Bremsstrahlung-induced Auger peaks

For highly-polar nitro-substituted aromatic compounds, characterization of the nitro-N 1s core-hole states from the standpoint of intramolecular charge-transfer (CT) interaction is obtained through analysis of the energy difference between the satellite and the main peak, and the relative intensity of the satellite compared to the main peak.Two-dimensional configuration interaction (CI) is considered between the no-bond (covalent) and the CT (ionic) structures in both the initial and the final states. The off-diagonal element between these two structures is given in terms of the core resonance integral between the ring carbon and the nitro nitrogen, and can therefore be reduced by steric hindrance, and is also closely related to the π-electron density at the adjacent carbon in the donor orbital. The absolute magnitude of the off-diagonal element and the ionization potential of the donor are important factors determining the relative intensity of the CT satellite.It was found that in the case of some nitro-N 1s core-hole ions the CT structure is located below the no-bond structure, and that two-dimensional CI calculations are valid as far as the ground and the CT states are concerned.     

Highlights from the Pierre Auger Observatory

The Pierre Auger Observatory is the world’s largest cosmic ray observatory. Our current exposure reaches nearly 40,000 km ²sr and provides us with an unprecedented quality data set. The performance and stability of the detectors and their enhancements are described. Data analyses have led to a number of major breakthroughs. Among these, we discuss the energy spectrum and the searches for large-scale anisotropies. We present analyses of our X _max data and show how it can be interpreted in terms of mass composition. We also describe some new analyses that extract mass-sensitive parameters from the 100 % duty cycle surface detector (SD) data. A coherent interpretation of all these recent results opens new directions. The consequences regarding the cosmic ray composition and the properties of ultrahigh-energy cosmic ray (UHECR) sources are briefly discussed.     

Auger photoelectron coincidence spectroscopy

Auger photoelectron coincidence spectroscopy (APECS) is a technique that provides us with unique information and a chance to gain insight into the significance of processes in the Auger spectra of atoms in solids. Hence it is a great aid in our understanding of the Auger process in atoms where electron correlations are strong. Despite the first demonstration of the technique more than 20 years ago, there are still very few working experiments. The reasons why, and the ways forward are discussed.     

Auger spectrum of ammonia

The Auger spectrum of ammonia has been measured in the vapor phase, completing the isoelectronic series Ne, HF, H₂O, NH₃, and CH₄. Eleven transitions are possible, but three leading to triplet final states are expected to be very weak. Two others are expected on theoretical grounds to be unresolvable. The spectrum showed seven resolvable peaks, in keeping with these expectations. The peaks range in width from 2.2 to 7.4 eV, the narrowest being associated with vacancies in non-bonding lone-pairs and the broadest with vacancies in bonding orbitals. There is reasonably good agreement between the experimental results and recent theoretical calculations for both energy and intensity.     

On the restoration of Auger line shapes

A simple and efficient method to deduce the true Auger spectrum from the raw data is presented. This method is an application to Auger spectroscopy derived from signal processing procedures employed in different fields. It appears particularly promising when complex spectra are analyzed as it avoids spurious information to be introduced. The method is also compared with the more-standard van Cittert's approach. An experimental check of the common assumption that an Auger electron experiences almost the same energy-loss mechanisms as a nearly elastically reflected primary electron at the same energy is given.     

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.     

Quantum coherence in the time-resolved Auger measurement

We present a quantum mechanical model of the attosecond-XUV (extreme ultraviolet) pump and laser probe measurement of an Auger decay [Drescher et al., Nature (London) 419, 803 (2002)]] and investigate effects of quantum coherence. The time-dependent Schr?dinger equation is solved by numerical integration and in analytic form. We explain the transition from a quasiclassical energy shift of the spectrum to the formation of sidebands and the enhancement of high- and low-energy tails of the Auger spectrum due to quantum coherence between photoionization and Auger decay.     

On the satellite in the Auger electron spectrum

The satellite line profile in the C–VV Auger electron spectroscopy spectrum depends upon how the initial core hole state and the final multiple hole state are created. Even if the numbers of the holes in the satellite states are the same, the Auger electron spectroscopy spectral profiles of the final multiple hole states are different. The localization of the three hole satellite state is discussed.     

Crystalline effects on Auger emission; The influence of the electron channeling of the probe in scanning Auger microscopy

The modification of the Auger emission by the electron channelling of the probe is studied in silicon. An important effect is measured on the amplitude of the KLL Auger peak; the height of this peak is modified by a factor close to 2 when the channelling of the probe is changed using the rocking beam method. Due to the experimental conditions chosen, it is possible to interpret the results using the dynamical theory with two wave approximation.     

Interatomic Auger processes and the density of states

The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KVV peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of O, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L_2,3 VV and KVV Auger peaks of Mg and O in MgO. Due to the strong ionic character of this compound, the L_2,3 VV peak of Mg⁺⁺ is mainly due to interatomic processes between Mg⁺⁺ and O⁼ ions, whereas the KVV peak of O is mainly due to interatomic processes. This analysis shows that good agreement exists between the L_2,3VV Mg⁺⁺ Auger peak and the self-convolution of MgO density of states, whereas the KVV Auger peak of O⁼ is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.     

