A many-body theory of the Ti M2,3-VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of TiO2(1 1 0)

We calculated the Ti M_2,3-VV Auger-photoelectron coincidence spectroscopy spectrum of TiO₂(1 1 0) by a many-body theory. The spectral main line is governed by the DOS of the two O 2p holes living longer than the Ti ∣cd²L²〉 → ∣L²〉 super Coster-Kronig (sCK) decay. The two O 2p holes are created by the charge transfer core-hole screening at the Ti atomic site. Here c and L are the Ti 3p hole and the ligand O 2p hole, respectively. Analysis of the spectrum shows that the two (or three) CT O 2p holes in the π bonding states are localized, whereas those in the σ bonding states are delocalized. The three localized CT O 2p (π) holes in ∣cd³L³〉 in Ti M_2,3 main line (or satellite) of TiO₂(1 1 0) live longer than the Ti ∣cd³L³〉 → ∣d¹L³〉 sCK decay so that the Coulomb repulsion from the surrounding Ti ions gives the O⁺ ion desorption from the surface.     

Anomalous auger shifts in chemisorption on tungsten

Substrate Auger line energy shifts for monolayer gas adsorption are readily detected. The observed shifts in tungsten are incompatible with predicted chemical-shift trends and hence relaxation effects must be considered. It is proposed that transition-metal s electrons play a decisive role in core-hole screening. The measurement of substrate Auger line energy shifts on gas adsorption may thus provide information on the contribution of these electrons to chemisorption.     

Auger decay of molecular double core-hole state

We report on theoretical Auger electron kinetic energy distribution originated from sequential two-step Auger decays of molecular double core-hole (DCH) state, using CH(4), NH(3), and H(2)CO molecules as representative examples. For CH(4) and NH(3) molecules, the DCH state has an empty 1s inner-shell orbital and its Auger spectrum has two well-separated components. One is originated from the 1st Auger transition from the DCH state to the triply ionized states with one core hole and two valence holes (CVV states) and the other is originated from the 2nd Auger transition from the CVV states to quadruply valence ionized (VVVV) states. Our result on the NH(3) Auger spectrum is consistent with the experimental spectrum of the DCH Auger decay observed recently [J. H. D. Eland, M. Tashiro, P. Linusson, M. Ehara, K. Ueda, and R. Feifel, Phys. Rev. Lett. 105, 213005 (2010)]. In contrast to CH(4) and NH(3) molecules, H(2)CO has four different DCH states with C1s(-2), O1s(-2), and C1s(-1)O1s(-1) (singlet and triplet) configurations, and its Auger spectrum has more complicated structure compared to the Auger spectra of CH(4) and NH(3) molecules. In the H(2)CO Auger spectra, the C1s(-1)O1s(-1) DCH → CVV Auger spectrum and the CVV → VVVV Auger spectrum overlap each other, which suggests that isolation of these Auger components may be difficult in experiment. The C1s(-2) and O1s(-2) DCH → CVV Auger components are separated from the other components in the H(2)CO Auger spectra and can be observed in experiment. Two-dimensional Auger spectrum, representing a probability of finding two Auger electrons at specific pair of energies, may be obtained by four-electron coincidence detection technique in experiment. Our calculation shows that this two-dimensional spectrum is useful in understanding contributions of CVV and VVVV states to the Auger decay of molecular DCH states.     

ESCA: Electron Spectroscopy for Chemical Analysis

A substance on which X-rays fall emits photoelectrons and Auger electrons. The energy spectra of the electrons emitted provide information about the electronic structure in the specimen, ranging from the innermost atomic levels and their dependence on the chemical environment to the molecular orbitals of the valence electrons and the band structure in solids. Electron spectra of this nature can now be recorded with high-resolution instruments; their analysis offers new aspects for investigation of chemical composition. The method of electron spectroscopy developed for this purpose, which has now been developed to a high degree of perfection, will be referred to in the following discussion as ESCA (Electron Spectroscopy for Chemical Analysis).     

