Inner-shell Ionization Of Atoms Research Articles

Differential phase contrast from electrons that cause inner shell ionization

Differential Phase Contrast (DPC) imaging, in which deviations in the bright field beam are in proportion to the electric field, has been extensively studied in the context of pure elastic scattering. Here we discuss differential phase contrast formed from core-loss scattered electrons, i.e. those that have caused inner shell ionization of atoms in the specimen, using a transition potential approach for which we study the number of final states needed for a converged calculation. In the phase object approximation, we show formally that differential phase contrast formed from core-loss scattered electrons is mainly a result of preservation of elastic contrast. Through simulation we demonstrate that whether the inelastic DPC images show element selective contrast depends on the spatial range of the ionization interaction, and specifically that when the energy loss is low the delocalisation can lead to contributions to the contrast from atoms other than that ionized. We further show that inelastic DPC images remain robustly interpretable to larger thicknesses than is the case for elastic DPC images, owing to the incoherence of the inelastic wavefields, though subtleties due to channelling remain. Lastly, we show that while a very high dose will be needed for sufficient counting statistics to discern differential phase contrast from core-loss scattered electrons, there is some enhancement of the signal-to-noise ratio with thickness that makes inelastic DPC imaging more achievable for thicker samples.

Application of spectrum smoothing to improve the accuracy of atomic inner-shell ionization cross sections impacted by electron near the threshold energy

In this study, the X-ray spectrum smoothing is applied to improve the reading precision of the experimental X-ray peak net counts. The experimental Sn L-shell X-ray production cross sections (XRPCSs) and Bi-Mαβ XRPCSs before and after smoothing are compared, respectively. The result shows that spectrum smoothing effectively improves the accuracy of the experimental XRPCSs. The after-smoothing present XRPCSs agree well with the distorted wave Born approximation (DWBA) theoretical XRPCSs and are consistent with the previously published XRPCSs within the experimental uncertainty. Finally, the factors affecting the comparison between the experimental and DWBA theoretical XRPCSs are analyzed.

Analysis on components of characteristic X-ray yield of pure thick targets by positron impact near threshold energy

In the process of low-energy positron impact with pure thick target, the atomic inner-shell ionization is induced by not only the incident positrons whose energy is completely deposited in the target but also the other particles produced during positron-target collision, namely, the backscattered positrons that deposited part of their energy before escaping the target, the annihilation photons and the secondary electrons. In this paper, the W-M characteristic X-ray yields of two pure thick targets with different diameters impacted by a positron accelerated by the negative high voltage to 5–9 keV have been measured. The contributions from annihilation photons, secondary electrons, backscattered positrons, and the non-uniform electromagnetic field in the target chamber to the characteristic X-ray yields have been evaluated by the realistic Monte Carlo simulation. Besides, Nagashima et al. (Phys. Rev. Lett., 92 (2004) 223201) neglected the contribution from annihilation photons, secondary electrons and backscattered positrons to the yields when calculating the cross-section by the characteristic X-ray yields of thick target impacted by a positron. Here we also caculated the contribution share of the above particles to the Cu-K and the Ag-L characteristic X-ray yields of pure thick target impact by positrons below 30 keV. The results show that the contributions from the above components are non-negligible. Hence, it is necessary to modify the experimental yields for obtaining the accurate yields of pure thick targets by non-backscattered positron impact.

Influence of multipole effects on the cross section and alignment following inner-shell ionization of atoms by a linearly polarized photon

On the basis of the fully relativistic Dirac-Fock treatment of single ionization by a linearly polarized photon with regard to the E1, M1, E2, M2, and E3 multipoles of the radiative field as well as their interference, we have assessed the influence of multipole effects on the cross sections and alignment parameters A20 and A22 of the residual ions, taking the 3p3/2 and 3d3/2 vacancies of selected Zn and Zn-like Kr6+, Cd18+, and Xe24+ ions as examples. Starting from the most general method of density matrix theory, we carried out specific analytic expressions in terms of the reduced matrix elements of multipole fields, corresponding to different ion-core states. It is shown that the multipole contributions to the photoionization cross sections can be considerable, the character of which becomes more evident as the incident energy and/or atomic number increases. These dramatic influences also lead to a remarkable increase in the alignment parameter A20 of the residual ions, yet virtually independent of the nuclear charge for A22. The present results in Coulomb gauge and Babushkin gauge excellently agreed with each other, suggesting that there is a high degree of convergence achieved in this study. Comparison of our results with experimental data and other theoretical predictions, when available, is made.

