Showing papers on "Auger published in 1991"

Epitaxial film crystallography by high-energy Auger and X-ray photoelectron diffraction

Abstract: We review recent work in the application of Auger and X-ray photoelectron diffraction at high electron kinetic energies to the problem of structure determination in ultrathin epitaxial overlayers. These closely-related techniques are based on the fact that outgoing Auger and photoelectrons from single-crystal specimens undergo elastic scattering and interference from near-neighbour atoms in the vicinity of the emitter. Such coherent diffraction leads to large intensity modulations as the detected emission direction is varied with respect to the crystal axes of the specimen. The measured modulations are readily interpreted by means of model quantum mechanical scattering calculation in which atomic coordinates in the epitaxial film are systematically varied. Such analyses provide several kinds of useful information, including growth modes accompanying heteroepitaxy, structural details of alloy and compound formation, and quantitative determination of tetragonal distortion at lattice-mismatched hete...

The past, present, and future of auger line shape analysis

Abstract: This review critically evaluates the suitability of Auger spectral line shape analysis as a source of electronic structure information Methods for extracting the true Auger line shape from the raw data and a theoretical framework for semiquantitative interpretation of that line shape are presented A wide range of recent applications concentrating on the line shapes of the low Z metals (Be, Li, Na Mg, and Al) the line shapes of C and Si, the transition metals, and, finally, those of the metal oxides and halides are considered Spectra for gas-phase molecules, adsorbed molecules and solids are examined Finally, new methods for controlling the background, the initial state, the spin polarization, and the angle of escape are discussed, along with requirements for improving the theory Over 350 references are included

Many dicationic states and two-hole population analysis as a bridge to Auger spectra : strong localization phenomena in BF3

Abstract: The outer valence double ionization spectrum of BF3 is investigated by computing 100 double ionization transitions with the Green’s function method. The results show that one‐site and two‐site hole localization on the fluorine atoms takes place to full extent and the energy split between clusters of states with similar hole distribution exceeds 10 eV. Double hole localization as a symmetry breaking phenomenon is discussed, pointing out the inherent inability of uncorrelated methods to describe it. A two‐hole population analysis of the dicationic states for the quantitative measure of their localized character is presented. The strong effects of hole localization on Auger transition rates are discussed, showing that the two‐hole population analysis can be used to develop a new statistical approach to the computation of Auger line shapes. Application of this method to the fluorine and boron KLL Auger spectra of BF3 permits, for the first time, an accurate reproduction of the spectral profiles and a complete interpretation of their features.

Band shape and vibrational structure in Auger spectra: Theory and application to carbon monoxide

Abstract: A time‐dependent approach to Auger spectra is presented and used to derive simple working equations for computing the vibrational broadening and the vibrationally induced shift of the peaks in the spectrum. The formulas give the explicit dependence of the vibrational envelope on the local details of the electronic potential energy surfaces of the intermediate and final states, providing interesting general insights which we discuss in detail. It is shown that, in polyatomic molecules, relevant interaction terms among different nuclear modes arise. The theory applies as well to other processes which involve a core ionized or core excited intermediate state like, for instance, x‐ray emission or resonant Auger decay. As a test application, the double ionization spectrum of CO is computed by the Green’s function method, and the new equations, together with a two‐hole population analysis of the pole strengths, are used to obtain theoretical Auger spectra. The experimental spectral profiles, characteristically shaped by the varying vibrational broadening and substantial energy shifts, are accurately reproduced, giving most peak positions to within a few tenths of eV. The results present very different vibrational effects for the carbon and oxygen spectra, showing the general inadequacy of interpretations based on vertical transition energies only. Hole localization in the dicationic states is discussed in the light of the population analysis results and used to estimate the electronic Auger transition rates.

Incident-beam effects in electron-stimulated Auger-electron diffraction.

Abstract: We have examined incident-beam effects in electron-stimulated Auger-electron diffraction (AED) on a cleaved GaAs(110) surface. The results indicate that incident-beam diffraction is significant in an AED experiment, and that the dissipative nature of the incident beam in contributing to the Auger process must be accounted for. We have developed a qualitative model that describes the trend of the polar-angle dependence of the Auger intensity for both the incident and exit beams. In calculating the diffraction features, we used a zeroth-order approximation to simulate the dissipation of the incident beam, which is found to adequately describe the experimental data.

