Angular Distribution Of Ejected Electrons Research Articles

Autoionization of strontium atom during excitation of 4p<sup>6</sup> shell

Background : Autoionization is an indirect ionization process that significantly increases the total ionization cross section of the atom. Measurements of electron autoionizing spectra in a broad range of impact energies and the determination of their total intensity allow obtaining the absolute value of the autoionization contribution (the autoionization cross section) and its energy behavior. Methods : The study of the ejected-electron spectra of strontium atoms were carried out by using an electron spectrometer. It consists of a source of the incident electron beam, an electron energy analyzer (of 127° electrostatic type deflector) and an atomic beam source. To minimize the influence of the anisotropy of the angular distribution of ejected electrons, the measurements were carried out at a magic observation angle of 54.7°. The incident and ejected-electron energy resolutions were about 0.4 eV and 0.07 eV, respectively. The increment step of the incident electron energy was 0.1 eV in the near-threshold energy region. The autoionization cross section was determined as the normalized sum of total intensities of ejected-electron spectra measured at different impact energies. Results : Autoionization cross section for 4p 6 shell of strontium atoms was determined in an incident-electron energy range from the lowest autoionization threshold (20,98 eV) up to 600 eV. Energy behavior of the cross section is characterized by the presence of the strong resonance structure in the threshold region of 21-30 eV and a broad maximum at approximately 100 eV. Cross section reaches its maximum value of 1.6×10 -16 сm 2 at 24.8 eV. We analyze the excitation and decay processes of the 4p 5 n 1 l 1 n 2 l 2 n 3 l 3 autoionizing states. They determine the magnitude and the energy behavior of the autoionization cross section. Conclusions : The strong resonant excitation of the levels in 4p 5 4d5s 2 , 4p 5 4d 2 5s, and 4p 5 5s 2 5p configurations determines the resonant behavior of the measured autoionization cross section at low impact energies. Shape and magnitude of the cross section at high impact energies are determined by the total contribution from dipole-allowed and singlets autoionizing levels in the configurations 4p 5 4d5s 2 , 4p 5 4d 2 5s.

Autoionization cross-sections of cesium and barium atoms for 5p<sup>6</sup> shell excited by electron impact

Background : The presence of a closed valence shell in alkaline-earth atoms makes the processes of their excitation and ionization as a whole much more complicated than for neighboring alkali atoms. In order to find patterns and differences in the course of autoionization in these two groups of atoms, it is interesting to compare their autoionization cross sections. Studies of the latter for alkaline-earth atoms have not been conducted until recently. In this paper, the 5p 6 autoionization cross sections of cesium and barium atoms excited by electrons were investigated in the impact-energy range from the excitation threshold of the 5p 6 subshells to 600 eV. Methods : The measurements of the ejected-electron spectra of Cs and Ba atoms were carried out by using an electron spectrometer consisted of a source of the incident electron beam, an electron energy analyzer (of 127 ° electrostatic type) and an atomic beam source. To minimize the influence of the anisotropy of the angular distribution of ejected electrons, the measurements were carried out at a magic observation angle of 54.7 ° . The incident and ejected-electron energy resolutions were about 0.4 eV and 0.07 eV, respectively. The increment step of the incident electron energy was 0.1 eV in the near-threshold energy region. The autoionization cross section of Cs and Ba atoms were determined as the normalized total intensities of ejected-electron spectra measured at different impact energies. Results : The autoionization cross sections for 5p 6 subshells in Cs and Ba atoms were determined in an incident-electron energy range from the lowest autoionization thresholds up to 600 eV. The energy behavior of both cross sections is characterized by the presence of the strong resonance structure in the threshold region of 12-23 eV and a broad maximum at approximately 100 eV. The cross sections reach their maximum value of 4.2×10 -16 сm 2 for cesium and 6.7×10 -16 сm 2 for barium at 14.8 eV and 17.4 eV, respectively. An analysis is made of the excitation and decay processes of the 5p 5 n 1 l 1 n 2 l 2 (Cs) and 5p 5 n 1 l 1 n 2 l 2 n 3 l 3 (Ba) autoionizing states that determine the magnitude and the energy behavior of the autoionization cross sections. Conclusions : The strong resonant excitation of the lowest levels in 5p 5 6s 2 , 5p 5 5d6s configurations in cesium and 5p 5 5d6s 2 , 5p 5 5d 2 6s,6p,6d,7d, 5p 5 6s 2 6p,7s,7d and 5p 5 5d6s6p;7s,7d configurations in barium determines the resonant behavior of the measured autoionization cross sections at low impact energies. The shape and magnitude of the cross sections at high impact energies are determined by the total contribution from doublet autoionizing states in cesium and dipole-allowed autoionizing states in the lowest configurations 5p 5 5d6s 2 and 5p 5 5d 2 6s in barium.