Core-electron relaxation energy in small metallic particles

A general calculation method for screening in a finite electron gas proposed by Cini, is applied to study of core-hole relaxation energy in small metallic particles. In order to obtain quantitative results, a pseudopotential theory is developed for the core-hole perturbation which provides excellent agreement with the generally accepted excited-atom model in the bulk limit. The transition and noble metals treated by means of a semi-empirical extension of the method. The present calculation method of extra-atomic relaxation energies, involving an electron gas approximation for the conduction electrons allows straightforward applications of the method of Cini in the case of a finite-spherical metal particle. The relaxation energy is found to give an important contribution to core-hole binding energy shifts in small particles.     

X‐ray photoelectron spectroscopy and the pattern recognition method—their application to surface studies in CoPd alloys

In the present work, polycrystalline CoPd alloys in varying range of bulk atomic percent composition (Co30Pd70, Co50Pd50 and Co70Pd30) are investigated by means of X‐ray photoelectron spectroscopy (XPS). The results of conventional XPS quantitative multiline (ML) approach are compared to the results obtained on the basis of XPS lines shape analysis, where the selected XPS or X‐ray induced Auger electron (XAES) transitions, are processed using the pattern recognition method known as the fuzzy k‐nearest neighbour (fkNN) rule. The fkNN rule is applied to the following spectra line shapes: Pd MNV, Co 2p, Co LMM, Pd 3d and valence band, analysing electrons in a varying range of selected kinetic energies. Both methods showed the surface segregation of Pd in Co30Pd70 and Co50Pd50 alloys. The results of the ML, the binding energy shift (ΔBE) analysis and the fkNN rule remained in agreement. Discrepancies in quantitative results obtained using different approaches are discussed within the accuracy of the applied methods, differences due to mean escape depth (MED) of electrons in considered transitions, their depth distribution function, the sensitivity of electron transition line shape on the environmental change (weaker effect for the inner shell transitions, and stronger effect for the outer shell transitions and Auger electron spectroscopy (AES) electrons transitions) and the non‐uniform depth profile concentrations. Copyright © 2005 John Wiley & Sons, Ltd.     

Auger decay calculations with core-hole excited-state molecular-dynamics simulations of water

We report a new theoretical procedure for calculating Auger decay transition rates including effects of core-hole excited-state dynamics. Our procedure was applied to the normal and first resonant Auger processes of gas-phase water and compared to high-resolution experiments. In the normal Auger decay, calculated Auger spectra were found to be insensitive to the dynamics, while the repulsive character of the first resonant core-excited state makes the first resonantly excited Auger decay spectra depend strongly on the dynamics. The ultrafast dissociation of water upon O(1s)-->4a(1) excitation was analyzed and found to be very sensitive to initial vibrational distortions in the ground state which furthermore affect the excitation energy. Our calculated spectra reproduce the experimental Auger spectra except for the Franck-Condon vibrational structure which is not included in the procedure. We found that the Auger decay of OH and O fragments contributes to the total intensity, and that the contribution from these fragments increases with increasing excitation energy.     

Auger photoelectron coincidence spectroscopy using synchrontron radiation

The technique of Auger-photoelectron coincidence spectroscopy (APECS) is described and illustrated with a case study of the Cu(100) 3p and M₂₃VV spectra. APECS offers many advantages over the conventional singles spectroscopy such as isolating overlapping spectral features, reducing secondary electron background, and revealing new decay modes. In the coincidence Cu Auger spectra discussed here, the multiplet structure of the quasi-atomic 3d⁸ Auger final state is clearly observed, as well as different intensities for the multiplet components for the p_1/2 and p_3/2 transitions. Furthermore, the spectra reveal evidence for a Coster-Kronig decay channel for 3p_1/2 core holes, and illustrate that the sum of the Auger electron and photoelectron kinetic energies is conserved. Possible technical improvements that can increase the counting efficiency are also discussed.     

