Excitation Of Inner-shell Electrons Research Articles

Photoionization of singly and doubly charged aluminum ions in the extreme ultraviolet region: Absolute cross sections and resonance structures

Absolute measurements of the photoionization cross sections of ${\mathrm{Al}}^{+}$ ions ranging from 20 to 160 eV and of ${\mathrm{Al}}^{2+}$ ions from 70 to 105 eV have been performed utilizing a merged-photon--ion-beam setup with synchrotron radiation from an undulator. The continuum cross sections are in good agreement with published theoretical predictions. Structures resulting from excitation of valence or inner-shell electrons are resolved, identified, and compared with assignments from earlier spectral measurements. The analysis of the spectra is performed with the support of multiconfigurational atomic structure calculations.

Study of Ar Ions Kβ Profile with the Help of an Electron Beam Ion Trap

The Kβ transitions of Ar ions, excited in an electron beam ion trap, were detected with the help of an x-ray crystal spectrometer. The spectrum was analyzed by using Hartree–Fock calculations with Cowan computer code. High-energy features above 3190 eV, belonging to the Kβ transitions in trapped Ar III and Ar IV ions, were observed in the spectrum. An unfitted maximum at 3192 eV was interpreted as the result of 1s–3d excitation of inner shell electrons in Ar II ions.

Photoluminescence of erbium-doped silicon: Excitation power and temperature dependence

Photoluminescence measurements have been made on float-zone and Czochralski-grown silicon samples which were doped with erbium by ion implantation. The characteristic luminescence spectra in the wavelength range between 1.5 and 1.6 μm were observed. Differences in the multiple line structure of the spectra indicated that the active luminescent centers have different symmetries and atomic structure. The dependence of the photoluminescence intensity on the laser excitation power and on the temperature was measured. Results are discussed on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, the excitation of 4f inner-shell electrons of the erbium ions, and their subsequent decay by light emission. To obtain a quantitative agreement between model analysis and experimental data the consideration of Auger processes by which erbium-bound excitons and erbium ions in excited state can decay by dissipating energy to conduction band electrons appears to be required. From the temperature dependence two activation energies are derived which are associated with the exciton binding energies and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. A good quantitative agreement can be obtained for suitable values of the model parameters. The luminescent properties of the samples of the different types of crystalline silicon are remarkably similar.

Inner-shell soft X-ray lasers in Ne-like ions driven by optical field ionization

A novel short-wavelength laser based upon monopole excitation of inner-shell electrons in Ne-like ions following optical field ionization with circularly polarized radiation is discussed. Calculations of the small-signal gain are presented for one such ion, Ne-like Ar, for which a small-signal gain of 13.7 cm −1 is predicted on the 2s→2p hole transition at 15.8 nm for an Ar pressure of 50 mbar. Extensions of this approach to other inner-shell transitions are suggested.

Desorption of fragment ions from condensed Si(OCH3)4 by localized inner-shell electron excitation at the silicon, oxygen, and carbonKedges

Desorption of fragment ions from condensed Si(OCH3)4 (tetramethoxysilane) following localized inner-shell electron excitation at the silicon, oxygen, and carbon K edges has been investigated using monochromatized synchrotron radiation in the soft x-ray region. For Si K-edge excitation, the predominant species of the desorbed ions were Si(OCH3)n+ where n=2, 3, 4, which resembles the cracking pattern of gas-phase molecules excited by 70 eV electrons. In contrast, the CH3+ ions were the main species of the desorbed ions for the C and O K-edge excitations. The photon-energy dependencies of the CH3+ yield at the C K edge revealed that the CH3+ desorption happens only at the resonant excitation from C 1s to the σ* orbit localized at the C–O bond, which suggests that the core-to-valence resonant excitations are localized around the C–O bond until the C–O bond breaks. These results shed light on the possibility of low-temperature SiO2 deposition on a silicon surface by an x-ray-induced photochemical reaction through selective C–O bond scission using monochromatized synchrotron radiation.