Experimental study on Ti K shell ionization cross sections induced by 8-9.5 keV positrons

Due to lack of experimental data of the inner shell ionization cross sections induced by low-energy positron, advanced theoretical models developed in recent years cannot be correctly evaluated, and the application of slow positron beam technique is greatly limited. Here we present the method of obtaining reliable experiment data of atomic inner-shell ionization cross section by positron impact. In this work, the slow positron beam device is used to generate 8-9.5 keV positron beams impacting on a pure thick Ti target, and the silicon drift detector (SDD) is adopted to collect the X-ray spectra produced by positrons impacting on thick Ti target, and the incident positron numbers are obtained by applying an HPGe detector to on-line collect annihilation photons. Then the experimental characteristic X-ray yields of Ti K shell impacted by 8-9.5 keV positron could be acquired. Meantime, the simulated characteristic X-ray yields are acquired by the PENELOPE program simulating the experiments. In the comparison between the experimental yields and the simulated yields based on two sets of different inner shell ionization cross section database in the PENELOPE code, i.e. the optical data model (ODM) and the distorted-wave Born approximation model (DWBA), there is a large difference between the simulated data from the ODM theoretical model and the experimental values, while the simulated yields from the DWBA theoretical model are in good agreement with the experimental results. Accordingly, a correction factor is introduced to modify the DWBA theoretical model database which is used in the PENELOPE, and then the experimental process is re-simulated. When the simulated yields and the experimental yields are in the highest consistence, the reliable Ti K shell ionization cross sections impacted by 8-9.5 keV positron could be obtained. The biggest advantage of using this method to obtain atomic inner-shell ionization cross section impacted by positron is that the effects of the multiple scattering of incident positrons in the thick target, from the bremsstrahlung and annihilation photons, and other secondary particles on the experimental characteristic X-rays do not need calculating (the calculation method that has been developed previously cannot give the more correct result about the contribution of the multiple scattering of incident positrons, from the bremsstrahlung and annihilation photons, and other secondary particles to characteristic X-rays).

Feasibility study of thin films deposited on a self-supporting carbon grid substrate target on the measurement of atomic inner-shell ionization cross-sections impacted by 3–30 keV electrons

In this paper, we have measured the , and X-ray production cross-sections of the W element and the K-shell ionization cross-sections of the Al element impacted by 3–30 keV electrons. A new kind of target consisting of a thin film deposited on a self-supporting carbon grid substrate has been used in this work. The self-supporting carbon grid substrate is made of several layers of super-aligned carbon nanotube arrays (the mass thickness of each layer of super-aligned carbon nanotube arrays is 20 μg/cm2). As a comparison, three kinds of targets consisting of a) the studied element deposited onto four layers of super-aligned carbon nanotube arrays and separated from each other by one layer of single carbon atoms, b) the studied element deposited directly onto four layers of super-aligned carbon nanotube arrays, and c) the studied element deposited directly onto two layers of super-aligned carbon nanotube arrays, were respectively prepared to figure out which type is fit for the measurement of inner-shell ionization cross-sections induced by 3–30 keV electrons. The experimental data are compared with the distorted-wave Born approximation (DWBA) theoretical predictions and with the available experimental data from other references. The results show that the first and the second targets are both suitable for the inner-shell ionization cross-section measurements by 3–30 keV electrons, but the third target is not. Besides, we present the M-shell X-ray production cross-sections of the W element by 5–30 keV electrons in this paper for the first time.

Suppression of Exponential Electronic Decay in a Charged Environment

Inner-shell ionization of atoms and molecules leads to the creation of highly excited ionic states that often decay by electron emission. The dynamics of the decay is usually assumed to be exponential and the process is characterized by a decay rate. Here we show that in a multiply ionized cluster created by interaction with a high-intensity free-electron laser (FEL) radiation, trapping of the emitted electron by the neighboring ions changes the character of the decay dynamics qualitatively to the extent that it can become oscillatory instead of exponential. Implications of the predicted effect on Coster-Kronig and interatomic Coulombic decay processes induced by FELs are investigated.

Multiple elastic scattering of core-loss electrons in atomic resolution imaging

We consider the elastic scattering of a fast incident electron both prior and subsequent to its involvement in an atomic inner-shell ionization (core-loss) event in a crystal. By using numerical simulations, it is shown that elastic scattering subsequent to ionization can strongly affect the qualitative features of atomic resolution core-loss images of crystals recorded in the scanning transmission electron microscope (STEM). This conclusion holds even for a thin crystal foil and for a relatively large detector, which is matched to the probe-forming aperture in an aberration-corrected STEM. Such a conclusion is potentially very important for the interpretation of experimental core-loss images. We also introduce an approximate model that incorporates the effects of elastic scattering subsequent to ionization in the case of a small detector.