Application of a post-collisional-interaction distorted-wave model for (e, 2e) of some atomic targets and methane

Recently Isik et al (2016 J. Phys B: At. Mol. Opt. Phys. 49 065203) performed measurements of the triple differential cross sections (TDCSs) of methane by electron impact. Their data clearly show that post-collisional interaction (PCI) effects are present in the angular distributions of ejected electrons. A model describing the ejected electron by a distorted wave and including PCI is applied for the single ionization of atomic targets and for methane. Extensive comparisons between this model and other previous models are made with available experiments.

Ionization of small molecules induced byH+, He+, andN+projectiles: Comparison of experiment with quantum and classical calculations

We report the energy and angular distribution of ejected electrons from $\mathrm{C}{\mathrm{H}}_{4}$ and ${\mathrm{H}}_{2}\mathrm{O}$ molecules impacted by 1 MeV ${\mathrm{H}}^{+}, \mathrm{H}{\mathrm{e}}^{+}$, and 650 keV ${\mathrm{N}}^{+}$ ions. Spectra were measured at different observation angles, from 2 to 2000 eV. The obtained absolute double-differential electron-emission cross sections (DDCSs) were compared with the results of classical trajectory Monte Carlo (CTMC) and continuum distorted wave, eikonal initial state (CDW-EIS) calculations. For the bare ${\mathrm{H}}^{+}$ projectile both theories show remarkable agreement with the experiment at all observed angles and energies. The CTMC results are in similarly good agreement with the DDCS spectra obtained for impact by dressed $\mathrm{H}{\mathrm{e}}^{+}$ and ${\mathrm{N}}^{+}$ ions, where screening effects and electron loss from the projectile gain importance. The CDW-EIS calculations slightly overestimate the electron loss for 1 MeV $\mathrm{H}{\mathrm{e}}^{+}$ impact, and overestimate both the target and projectile ionization at low emitted electron energies for 650 keV ${\mathrm{N}}^{+}$ impact. The contribution of multiple electron scattering by the projectile and target centers (Fermi shuttle) dominates the ${\mathrm{N}}^{+}$-impact spectra at higher electron energies, and it is well reproduced by the nonperturbative CTMC calculations. The contributions of different processes in medium-velocity collisions of dressed ions with molecules are determined.

Branching ratio and angular distribution of ejected electrons from Eu 4f76p1/2nd auto-ionizing states**Project supported by the National Natural Science Foundation of China (Grant No. 11174218).

The branching ratios of ions and the angular distributions of electrons ejected from the Eu 4f76p1/2nd auto-ionizing states are investigated with the velocity-map-imaging technique. To populate the above auto-ionizing states, the relevant bound Rydberg states have to be detected first. Two new bound Rydberg states are identified in the region between 41150 cm−1 and 44580 cm−1, from which auto-ionization spectra of the Eu 4f76p1/2nd states are observed with isolated core excitation method. With all preparations above, the branching ratios from the above auto-ionizing states to different final ionic states and the angular distributions of electrons ejected from these processes are measured systematically. Energy dependence of branching ratios and anisotropy parameters within the auto-ionization spectra are carefully analyzed, followed by a qualitative interpretation.