Angular distribution in resonant Auger decay

Abstract: The angular distribution of Auger electrons in the decay of resonantly excited atomic states is studied theoretically. The two-step model is used for a description of the Auger process with the excited electron as a spectator. Expressions for the angular anisotropy parameters in the jK and jj coupling schemes are obtained for excitation of a closed-shell system. A close relation between the angular anisotropies of the resonant Auger and normal Auger processes is found within the framework of the spectator model. Numerical calculations for resonant Auger transitions in Ar, Kr and Xe are presented and compared with recent experiments.

Angular anisotropy and spin polarization of Auger electrons from noble-gas atoms

Abstract: The angular anisotropy coefficients and spin polarization parameters of Auger electrons for the transitions L3M2.3M2.3 in Ar, M4.5N2.3N2.3 in Kr and N4.5O2.3O2.3 in Xe atoms are calculated. The relativistic Dirac-Fock model with intermediate coupling is used for the description of the initial and final state of the Auger transition. The results of calculation are compared with earlier theoretical predictions and recent experimental data in the case of angular anisotropy. With a few exceptions good agreement with the experimental anisotropy is found.

Auger parameter shifts in the case of the non-local screening mechanism: Applications of the electrostatic model to molecules, solids and adsorbed species†

Abstract: A simple electrostatic model is applied to predict and rationalize the auger parameter shifts in molecules, solids and adsorbed species. The model is valid in the case of the non-local screening mechanism, i.e. when the screening of the core hole is due to the polarization of the nearest-neighbour ligands (no charge transfer from the ligands to localized electronic levels of the atom with the core hole, as occurs with the main peak of the core-ionized heavier 3dn transition metal ions or the light lanthanide ions). To good approximation it is shown that: (1) the Auger parameter shift is a function of the number, distance, electronic polarizability and local symmetry of the first-neighbour ligands of the core-ionized atom; (2) the dipole–dipole interactions can play a key role in determining the extent of the extra-atomic relaxation energy and hence of the Auger parameter shift.

Resonance Auger and autoionization processes in solid lanthanum after 4d to 4f resonant excitation by synchrotron radiation

Abstract: The decay processes of metallic lanthanum after resonant 4d to 4f excitation by synchrotron radiation have been studied by means of electron spectroscopy. Excitation to the 4d94f1(3D1) state, which is below the 4d ionization threshold, was found to decay predominantly through N4,5O2,3O2,3 and N4,5O2,3V spectator resonance processes. The energy shift relative to the corresponding normal Auger energies was 1.6+or-0.3 eV in the case of NOO resonance Auger decay, whereas no energy shift was found for the NOV resonance Auger electrons. The excitation to the main 4d to 4f giant resonance was found to decay through autoionization leading to a one-hole final state 5s-1 or 5p-1, or through normal Auger processes. Autoionization processes are strong on the low energy side of the resonance, but when the excitation energy increases, normal Auger processes become more and more dominant, indicating a change in the nature of the giant resonance.

Auger electron spectra of molecules: the first row hydrides

Abstract: A semi-empirical molecular orbital approach primarily developed to predict the Auger energies and transition rates associated with complex polyatomic molecules has been evaluated by reference to the first row hydrides. For this suite of molecules, spanning a range of symmetry classes, high resolution experimental spectra and ab initio energy calculations are available for comparison. The method proposed provides a good first estimate for transition rates. Relative Auger energies are less satisfactory, especially when electron correlation effects are strong. Absolute kinetic energies are too low, principally due to the neglect of electronic relaxation. The usefulness of the approach is mainly for the interpretation of spectra for larger systems where ab initio calculations are not readily accomplished. A need for further experimental measurements obtained using photoelectron-Auger electron coincidence studies is identified to remove uncertainties associated with the extent of satellite contributions present in all the spectral profiles so far recorded. In the present work the satellite contribution to the methane Auger spectrum is estimated to be 16% of the total intensity compared with an earlier estimate in the literature of 49%.

Symmetric versus asymmetric collisions in ion-induced Auger emission from silicon

Abstract: We investigate the production of Si 2p Auger electrons during bombardment of Si surfaces by 1.0–5.1 keV Ar+ ions. Yields of Auger electrons emitted at ∼ 146° with respect to the ion beam are measured as a function of ion energy. We observe a threshold of ∼ 3.8 keV for formation of a double L vacancy in Si, which is due to ArSi collisions. This value is higher than those of previous studies, which were presumably influenced by contamination of the Ar+ beam with multiply charged ions, as follows from our measurements of the variation of the Auger yields with the energy of the electrons in the ion source. We conclude that Auger emission in our energy range is dominated by symmetric collisions involving a fast Si recoil, with excitation in asymmetric, ArSi collisions growing in importance above the ∼ 3.8 keV threshold. The conclusion is also supported by the appearance of additional, Doppler shifted, Auger peaks at high projectile energies. Our threshold value for ArSi collisions is in good agreement with previous studies of gas-phase collisions, and should be used to reevaluate recent computer simulations which fail to predict the observations.