Thick-target method in the measurement of inner-shell ionization cross-sections by low-energy electron impact

In this paper, we have studied the thick-target method for the measurements of atomic inner-shell ionization cross-section or X-ray production cross-section by keV electron impact. We find that in the processes of electron impact on the thick targets, the ratios of the characteristic X-ray yields of photoelectric ionization by bremsstrahlung to the total characteristic X-ray yields are Z-dependent and shell-dependent, and the ratios also show the weak energy-dependence. In addition, in the lower incident energy region (i.e. U < 5–6), the contribution from the rediffusion effect and the secondary electrons can be negligible. In general, the thick-target method can be appropriately applied to the measurements of atomic inner-shell ionization cross-sections or X-ray production cross-sections by electron impact for low and medium Z elements in the lower incident electron energy (i.e. U < 5–6). The experimental accuracies by the thick-target method can reach to the level equivalent or superior to the accuracies of experimental data based on the thin-target method. This thick-target method has been applied to the measurement of K-shell ionization cross-sections of Ni element by electron impact in this paper.

Nonstationary theory for short-pulse near-threshold photoionization of inner atomic shells

A theoretical description of near-threshold ionization of inner atomic shells by short electromagnetic pulses is developed. It is based on a nonstationary approach to the Fano-Feshbach problem of photoexcitation with simultaneous Auger decay of the produced vacancy. We suggest a method of solving the system of time-dependent Schr\odinger equations describing a resonant state embedded into continuum. The method gives a nonstationary description of the post-collision interaction in near-threshold photoionization including the recapture of the slow photoelectron in high Rydberg states. As an example, $1s$ photoionization in Ne with the following $\mathit{KLL}$ Auger decay is considered. The influence of the duration of the pulse on the recapture probability and shape of the emitted electron peak is studied.

Relativistic effects in atomic inner-shell ionization by a focused electron probe

A relativistic theory of atomic inner-shell ionization is presented in a form directly applicable to the calculation of the electron energy loss signal obtained using a focused electron probe in a scanning transmission electron microscope. Expressions are given for the implementation of the theory assuming a central potential atomic model. Analytical and numerical calculations are used to demonstrate that, for points in the diffraction plane lying close to the optic axis, the difference between the relativistic and nonrelativistic theories can be significant, even for an incident energy of $100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ (usually considered too low an energy for relativistic effects to be important). Implications for the quantitative matching of experimental and simulated data are discussed.

Multislice theory of fast electron scattering incorporating atomic inner-shell ionization

It is demonstrated how atomic inner-shell ionization can be incorporated into a multislice theory of fast electron scattering. The resulting theory therefore accounts for both inelastic scattering due to inner-shell ionization and dynamical elastic scattering. The theory uses a description of the ionization process based on the angular momentum representation for both the initial and final states of the atomic electron. For energy losses near threshold, only a small number of independent states of the ejected atomic electron need to be considered, reducing demands on computing time, and eliminating the need for tabulated inelastic scattering factors. The theory is used to investigate the influence of the collection aperture size on the spatial origin of the silicon K-shell EELS signal generated by a STEM probe. The validity of a so-called local approximation is also considered.

Two-Photon Excitation and Ionization of Inner Atomic Shells

Two-Photon Excitation and Ionization of Inner Atomic Shells

Two-photon excitation and ionization of inner atomic shells

Absolute values of the cross section for two-photon one-electron excitation and ionization of the 1s shell of the Ne atom are calculated taking into account the relaxation effect of the atomic core in the 1s-vacancy field. The results of calculation can be used for predictions.

Ultrafast electron dynamics and inner-shell ionization in laser driven clusters

The ionization dynamics of small rare-gas clusters in intense, ultrafast laser fields are studied via classical trajectory Monte Carlo simulations. Our results indicate that for similar laser pulses the charge states reached by atoms in a cluster can be significantly higher than those for atoms in the gas phase. The ionization enhancement is strongly dependent on the cluster density and exhibits a rapid increase in charge state once the laser intensity has reached the threshold for single ionization. This 'ionization ignition model' is driven by the combination of the laser field and the strong field from the ionized cluster atoms. Approximate atomic inner-shell ionization probabilities are calculated for several cluster densities and peak laser intensities and provide evidence for the generation of inner-shell holes on an ultrafast time scale. This is a necessary condition for the generation of x-ray pulses with temporal widths comparable to that of the driving laser pulses.