Three dimensional particle-in-cell simulations of electron beams created via reflection of intense laser light from a water target

We present 3D Particle-in-Cell (PIC) modeling of an ultra-intense laser experiment by the Extreme Light group at the Air Force Research Laboratory using the Large Scale Plasma (LSP) PIC code. This is the first time PIC simulations have been performed in 3D for this experiment which involves an ultra-intense, short-pulse (30 fs) laser interacting with a water jet target at normal incidence. The laser-energy-to-ejected-electron-energy conversion efficiency observed in 2D(3v) simulations were comparable to the conversion efficiencies seen in the 3D simulations, but the angular distribution of ejected electrons in the 2D(3v) simulations displayed interesting differences with the 3D simulations' angular distribution; the observed differences between the 2D(3v) and 3D simulations were more noticeable for the simulations with higher intensity laser pulses. An analytic plane-wave model is discussed which provides some explanation for the angular distribution and energies of ejected electrons in the 2D(3v) simulations. We also performed a 3D simulation with circularly polarized light and found a significantly higher conversion efficiency and peak electron energy, which is promising for future experiments.

Dynamic properties of Eu 4f76p1/2ns autoionization process

To explore the dynamic properties of Eu 4f76p1/2ns autoionization process, the autoionization branching ratios of ions and the angular distributions of ejected electrons from the Eu 4f76p1/2ns (n=7, 9) autoionizing states are systematically investigated with the combination of the three-step isolated-core excitation (ICE) and the velocity-map imaging techniques The Eu 4f76sns Rydberg states are populated via a two-step laser excitation, from which the Eu 4f76p1/2ns autoionizing states are excited by the wavelength of the third laser around the Eu 6s+6p1/2+ ionic resonance in order to obtain autoionization spectra and the velocity-map images of ejected electrons from the Eu 4f76p1/2ns autoionizing states. Once the velocity-map images have been measured, both the energy distribution and angular distribution of ejected electrons can be acquired. Moreover, the spectra of the branching ratios and the anisotropic parameters within the autoionization resonances are also measured to observe their energy dependence and the relation with the autoionization spectra. Comparisons of the observed spectra of 4f76p1/2ns autoionizing states with n = 7, 8, and 9 manifest that the ICE technique is more suitable for the higher-n members of autoionization series. It is found that the Eu atoms in the 4f76p1/2ns (n = 8, 9) autoionizing states mainly decay into 4f75d+(9D) ionic state, leading to the population inversion between 4f75d+(9D) and 4f76s+ (7S) or 4f76s+ (9S) ionic states, which is significant for developing the autoionization laser. The angular distributions of the ejected electrons from the Eu 4f76p1/2ns autoionizing states show simple patterns at the energy points corresponding to the peaks of autoionization spectra, and have complicated patterns in the energy regions off the peaks of autoionization spectra, especially in the regions corresponding to the sharp increase or decrease in the autoionization spectra. The above phenomena can be explained with the strength of configuration interaction among different autoionization series converging to different ionic states, which is fluctuated within the energy region of autoionization spectra. In addition, within the autoionization resonance both the spectra of branching ratios and anisotropic parameters vary irregularly, and no obvious correlation with the spectra of 4f76p1/2ns autoionizing states can be found.

The VMI study on angular distribution of ejected electrons from Eu 4f76p1/26d autoionizing states**Project supported by the National Natural Science Foundation of China (Grant No. 11174218).

The combination of a velocity mapping imaging technique and mathematical transformation is adopted to study the angular distribution of electrons ejected from the Eu 4f76p1/26d autoionizing states, which are excited with a three-step excitation scheme via different Eu 4f76s6d 8 DJ (J = 5/2, 7/2, and 9/2) intermediate states. In order to determine the energy dependence of angular distribution of the ejected electrons, the anisotropic parameters are measured in the spectral profile of the 6p1/26d autoionizing states by tuning the wavelength of the third-step laser across the ionic resonance lines of the Eu 6s+ → 6p+. The configuration interaction is discussed by comparing the angular distributions of ejected electrons from the different states. The present study reveals the profound variations of anisotropic parameters in the entire region of autoionization resonance, highlighting the complicated nature of the autoionization process for the lowest member of 6p1/26d autoionization series.