Scaling of relativistic Auger rates with Z for ions with two electrons

Abstract: Corrections to Auger rates for two-electrons ions with high Z values are obtained by using relativistic wavefunctions and the Breit interaction. The authors compare Auger rates including these corrections with results of non-relativistic calculations. A strong Z dependence of the corrections proportional to Z2 and Z4 is found.

Energy dependence of the different electron emissions backscattered from polycrystalline molybdenum

Abstract: The energy dependence of the emission yield of electrons backscattered from a clean polycrystalline molybdenum sample has been studied using a dispersive hemispherical analyser (incidence angle theta =70 degrees ; collection angle phi =0 degrees ). True secondaries, Auger electrons, elastic peak and background detected in the N(E) mode were investigated. Experimental results related to the elastic scattering of primary electrons are in good agreement with theoretical treatments (simple diffusion model and Monte Carlo simulation of multiple scattering). It is found that elastically reflected electrons mainly arise from single collision processes with a maximum of the emission yield at a primary energy Ep approximately=300 eV. The behaviour of the Mo-MNN Auger signal normalized by the primary current is compared to the energy dependence of the Auger peak/background ratio. These two normalization procedures lead to some differences mainly due to the transfer of the inelastic contribution of the background toward higher kinetic energies when Ep is increased. The dependence on the primary energy of the true secondary emission is closely related to the one recorded for the total backscattered coefficient deduced from the measurements of primary and sample currents. Threshold effects at an energy corresponding to the Mo-3d inner level are hardly detected. Furthermore, a rough estimation of the relative contributions of the different electrons involved in the N(E) backscattered spectra is proposed.

Angular distribution and spin polarization for resonant Auger emissions and for open shell transitions

Abstract: A model is developed to calculate anisotropy coefficients for resonant Auger emissions and for normal Auger emissions on elements with one electron in an open subshell. By applying some simple restrictions to the model the author connects the required matrix elements with those for the case of closed subshells which are already analysed. For an intermediate state with Jf=0 the angular distribution becomes independent of the matrix elements. The angular distribution parameters are calculated for these cases.

Auger electron angular distributions from surfaces: direct comparison with isoenergetic photoelectrons.

Abstract: Angular distribution patterns of Auger electrons and of photoelectrons from a Cu (001) surface were measured at the same electron kinetic energy. These measurements reveal that the low kinetic energy angular distributions for Cu Auger electrons and Cu 3p{sub 3/2} photoelectrons differ substantially. This direct comparison between the photoelectron and Auger electron angular distributions demonstrates that, in some circumstances, the Auger process produces a complicated source wave whose nature must be explored before Auger angular distributions can be used for surface structure analysis. 28 refs., 2 figs.

Alignment of Ar+ (2p −1 2 P 3/2) ions after electron impact ionisation in the range 1 keV to 268 eV

Abstract: We have measured the alignmentA 20 of Ar+(2p −1 2 P 3/2) ions after electron impact ionization in the range of primary electron energyE 0=1000...268 eV (range of excess energyE 1=750...19.5 eV) via the anisotropic angular distribution ofL 3−M 2,3 M 2,3(1 S 0) Auger electrons. On decomposing the Auger spectra into their components special care was taken by including the effect of the postcollision interaction on the shape of Auger lines. The present alignment values forE 0≧350 eV agree well with previously existing experimental values of DuBois and Rodbro and with theoretical DWBA results of Berezkho and Kabachnik, but forE 0 0.

Neutralization of H-like ions at a W-surface: via auger cascades?

Abstract: We present the electron spectra for 07+ on a polycrystalline tungsten surface at collision energies between 100 eV and 100 keV. We find clearly resolved structures from projectile KLL-and LMM Auger electrons. The relative intensities of these peaks show that the n=2 level is partly populated by ‘side-feeding’ processes. The time dependence of the KLL peak and the LMM peak can be described by a single exponential.