Energy-filtered high-resolution Electron Microscopy of nanostructured materials

The interaction between an incident electron and the atoms in condensed matter results in various inelastic scattering processes. Thermal diffuse scattering or phonon scattering is the result of atomic vibrations in crystals. This process does not introduce any significant energy-loss (&lt; 0.1 eV) but produces large momentum transfer. Valence-loss (or plasmon for metals and semiconductors) excitation, which characterizes the transitions of electrons from the valence band to the conduction band, involves an energyloss in the range of 1 -50 e V. Atomic inner-shell ionization is excited by the energy transfer of the incident electron, resulting in an ejected electron from the deep-core states. Continuous energy-loss spectra can also be generated by an electron which penetrates into the specimen and undergoes collisions with the atoms in it, resulting in Bremsstrahlung and leading to emission of x-rays with continuous energy. The electron Compton scattering refers to the collision of the incident electron with an electron belonging to the specimen. In an electron energy-loss spectrum (EELS), the zero-loss peak is composed of elastically and thermal diffusely scattered electrons. The low-loss region is dominated by valence-excitations.

Inner-shell ionization of atoms by electron, positron and photon impacts

Plane wave Born approximation with Coulomb, relativistic and exchange corrections is employed to obtain L1-, L2- and L3-subshell ionization cross sections of several atoms due to electron and positron impacts for projectile energy varying from the threshold of ionization to 60 times the threshold energy. Photoionization cross sections for all the three L-subshells of the atoms are also calculated using the hydrogenic approximation for the atomic wave functions. For L3-subshell the present cross sections due to electron impact are in good agreement with a number of experimental data for different atoms over the entire energy range investigated. For L1- and L2-subshells the present calculations yield qualitative agreement with the experimental data. The agreement between the present results and the limited experimental data for positron impact is also satisfactory. The hydrogenic approximation for the L-subshell photoionization is found to be good at small photon energies but it underestimates the cross sections at large photon energies.

Characteristic and non-characteristic X-ray emission by lepton impact

Recent progress of characteristic and non-characteristic X-ray emission in collisions of leptons (electrons, positrons) with target atoms is reviewed. We discuss, in particular, i) the differences between electron and positron impact in the ionization of inner atomic shells, ii) the two-photon bremsstrahlung emitted in collisions of electrons with heavy atoms, and iii) the role played by stimulated emission and absorption of photons during electron collisions in a strong laser field.

Post-collision interaction in inelastic ion-atom scattering

The ion-atom inelastic scattering processes including (i) excitation of autoionization states of atoms by ion impact, and (ii) ionization of inner atomic shells are considered. The Coulomb interaction of particles in the final state of such resonance processes (the post-collision interaction — PCI) exerts a strong influence on the energy and angular distributions of the particles. The theoretical models of PCI and experiments that make it possible to observe the PCI effects in different kinematic regions are discussed in this survey. The characteristics of PCI are considered by comparing photon and electron impact processes. Coincidence and noncoincidence experiments as well as the dependence of the lineshift on the incoming ion energy are discussed. The PCI effects are considered also in the kinematic region, where the relative velocity of a scattered ion and an autoionization electron is close to zero. In this case PCI results in the so-called Coulomb focusing of the autoionizing electrons and the oscillatory structure of the cross section. The interference between the direct and resonance scattering processes is considered, and possibilities of its experimental observation are discussed.

Biological effects of the Auger emitter iodine-125: a review. Report No. 1 of AAPM Nuclear Medicine Task Group No. 6.

The biological implications of Auger electron cascades following inner shell ionization of atoms have been of interest for over 25 years. By virtue of their decay via orbital electron capture and/or internal conversion, several biomedical radionuclides emit numerous low-energy electrons spontaneously. The biological effects of such radionuclides incorporated into tissues cannot be predicted a priori because of the highly localized patterns of energy deposition by the electrons. Results of extensive research using Iodine-125 as a model Auger electron emitter are now available. This article presents an up-to-date review of the physical and radiobiological data on this Auger emitter. Valuable concepts concerning the action of internal Auger emitters are identified phenomenologically, and questions that need to be answered are indicated. The present understanding provides a scientific basis toward estimation of risk associated with Auger emitters used in diagnosis, and suggests potential applications to therapy.