Inner-shell Ionization Research Articles

Development of symmetry-resolved zero-kinetic-energy photoelectron spectroscopy for probing multielectron processes

A new experimental setup for probing multielectron processes in molecular inner-shell ionization regions has been developed. Symmetry-resolved zero-kinetic-energy (ZEKE) spectra have been measured by scanning the photon energy along with monitoring the intensity of the coincidence signals between ZEKE electrons and fragment ions detected at 0° and 90° relative to the electric vector of the light. The actual performance of the method is illustrated by using it to reveal the symmetry decomposition of the multielectron processes, such as double excitations and shake-up satellites, in the K-shell ionization region of nitrogen.

Inner-shell ionizations and satellites studied by the open-shell reference symmetry-adapted cluster/symmetry-adapted cluster configuration-interaction method

Open-shell reference version of the symmetry-adapted cluster (SAC) and SAC-configuration-interaction (CI) methods, termed open-shell reference (OR)-SAC and OR-SAC-CI methods, are developed and applied to inner-shell ionizations of CH4, NH3, H2O, and HF. The inner-shell ionization potentials and spectra calculated by the OR-SAC and OR-SAC-CI methods are in excellent agreement with the experimental data. Including both of the electron correlation and orbital relaxation is important for quantitative agreements. Timing comparisons with the SAC-CI general-R calculations that give similar high accuracies show an efficiency of the present OR-SAC and OR-SAC-CI methods.

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.

X-ray emission from multi-inner-shell excited states produced by high-intensity short-pulse X-rays

We study the double inner-shell ionization produced by high-intensity short-pulse X-rays and the X-ray emission from doubly inner-shell excited states of Mg (1s 22s 22p 43s 2) and Xe (1s 22s 22p 63s 23p 64s 24p 64d 85s 25p 6). The intensity and pulse width dependence on the X-ray emission is studied.

MOON for double beta decays and X-rays from WIMP nuclear interactions

Neutrino-less double beta decays ( 0 ν β β ) and direct observation of cold dark matter (DM) are of great interest for studying fundamental properties of neutrinos and weakly interacting massive particles (WIMPs). These are crucial for particle physics and cosmology beyond the standard electro-weak theory. The present seminar in the Erice neutrino school reports briefly (1) the effective neutrino mass studied by 0 ν β β , (2) the unique features and the present status of MOON (Molybdenum Observatory Of Neutrinos) for high-sensitivity 0 ν β β studies with 100Mo in the quasi-degenerate and inverted hierarchy regions, and (3) the direct detection of WIMPs by measuring atomic X-rays following inner-shell ionization by WIMPs nuclear interactions. The MOON project is carried out in collaboration with the MOON collaboration, and the X-ray work from WIMPs is done with Ch.C. Moustakidis and J.D. Vergados.

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.

Influence of formation path on the CH2BrCl2+ dissociation dynamics

To get further insight into the CH2BrCl site-selective fragmentation previously observed upon inner-shell ionization, we have performed high-resolution Br 3d and Cl 2p Auger and spin-orbit resolved Br 3d Auger spectra, and studied the dissociation properties of the CH2BrCl2+ dication formed at threshold by means of threshold electron pair-ion coincidence measurements. The key point is that the origin of site-specific bond breaking is found in the Auger decay itself, as it preferentially populates selected dication states. Whereas the predominance of the C-Br bond breaking is observed in both threshold and inner-shell studies, no signature of selective C-Cl rupture is reported for the dication formed at threshold.

Testing the ECPSSR theory and its modifications with ratios of antiproton-to-proton ionization cross sections

While fits to 90% of all ever measured K-shell ionization cross sections by protons are on the average within 10% of the ECPSSR [W. Brandt, G. Lapicki, Phys. Rev. A 23 (1981) 1717], this theory systematically overestimates the remaining 10% of all data (comprised mainly of the measurements on heavy target elements) in the slow collision regime of proton velocities less than 0.1 of the orbital velocity of the K-shell electron. For ionization of H and He, experimental ratios of antiproton-to-proton cross sections and benchmark theories thereof exist in the range around where the projectile and target electron velocities match and the cross sections peak. They confirm the excellent accuracy with which the antibinding/binding and antipolarization/polarization effects are accounted for in the perturbed-stationary state (PSS) treatment of the ECPSSR theory. It is shown that the K-shell ionization of heavy elements by MeV antiprotons, which were routinely extracted from the LEAR – or alternatively in ionization of light elements below 0.3MeV that will become available at the GSI’s future Facility for Low-Energy Antiproton and Ion Research (FLAIR) – would provide for a decisive and critical test between various modifications of the ECPSSR theory and any other theory of inner-shell ionization by protons and antiprotons.