State-selective double capture in collisions of bare ions with helium atoms at low energies. II. Ejected-electron spectra

Abstract: For pt.I, see ibid., vol.24, no.19, p.4215 (1991). The Auger electron spectra of double excited states of ions resulting from double capture in collisions of bare ions with helium atoms in the energy range of a few keV amu-1 are calculated. The electron capture amplitudes are obtained in the independent-electron approximation. By combining these amplitudes with the Auger decay amplitudes and taking into account the post-collision interaction effect and the interference from different states, the electron spectra as a function of the ejection angles are evaluated. Calculations have been carried out for the electron spectra of 2l3l' doubly excited states of C4+ from C6+-He collisions and of 3l3l' double excited states of O6+ from O8+-He collisions. The theoretical results are compared with measured Auger electron spectra.

Comparative study of the properties of ultrathin Si3N4 films with Auger electron spectroscopy, spectroscopic ellipsometry, and Raman spectroscopy

Abstract: Ultrathin Si3N4 films are characterized with Auger electron spectroscopy, spectroscopic ellipsometry (SE), and Raman spectroscopy (RS). It is shown that Ar+ sputtering induces preferential nitrogen sputtering which causes problems in the determination of the interface location. Film thickness measurements with the nondestructive SE technique result in film thicknesses systematically larger than those calculated from the Auger sputter profiling, for films thinner than 100 A. Furthermore, analysis of the SE and RS data shows that the films are dense and stoichiometric while the stress induced by the Si3N4 film on the Si substrate amounts to 2–3 kbar.

Correction of backscattering effects in the quantification of Auger depth profiles

Abstract: The calculation of surface composition form Auger peak heights requires a knowledge of the matrix effects. These are modifications to the yield of Auger electrons arising from inelastic mean free paths, the Auger backscattering factor and sample density effects. A method for approximating the Auger backscattering factor for films of any thickness and atomic number is described. The method is based upon the assumption that the contribution of backscattered primary electrons to the Auger yield will change in proportion to the change in the electron backscattering coefficient of the film with thickness. The auger backscattering factor so calculated varies smoothly from the value corresponding to the bulk material of the film to that of the substrate. Using known generic equations for bulk backscattering coefficients and factors, a simple algebraic expression is obtained. Comparison with experimental data obtained form both high and low atomic number films yields good agreement for various primary beam energies and angles of incidence.

Auger for soil cement composite pile

Abstract: PURPOSE:To promote improvement in kneadability, reduction in power consumption and shortening in drilling time by forming a soil cement composite pile in the lower part of a drilling blade with an auger provided with a small diametral precedence drilling blade. CONSTITUTION:A hollow pipe 1, inserted with an auger rod through, is vertically set up in a specified position on the ground, and each of blades 5, 7, 9 is made into a state of being opened wide with each shear pin 13, connecting the upper end of an auger fulcrum shaft 3. Next, an auger head 20 is rotated forward as spraying a solidifier such as cement milk or the like into the ground from a nozzle 11 of the auger fulcrum shaft 3 or another nozzle 11a of a shaft 3a, and thereby the hollow pipe 1 is going to be installed in a soil cement pole being created after drilling and stirring the ground and going down. When the ground is drilled up to the specified depth, the solidifier is selected, forming a foot protection part. Finally the hollow pipe 1 is locked, and when the auger rod 2 is pulled out, stirring blade 9 in a widened state, the drag preventing blade 7 and the drilling blade 5 are all folded downward by cutting of the shear pin 13 and pulled out and removed.

A new method for measurement of post-collision interaction effects in ion-atom collisions

Abstract: The authors present a new experimental method for determination of the post-collision interaction (PCI) distortions of the lineshape of Auger electrons excited in ion-atom collisions. The method is based on the assumption that the excitation of Auger electrons from a target atom by electron capture on impact of singly charged ions is not influenced by PCI in regard to the absence of charged outgoing particles. Consequently, the detection of Auger electrons following capture a reference line can be obtained which is free of PCI distortions. As a demonstration of the method they have studied the shape of the L3 -M2,3 2 (1 S0 ) Auger line of argon excited by 0.7 MeV protons detecting the electrons in coincidence with the neutralized H0 atoms and without it. Further non-coincidence measurements determining the asymmetry of the L2,3 -M2,3 2 Auger lines as a function of the proton energy and observation angle have also been made. The results are in a qualitative agreement with the semi-classical theory of van der Straten and Morgenstern.