Auger recombination of electron-hole drops

Phonon-assisted Auger recombination is calculated for indirect band gap semiconductors in the strongly degenerate case. It follows a reciprocal lifetime τ^?1=Cn² with C=7.19×10³¹ cm⁶ sec^?1 for Si and C=2.94× 10^?31 cm⁶ sec^?1 for Ge. These results are in good agreement with experimental values of the decay of electron-hole drops. Therefore one can conclude that phonon-assisted Auger recombination is the essential nonradiative recombination process in this case.     

K Auger transitions of the free sodium atom

TheK Auger spectrum of the free sodium atom, following electron impact ionization, was measured with 0.16% resolution using a sodium atomic beam as target. The absolute energies and relative intensities ofKLL, KLM and numerous satellite transitions have been determined. The absolute energies of theKLL transitions of the free sodium atom were found to be smaller by 13.1 eV than the corresponding energies found for sodium in Na₂O (relative to the Fermi level) by Fahlmanet al. Also the relativeKLL intensities differed in two cases indicating solid state and/or chemical effects. The present relative intensities agree better with theoretical values calculated by McGuire, Walters and Bhalla, and Chen and Crasemann than those of Na₂O. In an Appendix the natural line widths of theKLL andKM-LLM transitions of sodium have been calculated.     

Core-level Auger spectroscopy of the 4d metals

We report some of the results of extensive experimental and theoretical studies by our group of the high energy Auger spectra of the 4d metals and some of their alloys. We consider, separately, three aspects of these spectra: comparison of experiment with atomic theoretical calculations; the relationship between the Auger kinetic energies and the screening mechanisms in these metals; and the unique properties of satellites associated with these spectra, especially with the L_1,2,3M_4,5M_4,5 spectra. Comparison with atomic theory yields some discrepancies that should be taken into account in the formulation of the theories. Consideration of the Auger kinetic energies yields insight into screening mechanisms and suggests methods for extracting electronic structure changes in alloys from XPS and XAES shifts. Systematic studies of the satellites permit their identification as arising from shake-up processes without contributions from of Coster–Kronig processes, in contrast to studies of 3d metals, such as Cu. Synchrotron studies allow the observation of the transition from the adiabatic approximation regime to that of the sudden approximation.     

Beam-foil-excited Auger transitions in neon

Energy spectra of electrons ejected from collisions between a carbon foil and Ne projectiles with energies between 1.4 and 20 MeV have been measured. Continuous and discrete electron energy distributions are observed. Auger transitions of foil-excited Ne have been studied. Using relativistic Dirac-Fock multiconfiguration calculations, most of the measured Auger transitions have been identified.     

Auger spectra of lithium hydride

Auger electron emission spectra have been observed for lithium hydride in three conditions : (1) cleaved in vacuum, (2) prepared by the reaction of hydrogen gas with clean lithium metal, and (3) by annealing slightly oxidized lithium hydride single crystals in vacuum. The dominant Auger line (40 ± 1 eV) was found to be a KVV transition involving valence electrons from the anion and was indistinguishable from a similar transition for lithium oxide at room temperature. Lithium hydride surfaces lose hydrogen in vacuum causing the formation of a lithium metal phase at room temperature and a significant reduction in surface hydride stoichiometry at 600°C.     

The Auger spectra of uranium

TheL, M, andK-Auger andL-Coster-Kronig spectra of uranium were measured and are compared to other experimental and theoretical work. The meanL-shell fluorescence yield of uranium was measured as \(\bar \omega _L \) =0.53±0.06.      

Experimentally derived Auger transition probabilities in X-ray excited Auger electron spectroscopy (XAES)

The capability of Auger transition probabilities experimentally derived from X-ray excited Auger electron spectra in XPS were tested. The relative sensitivity factor (RSF) method has been employed in the quantification by AES (Electron excited Auger electron spectroscopy). However, the difference between experimentally derived RSF and theoretically calculated ones has been found in some reports. One of the great reason of the difference may be caused by the calculated values of the Auger transition yield which has been commonly employed without the consideration of the allotment of coupling scheme in the transition selected in the quantification, for instance, the allotment of each six coupling KL₁L₁, KL₁L₂, KL₁L₃, KL₂L₂, KL₂L₃, and KL₃L₃ in KLL transition. The employment of derived Auger transition probabilities reduce the difference between theoretically calculated RSF and experimentally derived one.     

Shapes and shifts in the oxygen Auger spectra

The oxidation of silicon and platinum single crystal faces, of polycrystalline supported catalysts and of some alloy surfaces has been studied by AES and as far as possible by LEED. A comparison of the oxygen Auger spectra obtained during the oxidation process with those found on oxides has been made; it shows that the modification of the fine structure of the oxygen Auger peaks gives some information about the binding state of oxygen. Two different structures, which compete one with the other, are described. In one case, a spectrum where three lines dominate is obtained; in the other case, a “quasi-atomic” spectrum characterized by five features is observed: multiplet splitting in the two-hole final state is predominant. Besides these differences in the fine structure of the Auger spectra one can notice shifts of several eV for the main feature. They have been correlated with the various observed LEED patterns. Physisorption, chemisorption, solution of oxygen in the metal lattice, growth of ordered or amorphous oxides are the different possibilities which are discussed.