Energy loss in medium-energy ion scattering: A combined theoretical and experimental study of the model system Y on Si(111)

The energy-loss spectrum associated with scattering of $100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{H}}^{+}$ ions from Y atoms on Si(111) has been investigated both experimentally and theoretically. Measurements were made from Y overlayers, and from the $\mathrm{Si}(111)(1\ifmmode\times\else\texttimes\fi{}1)$ two-dimensional silicide phase formed by Y on this surface, in various scattering geometries and with different surface preparations. Theoretical simulations were conducted based on calculations of the energy loss experienced in specific ion trajectories through the surface, using coupled-channel calculations to describe inner-shell ionization and excitation as a function of impact parameter. The experimental results indicate that additional broadening contributions arise from surface inhomogeneity and roughness, but for near-normal incident and outgoing trajectories the theory and experiment agree quite well. The calculations show that, even for the ideal two-dimensional silicide phase in which the Y atoms lie just below the surface, significant energy loss arises from interaction of the ions with surrounding Si atoms, leading to a complete loss of intensity at zero energy loss.

Dependence of shake probability on nuclear charge in Li-, Na- and K-like ions

In sudden perturbation approximation, the probability of the shake-up process accompanying inner-shell ionization is calculated for the isoelectronic sequences of Li-, Na- and K-like ions in the ground and excited np and nd states. Numerical solutions of Hartree–Fock equations and hydrogen-like radial orbitals are used. Very large differences between the results of both approximations for all ions and strong dependences on ion charge are obtained at the beginning of the isoelectronic sequences.

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.

Semiempirical formulae for electron-impact double-ionization cross sections of light positive ions

Electron-impact double-ionization processes of light positive ions from He- to Ne-like isoelectronic sequences, as well as of the heavy ions Arq+ (q = 1–7) and Kr q+ (q = 1–4) are considered for incident-electron energies E < 50 Ith where Ith is the threshold energy for double-electron ionization. On the basis of reliable experimental data and quantum-mechanical calculations, simple semiempirical formulae with three fitting parameters, by taking into account the contribution of direct double ionization and of inner-shell ionization processes, are obtained which describe the experimental cross sections within an accuracy of 20–30%. With this accuracy, the formulae suggested can be used for prediction of the double-ionization cross sections of positive ions with the nuclear charge Z ⩽ 26 in the electron energy range of E < 50Ith. According to the model suggested, the direct ionization cross section σdir (simultaneous ionization of two outer electrons) is scaled as I−3th against scaled electron energy E/Ith and contains only one fitting parameter. Two additional fitting parameters are obtained using the least-squares method in conjunction with numerically calculated single-electron inner-shell ionization cross sections. All fitting parameters are found to be constant for ions within a given isoelectronic sequence. The present analysis also provides a method for indirect determination of K-shell ionization cross sections for ions from Be-like to Ne-like sequences. The fluorescence yields ωK for a single K-shell vacancy in ions from Li-like to Ne-like sequences with nuclear charges 3 ⩽ Z ⩽ 26 are calculated as well.

Highly Charged Ion Research at the Livermore Electron Beam Ion Traps

Spectroscopy performed with the three Livermore electron beam ion traps is reviewed, which is continuing and complementing the innumerable contributions to atomic physics provided over the years by heavy-ion accelerators. Numerous spectrometers were developed that cover the spectral bands from the visible to the hard x-ray region. These enabled exhaustive line surveys useful for x-ray astrophysics and for systematic studies along iso-electronic sequences, such as the 4s–4p, 3s–3p, and 2s–2p transitions in ions of the Cu-I, Na-I, and Li-I sequences useful for studying QED and correlation effects as well as for precise determinations of atomic-nuclear interactions. They also enabled measurements of radiative transition probabilities of very long-lived (milli- and microseconds) and very short-live (femtosecond) levels. Because line excitation processes can be controlled by choice of the electron beam energy, the observed line intensities are used to infer cross sections for electron-impact excitation, dielectronic recombination, resonance excitation, and innershell ionization. These capabilities have recently been expanded to simulate x-ray emission from comets by charge exchange. Specific contributions to basic atomic physics, nuclear physics, and high-temperature diagnostics are illustrated.