Nondipole parameters of TDCS for electron impact ionization and drag current

Abstract A comprehensive study is undertaken of angular distributions of electron knock-out from atomic targets by fast electrons with a small transfer of momentum. The general expressions for the parameters of the triple differential cross-section of impact ionization in the optical limit are derived. The calculated parameters are compared with those of the angular distribution of electrons ejected from an atom in the process of photoionization. In these processes, when the multipole transitions are involved, the one-to-one correspondence between the photoionization and impact ionization parameters disappears. The nondipole transitions lead to the backward/forward asymmetry of the angular distribution of ejected electrons that is absent in the dipole approximation for ionization by both fast electrons and photons. Using the He atom as an example, the character of the asymmetry for these two processes is qualitatively different and the backward/forward asymmetry results in macroscopic directed motion of secondary electrons accompanying the passing of a fast electron beam through gas or plasma. The general formulas for this drag current are derived and applied to gaseous He.

Tracing fingerprints of young type interferences in angular distributions of ejected electrons from molecular targets

Interference patterns in angular distributions of ionized electrons due to coherent emission from the proximities of the nuclei of molecular hydrogen are investigated in (e,2e) collisions. It is shown that double and single differential cross sections show signatures of this effect.

Angular distribution of ejected electrons at the laser-cluster interactions

The histograms of deflection angles of electrons ejected from Xe clusters irradiated by femtosecond super-intense laser pulses are presented. The dependence of the angular distribution on the peak laser intensity, the pulse duration, and the cluster position is considered. A clear relationship between the final electron energy and the deflection angle is shown. The deflection angles are calculated by solving the relativistic equation of motion taking into account the Lorentz force and the Coulomb field of the ionized cluster. The ions in the cluster undergo sequential multiple ionization up to charge multiplicity Z = 26. The measurements of the electron angular distributions allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.

Angular distribution in two-photon double ionization of helium by intense attosecond soft-x-ray pulses

We investigate two-photon double ionization of helium by intense ({approx_equal}10{sup 15} W/cm{sup 2}) ultrashort ({approx_equal}300 as) soft-x-ray pulses (E=91.6 eV). The time-dependent two-electron Schroedinger equation is solved using a coupled channel method. We show that for ultrashort pulses the angular distribution of ejected electrons depends on the pulse duration and provides insight into the role of electron correlations in the two-electron photoemission process. The angular distribution at energies near the 'independent-electron' peaks is close to dipolar while it acquires in the 'valley' of correlated emission a significant quadrupolar component within a few hundred attoseconds.

A critique of finite Hilbert basis set calculations for the angular distribution of ionized electrons produced in p + H impact at 20 keV

We present a critique of a finite Hilbert basis set calculation of the angular distribution of ejected electrons in a p+H collision at 20keV.

Finite-Hilbert-basis-set calculations for the angular distribution of ionized electrons produced inp+Himpact at20keV

We present a different method of extracting the angular distribution of ejected electrons in an ion-atom collision from a two-centered finite Hilbert basis-set calculation. We obtain good agreement with experiment for a p+H collision at 20 keV if we include an interference between the target centered and projectile centered amplitudes.

Observation of interference effects in ejected electrons from16.0−keVe−−SF6collisions

The relative energy- and angle- dependent cross sections for emission of electrons from ${\mathrm{SF}}_{6}$ molecule by impact of $16\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ electrons have been measured. The angular distributions of ejected electrons are shown to exhibit an oscillatory structure which is suggested to arise due to an interference effect. The condition for interference effect for the present collision system has been examined and it is shown that the appearance of interference pattern takes place above a threshold energy of $65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the ejected electrons. The ejected electrons producing an interference structure are suggested to originate from two atomic centers of a transiently formed doubly ionized parent molecule, namely, ${\mathrm{SF}}_{6}^{2+}$. This extremely unstable ion suffers a Coulomb explosion and gives rise to many singly charged stable radical and atomic ions. The time of flight mass spectrometric results of our earlier work [Phys. Rev. A 67, 022704 (2003)] on partial ionization of ${\mathrm{SF}}_{6}$ molecule by impact of $16\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ electrons are found to support the existence of these stable ions.

