Changes in Auger spectra of Mg and Fe due to oxidation

Auger electron spectra of clean Mg and Fe surfaces have been investigated under UHV conditions. The main Auger peaks in the low energy Auger spectra of these elements are identified as due to L_2,3VV and M_2,3VV transitions for Mg and Fe respectively. Changes in the low energy spectra of these clean surfaces of Mg and Fe due to chemisorption of residual oxygen in the UHV system, were also studied. The results indicate that for each oxidised surface new larger Auger peaks appear at energies lower than the original main peaks in the clean spectra. The changes in the spectra are believed to be due to the energy shifts of inner energy levels and valence bands involved in the Auger transitions as an oxide is formed.     

Valence band and core level studies of InSb via photoemission measurements in the 20–70 eV range

We report photoemission measurements of valence band critical points, core level binding energies and spin-orbit splittings, and Auger processes using synchrotron radiation in the 20–70 eV range for InSb. Based on our studies of InSb and other semiconductors, several precautions when interpreting photoemission data in this energy range (e.g. Auger processes, matrix elements) are discussed.     

Émission électronique secondaire de cibles solides d'aluminium et de magnésium

The L-Auger and very low energy secondary electron spectra from Al and Mg solid samples have been studied. Interpretation of the different structures is discussed by using recent X-ray and photoelectron spectroscopic data. Especially the structures interpreted by other authors as plasmon gains in the Auger and in the true secondaries spectra are suggested to be due to other processes, respectively double ionization and interband transitions.     

Low energy Auger and loss electron spectra from magnesium and its oxide

Data have been obtained from Auger and energy loss processes in clean metallic Mg, Mg during stages of oxidation, and UHV cleaved MgO(100) surfaces. Particular attention has been paid to twenty features below 200 eV in the Auger spectra from these surfaces. A comparison of spectra from the metal, oxidised metal surface, and single crystal MgO has enabled estimates to be made of surface charging effects, and the MgO steady state surface potential is found to be near + 10 V above ground. All the Auger features are given assignments, two of which are interfacial processes involving ionic initial states and metallic final states. Several features in the low energy Auger spectrum are attributed to diffraction of true secondary electrons.     

Spectres Auger sous bombardement d'ions Ar+ de 60 keV de quelques éléments purs et de composés binaires

Auger spectra of different elements, alloys and compounds excited by 60 keV argon ions have been obtained. These spectra show a detailed structure with, in particular, a sharp Auger peak. This peak is interpreted as a superposition of two kinds of Auger transitions: the first one occurs with two electrons of the electronic band, the second, takes place from electrons situated on “quasi-atomic” outer shells. On the low energy side of Auger line, two equidistant peaks attributed to Auger electrons having suffered energy losses by ionization of electronic outer shell of target-atoms were observed. From Mg, Al, Si, P and S Auger transitions due to initial double L_ii,iii level ionization were detected. The interpretation of the creation of a vacancy in electronic inner shell at the origin of Auger effect by ionic bombardment is given from the electronic promotion model of Fano and Lichten.     

Deconvolution of auger electron spectra for lineshape analysis and quantitation using a fast fourier transform algorithm

A Fast Fourier Transform algorithm for the deconvolution of electron energy loss processes and instrumental broadening functions from Auger lineshapes is presented. This method uses a scaled electron backscatter spectrum at the energy of the Auger transition to approximate the spectral features in the data due to energy-loss processes and instrumental broadening effects (the sample-instrument response function, SIRF). Auger spectra, following deconvolution of the SIRF, show little variation to changes in the scaling of the low energy loss features of this backscatter spectrum, and little sensitivity to the choice of the truncation point of the SIRF in the frequency domain during apodization. Auger transitions for a series of sulfur oxyanions standards are comparable to transitions expected from independent molecular orbital calculations. The areas of the deconvolved oxygen (KLL) and sulfur (LMM) spectra were used to calculate oxygen-to-sulfur atomic ratios which were then corrected using standard theories for electron escape depth, electron impact ionization cross section and backscattering coefficient. These relative atomic ratios agree with the stoichiometry obtained from the molecular formulas of the standard compounds.     

Interatomic Auger transitions: A survey

A brief survey of the effects of interatomic Auger and Coster-Kronig transitions involving either the annihilation and/or ejection of electrons associated with the ligands nearest neighbor to the hole-state atom is presented. It is shown by way of example how these transitions may be used to interpret low energy Auger spectra of compound materials. Since these transitions affect the lifetime of certain core hole states, their probability should be reflected in their linewidths measured by x-ray photoelectron and x-ray emission spectroscopies. Dominant nonlifetime broadening processes generally preclude accurate determination of lifetime effects for most compounds u sing the former technique, but the latter spectroscopy may prove useful for studying such effects from the linewidths of characteristic x-rays and the intensities of multiple-hole x-ray satellite lines in certain materials.     

Extra-atomic relaxation energies and the Auger parameter

A simple procedure is used to derive several of the models that have been developed to understand relaxation energies, and especially the relationship between the Auger parameter and the extra-atomic relaxation energy. The approach shows clearly the assumptions that are made in these models and the next level of higher-order terms needed in the evaluation of extra-atomic relaxation energies from the Auger parameter. Numerical evaluation of these higher-order terms shows that they are probably small, except when comparison is made between atoms in very dissimilar environments. The problems that arise when the final orbitals of the Auger transition are not entirely in the core are considered.     

Auger electron spectroscopy study of 18/8 stainless steel oxidation

Auger electron spectra have been recorded for Cr, Mn and Fe from both clean and oxidised surfaces. Changes in the low energy MVV transitions of these elements were observed on oxidation and these effects have been used to characterise the surface oxidation product on an 18/8 stainless steel.     