Diffraction Channeling and the Production of Secondary Excitation

The pivotal role played by Archie Howie in the development of many areas of electron microscopy is universary acknowledged.Here I would like to highlight his contribution to the quantitative description of secondary excitations, which was an important influence on the development of Z-contrast imaging in zone-axis crystals. Secondary excitations are those such as x-ray emission which occur following a primary scattering event, in this case excitation of inner shell electrons. The first important concept to be realized by Archie was that dynamical diffraction and channeling are different manifestations of the same physical effect, namely, the multiple scattering of electrons within a crystal. Second was the realization that processes which are localized within the unit cell will show a dependence on diffraction conditions, such as incident beam orientation, and could therefore be described quantitatively using dynamical diffraction theory. Precisely the same theory was used to describe the orientation dependence of cathodoluminescenceThe development of the STEM for high resolution imaging was of course due primarily to Crewe and coworkers, with an annular detector to allow efficient detection of elastic scattering over a wide angular range.

Electronic structure and excited state dynamics of clusters: What can we learn from experiments with synchrotron radiation?

Abstract The investigation of the electronic structure and dynamics of clusters is a topic of current interest. Spectroscopy with vacuum ultraviolet (VUV) photons has two aspects: (i) systems with a large band gap can be studied and (ii) excitation of inner shell electrons allows us to obtain element-specific information. This paper gives an overview of recent developments in the field of cluster research with VUV radiation. Two topics will be discussed in detail: (i) Electronic excitation and relaxation dynamics in helium clusters studied with fluorescence spectroscopy. It turned out that desorption of electronically excited atoms and molecules is an important relaxation process. (ii) Inner shell photoionisation of NaCl clusters produced with a new pick-up cluster source. The near edge absorption structure (XANES) at the Cl 2p edge contains information on the geometric structure of the clusters.

Photoluminescence of erbium-doped silicon: Excitation power dependence

The intensity of the photoluminescence of erbium in silicon is analyzed by a model which takes into account the formation of free excitons, the binding of excitons to erbium ions, the excitation of inner-shell 4f electrons of erbium ions and their subsequent decay by light emission. Predictions of this model for the dependence of luminescence intensity on laser excitation power are compared with experimental observations. The results for float-zone and Czochralski-grown silicon, in which erbium is introduced by implantation with or without oxygen co-implantation, are remarkably similar. To obtain agreement between model analysis and experimental data it is necessary to include in the model terms describing energy dissipation by an Auger process of both the erbium-bound excitons and the erbium ions in excited state with free electrons in the conduction band. A good quantitative agreement is achieved.

Diamond-like bonds in amorphous hydrogenated carbon films induced by x-ray irradiation

Diamond-like carbon films have been irradiated with x-rays from a synchrotron radiation source. Raman measurements showed that the fraction of tetrahedrally coordinated (sp3) bond in the film increased with the increase of x-ray irradiation time, whereas it decreased as the annealing temperature increased. It was confirmed by electron spin resonance measurements that the number of spins did not change by x-ray irradiation. These results indicated that the transition from sp2 to sp3 bond takes place by nonthermal Coulomb repulsive rearrangement of atoms after inner-shell electron excitation.

70 years of Auger spectroscopy, a historical perspective

The discovery of the nonradiative decay of an inner-shell ionized atom by the emission of an electron was made by P. Auger in 1925. The first investigation of Auger electrons by means of a magnetic electron spectrograph was done by Robinson and Cassie in 1926, this marks the birth date of Auger spectroscopy. The following 70 years of Auger spectroscopy of atoms will be divided into three periods. In the first period (1926–1960; Section 2) Auger spectroscopy was mainly connected with β-ray spectroscopy where inner-shell ionization in atoms in the solid state was caused either by γ-conversion or by electron capture. In the second period (beginning in 1960; Section 3) Auger spectroscopy developed independently of the availability of radioactive sources by using external excitation of gaseous atomic or metal vapor targets. The third period (beginning in 1977/78; Section 4) is characterized by the use of synchrotron radiation with its outstanding properties of tunability, polarization and narrow-band high-intensity for the excitation and ionization of inner-shell electrons.