Angular distribution of ejected electrons from Ba 6p ns ( J=1) autoionizing states

The asymmetry parameters of Ba 6p j ns ( j=1/2, 3/2 J=1) autoionizing states, which characterizes the angular distribution with the form ( σ/4 π)[1+ βP 2(cos θ)], have been investigated, using multichannel quantum defect theory, combined with R-matrix method. Furthermore, energy dependence of asymmetry parameters of electrons ejected from Ba 6p j ns autoionizing states to the 6s 1/2, 5d 3/2, 5d 5/2, and 6p 1/2 ion states has been studied. The results indicate the 6p 1/2ns and 6p 3/2ns series have different characteristics in their angular distributions of ejected electrons. Our theoretical results not only agree well with the previous experimental ones, but also predict some new phenomena.

Energy and angular distribution of relativistic electrons in the tunnelling ionization of atoms by circularly polarized light

Simple analytic expressions are obtained for relativistic electron energies and angular distributions produced in tunnelling ionization of atoms by intense laser radiation. Circularly polarized radiation is considered, using the Landau-Dykhne approach. Major differences have been found as compared with the previously investigated case of linearly polarized laser radiation: a circularly polarized intense field produces mainly relativistic electrons, while a linearly polarized intense field produces mainly non-relativistic electrons. In the limit of a weak field we obtain well known expressions for the non-relativistic energy spectrum and angular distribution of ejected electrons in the tunnelling ionization of atoms. (Limited, of course, to the case where the so-called Keldysh parameter corresponds to the tunnelling regime of ionization.) Simple expressions have been found for: (1) the angle between the direction at which most of the relativistic electrons are ejected and the plane of polarization of the laser radiation; (2) the energy spectrum at this angle; (3) the width of the energy spectrum and the position of its maximum; (4) the angular distribution near this angle and the width of this distribution. Our simple purely analytical results for energy and angular distributions are in agreement with the numerical derivations of Reiss based on the strong field approximation.

Penning ionization electron spectroscopy of the C2H2 molecule by Ne∗ (33P2, 33P0) metastable atoms

Abstract The present paper deals with Penning ionization electron spectroscopy (PIES) of C2H2 by Ne∗ (33P0 and 33P2) atoms. The observed vibrational population of the X 2IIu (C2H2+) state is in agreement with the Franck-Condon factors. The measured angular distribution of ejected electrons is found to be isotropic. This behaviour is consistent with the existence of a process competing with the covalent channel. This is also in agreement with results published on the measurement of ionization cross-section as a function of collision energy. An excitation transfer process was put forward to explain the entire experimental results obtained for the Ne∗C2H2 system. An indirect process via Rydberg states converging to the A2Σg+ state leads to a vibrational population that is not of the Franck-Condon type. However, an excitation transfer via the (2σ u −1 → 1π g 1 )π ∗ shape resonance can explain the Franck-Condon vibrational population, the large absolute value and the flattened behaviour of the ionization cross-section with respect to the variation of both collision energy and detection angle.

Ionization of Li by fast highly charged ion impact

Angular distributions of ejected electrons in fast N 7+ + Li collisions have been measured. By calculating the individual amplitudes in a multipole expansion, we evaluate the relative importance of dipole transitions. Using a Li target, we find the relative importance of dipole transitions to depend strongly on the initial state.

Ionization rates and energy and angular distributions at the barrier-suppression ionization of complex atoms and atomic ions

Analytical expressions are obtained for energy and angular distributions of ejected electrons at the barrier-suppression and tunneling ionization of complex atoms and atomic ions by low-frequency strong electromagnetic radiation. The results reduce to previously known expressions in the case of the ground state of the hydrogen atom. Both linear and circular polarizations of the electromagnetic field are considered. The ionization rates are found by integration over angles and energies of the ejected electron in the case of barrier-suppression ionization of complex atoms and atomic ions. The barrier-suppression results reduce correctly to the tunneling results of the Ammosov–Delone–Krainov approach in the limit of weak fields compared with the barrier-suppression fields.