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.     

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.     

Surface modification by ion chemical and physical erosion

Physical and chemical ion erosion surface interactions are observed for various elements and composite materials such as Si, B, C, TiB₂, WCCo cermet, and stainless steel which have been bombarded by low energy (50–1800 eV) hydrogen ions. Extrapolations of hydrogen erosion yields to zero hydrogen ion energy indicate that compounds or elements which are capable of forming volatile compounds with hydrogen undergo selective removal from surface and near surface regions. Auger depth profiling indicates that carbon can be selectively removed from near surface regions of cemented carbides and carburized stainless steel. Similarly, boron can be selectively removed from TiB₂. The results of this study may be extended to a variety of materials in which surface composition modifications are of interest.     

PCA-Based Analysis of X-Ray-Excited Auger Spectra from Non-ordered Ag(110)

Principal-components analysis (PCA) followed by factor analysis enables one to decompose the structure of Auger lines originating from large effects such as energy shifts induced by chemical effects. The aim of the present contribution is to show that PCA can also be effectively used for detection of composed structure in a set of Auger spectra even if the observed changes in line shape are very subtle. The analysed set of X-ray-excited MNN Auger spectra from Ag(110) shows a clear correlation between peak shifts and peak widths. This correlation can be explained as a result of the composed structure of the recorded Auger lines. It is suggested that the resultant Auger lines may consist of a number of constituents, each referred to Ag atoms differing in the value of their co-ordination number.     

Extremely high energy cosmic rays

The evidence for the existence of cosmic rays with energies in excess of 10²⁰ eV is now overwhelming. There is so far no indication of the GZK cutoff in the energy spectrum at 5 × 10¹⁹ eV. This conclusion is not firm for lack of statistics. A cutoff would be expected if the sources of the cosmic rays were distributed uniformly throughout the cosmos. The sources of cosmic rays with energy above the GZK cutoff must be at a distance ≤ 100 Mpc, and if they are protons they are very likely to point to these sources. There are no easy explanations how known astrophysical objects can accelerate protons (or atomic nuclei) to these energies. This difficulty has led to speculation that there may be exotic sources such as topological defects which produce these energetic cosmic rays directly along with a copious supply of neutrinos of similar energy. The fluxes of these cosmic rays is very low and large instruments are required to observe them even with modest statistics. One such instrument, the Pierre Auger Observatory, is described. It is designed for all-sky coverage and the construction of its southern site will begin in Argentina in 1999.     

Mechanisms of Auger recombination in quantum wells

The main mechanisms for the Auger recombination of nonequilibrium carriers in semiconductor quantum-well heterostructures are investigated. It is shown for the first time that there are three fundamentally different Auger recombination mechanisms in quantum wells: 1) a threshold-free mechanism, 2) a quasithreshold mechanism, and 3) a threshold mechanism. The rate of the threshold-free process has a weak temperature dependence. The rate of the quasithreshold Auger process exhibits an exponential temperature dependence. However, the threshold energy depends significantly on the quantum-well width and is close to zero for narrow quantum wells. It is shown that the threshold-free and quasithreshold processes are dominant in fairly narrow quantum wells, while the quasithreshold and threshold Auger processes are dominant in wide quantum wells. The limiting transition to a three-dimensional Auger process is accomplished for a quantum-well width tending to infinity. The value of the critical quantum-well width, at which the quasithreshold and threshold Auger processes combine to form a single three-dimensional Auger recombination process, is found. Zh. éksp. Teor. Fiz. 113, 1491–1521 (April 1998)     

On the low energy tail of the electron-hole drop recombination spectrum

The luminescence line shape of electron-hole drops in germanium is calculated assuming that the final state of the radiative transition is lifetime broadened due to Auger processes in the degenerate bands. This level broadening can account for the existence of the experimentally observed low energy tail in the recombination spectrum. The theoretical line shape is in good agreement with experimental results.     

(e,2e) and (e,3-1e) coincidence experiments for studying the PCI effect of low energy ionizing electrons in the Auger process of Ar

Our (e,3-1e) measurements for studying the post-collision interaction (PCI) after electron impact inner shell ionization of argon were continued and completed at different energy conditions. Emitted LMM Auger electrons are detected in coincidence with the ionizing scattered electrons and the energy of the slow PCI inducer ejected electron was calculated from energy conservation. Particularly the effect of the very low energy (i.e. 0–5 eV) ejected electrons (strongly asymmetric energy sharing) is studied at 500 and 460 eV primary electron energies. In the latter case, the background caused by outer-shell electrons was measured by itself and then removed from the coincident spectrum. Nevertheless, the evaluation of PCI distorted Auger lines is still considerably disturbed by the resonant Auger electrons from the high Rydberg states, their (e,2e) contribution was estimated in the paper.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

X-ray excited auger spectroscopy: selecting the optimum sharpest auger peaks excited by Al Kα, Ag Lα and Ti Kα X-ray radiations

XAS is developing, but, because it suffers from the fact that the “standard” Mg/Al dual anode X-ray gun usually limits the generation of intense Auger peaks to about half of the elements, this is a relatively slow development. It is necessary to obtain the optimum sharpest Auger peaks for high resolution XAS and for the precise determination of Auger Parameters. Therefore, a suitable unmonochromatic X-ray source should be selected for different elements or different transitions of an element. The Quadranode X-ray gun with the ability to emit three ranges (lower, medium and higher) of energy of X-ray photons is an appropriate candidate for modern XAS and XPS applications.