The influence of inner-shell electron promotion on charge exchange processes in low energy ion scattering from surfaces

The influence of inner-shell electron promotion on charge exchange in low energy (1–7 keV) Ne + ions scattered from the Cu (1 0 0), Ni (1 0 0) and Fe (1 1 0) surfaces has been studied systematically. The yield of Ne + ion scattered from these surfaces has been measured as a function of incident ion energy under various scattering geometries. The relative Ne + ion fraction, which is proportional to the normalised ion yield divided by the differential scattering cross section, is studied and an empirical formula for relative ion fraction has been extracted. The formula combines the charge exchanges along the incoming trajectory, during the close encounter, and along the outgoing trajectory into one simple expression. It can be concluded that inner-shell electron excitations during close encounters contribute significantly to the charge exchange in the scattering systems studied in this work.

Importance of relativistic effect on inelastic scattering cross-sections for quantitative microanalysis

For quantitative microanalysis of specimens using core-loss spectra, elemental maps by electron energy-loss spectroscopy or energy-filtering transmission electron microscopy, accurate values of partial cross-sections are required. Upon increasing the incident electron energy, the relativistic effect including retardation and magnetic interactions on inelastic scattering cross-sections can not be ignored. In the present work, we report the direct observation of the relativistic effect as it appears in the angular distribution of inner-shell electron excitation by using the energy-filtering diffraction technique performed with a high-voltage electron microscope (JEOL-ARM1000). The experimental result confirms a theoretical prediction proposed previously. The importance of the relativistic correction on partial cross-section is discussed in relation to quantitative microanalysis.

Resonant Photoemission in Organic Polymers at the CK-edge.

Resonant photoemission is a valence band photoemission process following the excitation of inner shell electron to a particular unoccupied state. This reveals information on both occupied and unoccupied states involved in the decay process, and correlation between the two. Although resonant photoemission in metals and their compounds have been studied quite extensively, few studies have been done for organic polymers. In this paper, resonant photoemission in poly (α-methylstyrene) is described, as an example of the results for organic polymers, as well as reviewing earlier works. The results showed that the resonant enhancement is clearly observed for the decay following the excitation to the lowest π* band. Especially, the photoemission from the highest occupied band is largely enhanced. In addition, the energy shift of a valence peak was observed, suggesting the very fast relaxation within the lowest π* band, which is peculiar to solids.

Radiation damage and spatial resolution in the EXELFS of inorganic materials

EXELFS, the extended energy loss fine structure superimposed on the high energy loss side past an inner-shell electron excitation in an EELS spectrum, contains quantitative information about the local structure around the scattering atom from which the core electron is excited, similar to that contained in the x ray absorption fine structure (XAFS). Main advantages of the EXELFS technique over XAFS include the relative ease in the former to study the local environments of low Z elements coupled to the higher spatial resolution, imaging and diffraction capabilities of the TEM.Radiation damage to the sample caused by the energetic electron beam, however, has been an important concern to whether EXELFS can be safely applied to study materials at the nanometer scale. To investigate this question we chose to study Al, SiC and MgO as representatives of three different classes of binding: metallic, covalent and ionic, with the belief that these should exhibit different sensitivities to different mechanisms of radiation damage and hence will be fair representatives of a wide variety of materials.

Inelastic energy loss and inner-shell electron promotion in low energy Ne ions scattered from Fe and Ni surfaces

Inelastic energy loss and inner-shell electron promotion of Ne + ions scattered from Fe(110) and Ni(100) surfaces have been studied systematically in the energy range from 1 to 7 keV at large scattering angles. The inelastic energy loss shows a threshold-type greater increase with incident energy in both scattering systems. Similar threshold-type behaviour is also observed in the scattered Ne + ion yield, the scattered Ne ++ ion yield and the peak fwhm. The threshold incident energy for such phenomena is found to increase with the atomic number Z of the target. It is suggested that excitations of the Ne 2s electrons occur during the close collision of the Ne with Fe or Ni atoms, resulting in a sudden increase in the inelastic energy loss. The excitation probability is found to depend on the atom position of the target relative to the swapping points of the Ne inner-shell energy levels. The ionization of the doubly excited Ne∗∗ particles alters the charge exchange behaviour of the Ne projectiles on their outgoing trajectories, leading to a remarkable increase in the scattered ion yield. The results show that inner-shell electron excitations could be significant in low energy ion surface scattering and may play an important role in the charge exchange process.