Fast He2+ ion irradiation of DNA loaded with platinum-containing molecules

Purpose: The association of radiotherapy and chemotherapy is an attractive approach to improve the therapeutic index of the treatment of tumors. A lot of work has been devoted to investigate the effects of X-ray, γ-ray and neutron irradiation of DNA or living cells loaded with different chemical compounds containing heavy atoms like platinum. No such studies exist presently when fast atomic ions are chosen as ionizing particles. In the present work, we investigate quantitatively the increase of DNA breaks in complexes of plasmid-DNA loaded with platinum atoms under irradiation by fast atomic He2+ ions.Materials and methods: DNA Plasmids (pBR322) are incubated in solutions containing different concentrations of terpyridine platinum (PtTC). In some preparations, dimethyl sulfoxide (DMSO), a free radical scavenger, has been added in order to investigate the role of the free radicals. The complexes of DNA plasmids loaded with high-Z atoms are irradiated under atmospheric conditions by He2+ ions at an energy of 143 MeV/amu and a linear energy transfert (LET) of 2.24 keV/μm. Analysis of DNA damage – single and double strand breaks – is made by electrophoresis on agarose gels.Results: The results show a significant increase in DNA strand breaks when platinum is present, indicating a radiosensitization by the high Z atoms. The increase in DNA damages is attributed to inner-shell ionization of a platinum atom by secondary electrons emitted along the He2+ tracks followed by an Auger deexcitation, leading, thus, to a local amplification of the radiative effects close to the DNA. The contributions of scavengeable – solvant mediated – indirect effects and non-scavengeable effects (direct ionization) are quantitatively evaluated.Conclusion: Enhancement of DNA breaks in plasmids loaded with heavy atoms like platinum and irradiated by atomic ions are observed. This finding suggests an enhancement of cell death rate will occur under irradiation by atomic ions when the cells contain high-Z atoms located close to DNA due to the increase of the DNA breaks.

Strong electron correlation in Ca 2p, Sr 3d and Ba 4d core hole states investigated by means of photoelectron spectroscopy and MCDF calculations

Many-electron (correlation) effects play a dominant role in inner-shell ionization processes in free alkaline earth metal atoms. In this work calcium 2p, strontium 3d and barium 4d photoionization processes have been investigated with emphasis on the correlation effects. In order to understand the details of the spectra, a series of ab initio calculations based on multiconfiguration Dirac–Fock method have been performed. Theoretical predictions are compared with high-resolution synchrotron radiation induced electron spectra and the capability of the multiconfigurational computations in reproducing the experiment is discussed.

The backscattering factor in Auger-electron spectroscopy: New approach for an old subject

We describe an algorithm for the calculation of backscattering factors in Auger-electron spectroscopy that, unlike most previous analyses, is not based on the assumption of the probability of inner-shell ionization being independent of depth. Illustrative BF results from Monte Carlo simulations are presented for four Auger transitions (Si KL 23L 23, Cu L 3M 45M 45, Ag M 5N 45N 45, and Au M 5N 67N 67 in the corresponding elemental solids) with primary-electron energies between 0.5 keV and 10 keV, normal incidence of the primary electrons, and Auger electrons emitted at angles between 10° and 80° with respect to the surface normal. Our calculated backscattering factors agree with values obtained from an empirical formula of Shimizu [Jpn. J. Appl. Phys. 22 (1983) 1631] to better than 4% for the Si KL 23L 23 transition and better than 11% for the other three transitions. This degree of agreement is considered satisfactory on account of the fact that data of higher accuracy are now available for needed parameters. We find that the backscattering factor varies with Auger-electron emission angle. This variation can be up to 6.7% for primary energies between 3 keV and 10 keV.

C 1s and O 1s photoelectron spectra of formaldehyde with satellites: theory and experiment

Shake-up satellite spectra accompanying the C 1s and O 1s photoelectron main lines of formaldehyde were studied by the combination of high-resolution X-ray photoelectron spectroscopy and accurate ab initio calculations. The symmetry adapted cluster–configuration interaction (SAC–CI) general- R method finely reproduced the details of the experimental spectra and enabled quantitative assignments for the seven satellite bands: some were newly interpreted. The shake-up transitions were mainly attributed to the valence excitations accompanying the inner-shell ionization. The Rydberg excitations were found to be minor. Three-electron processes such as 1 s − 1 n − 2 π * 2 and 1 s − 1 π − 2 π * 2 were predicted in the low-energy region where the valence shake-up states such as 1 s − 1 n – σ * , π – π * exist.

