms are considered. As follows from the data on excitation dynamics of subvalence shells in an impact energy region from the lowest excitation threshold up to 600 eV, at threshold energies their electron excitation results exclusively from the formation of negative-ion resonances. At mean impact energies, the cascade population processes due to resonance excitation and subsequent radiative decay of the upper states play an essential role for the low-lying autoionizing states. By comparing with theoretical data, the absolute excitation cross sections for autoionizing states and negative-ion resonances were determined. The absolute autoionization cross sections as well as the autoionization contribution to the total single ionization cross section were also determined. Keywords: atom, ion, autoionization, resonances, ionization, effective cross section.

Target Structure-Induced Suppression of the Ionization Cross Section for Low-Energy Antiproton-Molecular Hydrogen Collisions: Theoretical Confirmation

Theoretical confirmation of the experimentally observed phenomenon [Knudsen et al., Phys. Rev. Lett. 105, 213201 (2010)] of target structure-induced suppression of the ionization cross section for low-energy antiproton-molecular hydrogen collisions is given. To this end a novel time-dependent convergent close-coupling approach to the scattering problem that accounts for all possible orientations of the molecular target, has been developed. The approach is applied to study single ionization of molecular hydrogen on the wide energy range from 1 keV to 2 MeV with a particular emphasis on low energies. Results for the orientation-averaged total single ionization cross section are compared with available experimental data and good agreement is found at low (<20 keV) and high (>90 keV) energies. A minor discrepancy is found within a small energy gap near the maximum of the cross section.

Excitation-autoionization cross section of alkali atoms by electron impact

The excitation-autoionization (EA) cross section of Na atoms was obtained experimentally in the form of normalized intensities of autoionizing spectra measured in the ejected-electron energy region between 25.5 and 33.7 eV for electron impact energies from the 2p6 excitation threshold at 30.77 up to 400 eV. The cross section exhibits the strong resonance behaviour in the near-threshold region and reaches the maximum value of 1.2 × 10−17 cm2 at 200 eV. By assigning the lines in ejected-electron spectra, the relative role of the 2p6 and 2s2 subshells as well as quartet and doublet levels in formation of the EA cross section was estimated. In particular, the autoionizing doublets in 2p53sns(n = 3 − 6), np(n = 3 − 6), nd(n = 3 − 4) configurations provide approximately 80% of the EA cross section at 200 eV impact energy. A comparative analysis of the EA cross sections for whole alkali group Li–Cs indicates that the near-threshold resonance behaviour of all cross sections is caused exclusively by the negative-ion resonances formed on the basis of the levels 1sn1l1n2l2 in Li and np5n1l1n2l2 in Na–Cs atoms. The relative EA contribution to the total single ionization cross section does not exceed 2% in Li and 5% in Na, and reaches 26% in K, 32% in Rb and 39% in Cs atoms.

Classical simulation of single-electron capture and ionization in ion-atom collisions

Classical trajectory Monte Carlo (CTMC) simulation method and the four-body formalism of boundary corrected continuum intermediate state (BCCIS-4B) approximations have been employed to determine total single electron capture cross section and total single ionization cross section in collisions of H+, He2+ and Li3+ with helium atom and energy in range of 30–200 keV/amu. Present theoretical study using CTMC method is based on three-body formalism, where interaction of the active electron in target ion has been approximated by a model potential containing both long and short range part. As a consequence, initialization of the problem has been made on a multiperiodic orbit in sharp contrast to singly periodic orbit occurring in hydrogenic model. In BCCIS-4B formalism, distortion in the final channel related to Coulomb continuum states of projectile ion and active electron in the field of residual target ion are included. Results so obtained from both methods are in very good agreement with available experimental findings.

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p5n1l1n2l2 autoionizing states. The cross section has two maxima, 1.8 × 10−16 and 2.2 × 10−16 cm2, at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.

ELECTRON IMPACT DISSOCIATIVE IONIZATION OF NITROSYL CHLORIDE

Electron impact dissociative ionization of nitrosyl chloride ( ClNO ) has been studied at the electron beam energy of 200 eV. The dissociation channels of up to triply ionized ClNO are investigated by two- and three-dimensional covariance mapping methods. The absolute cross-sections for the different dissociation channels are measured. No stable ClNO + or ClNO 2+ ions are observed in the mass spectrum. The most possible pathway for the dissociation of ClNO + is ClNO + → NO + + Cl . The total double ionization cross-section of ClNO is found to be 6.3% compared to the total single ionization cross-section. The main ion-pair dissociation channels for ClNO 2+ are ClNO 2+ → N + + O ++ Cl and ClNO 2+ → NO + + Cl +. The ClNO trications dissociate into either an ion pair or an ion triple, with comparable probabilities.

Electron impact total ionization cross sections of beryllium and boron isoelectronic ions

A simple user-friendly empirical formula is proposed to calculate the electron impact total single ionization cross sections of beryllium-like Be, B+, C2+, N3+, O4+, Ne6+, Fe22+, Hg76+, and U88+ and boron-like B, C+, N2+, O3+, Ne5+, Fe21+, Kr31+, and Xe49+ targets. A simplified and modified version of the Bell formula is used in the present model incorporating the ionic correction factor. The predicted ionization cross sections are compared with the available experimental and theoretical results. Excellent agreements are found for the incident energies ranging from threshold to 106 eV considered herein. The presented results achieve a level of agreement that is better than the calculations from the existing theoretical methods and empirical models. Hence, this model may be an excellent choice, due to its simplicity, for the practitioners in the fields of applied sciences and technologies.

Ionization and single electron capture in collision of highly charged Ar16+ ions with helium

This paper uses the two-centre atomic orbital close-coupling method to study the ionization and the single electron capture in collision of highly charged Ar16+ ions with He atoms in the velocity range of 1.2–1.9 a.u. The relative importance of single ionization (SI) to single capture (SC) is explored. The comparison between the calculation and experimental data shows that the SI/SC cross section ratios from this work are in good agreement with experimental data. The total single electron ionization cross sections and the total single electron capture cross sections are also given for this collision. The investigation of the partial electron capture cross section shows a general tendency of capture to larger n and l with increasing velocity from 1.2 to 1.9 a.u.

Fully Correlated Electronic Dynamics for Antiproton Impact Ionization of Helium

We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from 10 keV/amu to 1 MeV/amu. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found. At an impact energy of 1 MeV we also find that the double-to-single ionization ratio is twice as large for antiproton impact as for proton impact, confirming a long-standing unexpected experimental measurement.

Electron impact ionization of SiCl2 and SiCl

We measured absolute partial cross sections for the formation of all singly charged positive ions produced by electron impact on SiCl2 and SiCl from threshold to 200 eV using the fast-neutral-beam technique. Some of the cross section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV, which may be indicative of the presence of indirect ionization channels. Dissociative ionization channels are dominant for both species. The experimentally determined total single ionization cross sections for both species agree very well with calculated cross sections using the Deutsch-Mark (DM) formalism. A brief summary of the ionization cross sections determined for all four SiClx (x=1–4) species is given highlighting similarities and differences.