VUV-photoionization of laser-excited atoms

Recent results on the VUV-photoionization of laser-excited atoms obtained by photoelectron spectroscopy are reviewed. There are two groups of experiments: In the first group electron correlation phenomena, especially satellite emission, are studied in direct inner-shell photoionization processes of unpolarized atoms, whereas in the second group of experiments aligned excited atoms are prepared in order to probe the resonant excitation of inner-shell electrons to autoionizing resonances.

A molecular dynamics study of collisional excitation/deexcitation at ion-bombarded surfaces

On the basis of ab initio potentials, a three-dimensional classical trajectory model of the dynamics of energetic collision events for ion-bombarded alkali halide surfaces is introduced. In this model, a distance-of-closest-approach method is used to determine the occurrence of core excitation and electron capture. The results show that an excited sodium ion may be formed when a sodium 2p electron is promoted into thet 3s state during a hard Ar +Na + collision. This inner-shell electron excitation is shown to be an initiating process, which can result in the subsequent neutralization and Auger electron emission from Na in the solid. We give numerical estimates of the probability for both the excitation and neutralization events that produce an excited sodium atom with a 2p vacancy which undergoes Auger electron emission. In addition, the lifetime of the promoted electron in sodium is calculated by applying quantum perturbation theory. Our results may be useful in interpreting the data from a recent experiment, in which three interesting spectral features due to the Auger decay of the core excitation appear in the autoionization spectrum.

Excitation-autoionization through 3d-4l inner-shell excitations in Cu- to Kr-like ions and the effect on fractional-ion-abundance balance in coronal plasmas.

Excitation of inner-shell electrons into autoionizing levels followed by autoionization (EA) can enhance the total ionization rate. For ions isoelectronic to elements of the fourth row, the present results show that the enhancements can be much more important than those previously calculated for the preceding isoelectronic sequences. \ensuremath{\Delta}n=1 3d-4l inner-shell excitations, which are the dominant EA channels, are considered here. Detailed computations of EA rates were performed using the relativistic distorted-wave method, and the behavior of the EA rate coefficients along the Cu, Zn, Ga, and Kr isoelectronic sequences has been analyzed. In the other sequences from Ge to Br with configurations involving a very large number of levels, EA calculations have been performed for a few elements only. In view of an easy use of the data, and for application to other elements, approximate analytical expressions have been deduced for ionization energies, average inner-shell excited-configuration energies, total electron-impact excitation rates, as well as average branching ratios for autoionization. These data enable a direct evaluation of the EA rates in all the sequences from Cu to Kr for all the elements with 40\ensuremath{\le}Z\ensuremath{\le}92. Finally, the EA rate coefficients were introduced in the coronal ionization rate equations. Fractional ion abundances for Mo (Z=42), Xe (Z=54), Pr (Z=59), and Dy (Z=66) obtained with and without taking into account EA processes are presented. The results show a decrease of up to 40% in the temperature of maximum abundance for Kr- to Ni-like ions.

Inner-shell electron excitation in the methylmethoxysilanes

The effect of electron-donating substituents on the low-lying unoccupied valence orbitals in the series of silanes Si(CH 3) x (OCH 3) 4− x is investigated in an examination of electron-impact energy-loss spectra near the Si 2p ionization edge, and by means of ab initio calculations of the core-hole states populated by the inner-shell excitation. The substituent effect on the LUMO energies, as well as upon the 29Si NMR shieldings, is considered along with the consequences of valence—Rydberg mixing.

電子線分光型電子顕微鏡による有機結晶の研究

Electron spectro-microscope equipped with a parallel-EELS is a powerful tool for elemental analyses as well as an investigation of electronic structures on a local specimen area. The present paper shows an application of this method to organic crystals. Electron energy-loss near-edge structures (ELNES) observed on the carbon and the nitrogen K-edges of the chlorinated Cu-phthalocyanine and poly GeO-phthalocyanine were measured as a function of irradiation dose. From an analysis of the change of ELNES peaks, a sitedependent process in the radiation damage was proposed, which was realized due to the localized property of an inner-shell electron excitation.