Monte Carlo simulations of electron transport in solids: applications to electron backscattering from surfaces

We report results of Monte Carlo simulations to investigate the effects of backscattered electrons in scanning Auger microscopy (SAM) on the radial distributions of emitted Auger electrons. We considered the emission of copper M 3VV and L 3VV Auger electrons from a thin Cu overlayer on a substrate of silicon or gold for primary electrons with energies of 5 and 10 keV that were normally incident on the sample. The Cu layer was assumed to be sufficiently thin that there were no changes in the angular and energy distributions of primary and backscattered electrons passing through the overlayer. We report values of the information radius, r a P , from which a selected percentage P of the emitted Auger electron intensity originates. Values of r a P found here range from 119 Å (Cu L 3M 45M 45 Auger transition, E 0 = 5 keV, Au substrate, P = 80) to 6757 Å (Cu M 3VV Auger transition, E 0 = 10 keV, Si substrate, P = 95). For the same substrate, primary energy, and chosen value of P, values of r a P are larger for Auger electrons from the Cu M 3VV Auger transition than for the Cu L 3M 45M 45 Auger transition. In addition, values of r a P increase with primary energy and are larger for the Si substrate than the Au substrate. The values of r a P are generally much larger than the radius of the primary beam (assumed to be 50 Å here) on account of inner-shell ionizations by backscattered electrons. We also report values of the mean escape radius, 〈 r〉, that range from 82.5 Å (Cu L 3M 45M 45 Auger transition, E 0 = 5 keV, Au substrate) to 1169 Å (Cu M 3VV Auger transition, E 0 = 10 keV, Si substrate). Knowledge of r a P and 〈 r〉 is important in the analysis of fine features in SAM because appreciable Auger signal can be collected from the nearby region as well as from the feature of interest. Finally, we report Monte Carlo simulations of Auger line scans across the edge of a thin Cu overlayer on a Si or Au substrate. The shapes of the line scans depended only weakly on the Cu Auger transition, although the differences were more pronounced for the Si than the Au substrate. On account of backscattered electrons, the lateral distance corresponding to signal variations of 25% and 75% of the maximum intensity in a line scan varied from 53.6 Å (Cu L 3M 45M 45 transition, E 0 = 5 keV, Si substrate) to 75.1 Å (Cu M 3VV transition, E 0 = 10 keV, Au substrate).

Auger electron–ion coincidence studies to probe molecular dynamics

Electron analysis combined with ion mass spectrometry is shown to be a unique tool to understand fragmentation dynamics of core-excited molecules. This article describes in detail a new setup devoted to energy and angle correlations measurements between the emitted particles resulting from inner-shell ionization or excitation. The data collection system is based on a pair of position sensitive detectors mounted behind a double toroidal electron analyzer and a short time-of-flight ion spectrometer. Because all relevant information results in time measurements, a natural synchronization in the events recording is obtained. The optimized geometry for the ion extraction allows spatial focusing for the ion trajectories by means of inhomogeneous extraction fields while preserving the time focusing. The N 2 molecule has been used for full characterization of the setup whereas the CO 2 molecule illustrates the role of the intermediate resonant state in controlling the final dissociation pattern. The bending mode excitation is shown to emphasize the O + production, and the ion kinetic energy distribution is rationalized through an impulsive model.

Photopumping of XUV lasers by XFEL radiation

Within the next few years X-ray free-electron lasers (XFELs) now under construction are expected to generate highly collimated XUV pulses with 1013photons and a duration of 100 fs. Focusing this radiation to a spot some 10 μm in diameter generates intensities of up to 1016W/cm2. Such pump intensities make feasible the investigation of photopumped XUV lasers using this radiation. We present simulations taking into account two different mechanisms generating the gain: (1) photoionization with subsequent three-body recombination, which takes advantage of the monochromaticity of the pump radiation to generate very cold electrons; (2) inner-shell ionization in which transient inversion is obtained by generating a hole in an otherwise completely filled shell. The simulations show that under appropriate conditions both mechanisms generate very high gain. However, a number of further issues must be considered, such as the propagation of the pump pulse in the medium to be pumped.