Core-to-Rydberg band shift and broadening of hydrogen bonded ammonia clusters studied with nitrogen K-edge excitation spectroscopy

Nitrogen 1s (N 1s) core-to-Rydberg excitation spectra of hydrogen-bonded clusters of ammonia (AM) have been studied in the small cluster regime of beam conditions with time-of-flight (TOF) fragment-mass spectroscopy. By monitoring partial-ion-yield spectra of cluster-origin products, "cluster" specific excitation spectra could be recorded. Comparison of the "cluster" band with "monomer" band revealed that the first resonance bands of clusters corresponding to N 1s → 3sa(1)/3pe of AM monomer are considerably broadened. The changes of the experimental core-to-Rydberg transitions ΔFWHM (N 1s → 3sa(1)/3pe) = ~0.20/~0.50 eV compare well with the x ray absorption spectra of the clusters generated by using density functional theory (DFT) calculation. The broadening of the core-to-Rydberg bands in small clusters is interpreted as being primarily due to the splitting of non-equivalent core-hole N 1s states caused by both electrostatic core-hole and hydrogen-bonding (H(3)N···H-NH(2)) interactions upon dimerization. Under Cs dimer configuration, core-electron binding energy of H-N (H-donor) is significantly decreased by the intermolecular core-hole interaction and causes notable redshifts of core-excitation energies, whereas that of lone-pair nitrogen (H-acceptor) is slightly increased and results in appreciable blueshifts in the core-excitation bands. The result of the hydrogen-bonding interaction strongly appears in the n-σ* orbital correlation, destabilizing H-N donor Rydberg states in the direction opposite to the core-hole interaction, when excited N atom with H-N donor configuration strongly possesses the Rydberg component of anti-bonding σ* (N-H) character. Contributions of other cyclic H-bonded clusters (AM)(n) with n ≥ 3 to the spectral changes of the N 1s → 3sa(1)/3pe bands are also examined.     

Effect of functional group (fluorine) of aromatic thiols on electron transfer at the molecule-metal interface

Electron transfer at the molecule-metal interface of self-assembled monolayers of 1,1';4',1'-terphenyl-4'-thiol (BBB) and its partially fluorinated counterpart (BFF: p-thiophenyl-nonafluorobiphenyl) on Au(111) is investigated by core-hole clock spectroscopy. Ultrafast electron transfer at the BBB/Au(111) interface in the low-femtosecond regime (on the same time scale as the C 1s core-hole lifetime, approximately 6 fs) was observed. In contrast, for BFF/Au(111), the interface electron transfer was forbidden during the core-hole decay. This strongly suggests that fluorination of phenyl rings significantly enhances the localization of the excited electrons in the LUMO.     

Dynamic screening and interference effects in X-ray and auger emission spectra

We study X-ray and Auger emission spectra from initial core holes which are strongly influenced by many-electron effects of giant Coster-Kronig type. In particular we include the dynamics of relaxation and decay processes in a systematic manner both in the spectral function of the initial-state core-hole (self-energies) and in the emission matrix elements (vertex corrections). We explicitly demonstrate that proper dressing of the emission matrix element guarantees that the sum of all branching ratios become unity (conservation of probability). Finally, we present numerical calculations of the 4p ^?1?4d ^?1 X-ray emission spectrum in Xe, illustrating the importance of a systematic treatment of self-energy and vertex corrections.     

Site-selected Auger electron spectroscopy of N2O

The N 1s Auger spectra for the two nonequivalent N atoms in N2O have been measured via Auger electron-photoelectron coincidence spectroscopy. The site-selected Auger spectra are compared with the normal Auger spectrum and with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. Such effects are found to be crucial factors in determining the different band shapes in the site-selected spectra.     

Fourier transform infrared (FT-IR) spectroscopy in bacteriology: towards a reference method for bacteria discrimination

Rapid and reliable discrimination among clinically relevant pathogenic organisms is a crucial task in microbiology. Microorganism resistance to antimicrobial agents increases prevalence of infections. The possibility of Fourier transform infrared (FT-IR) spectroscopy to assess the overall molecular composition of microbial cells in a non-destructive manner is reflected in the specific spectral fingerprints highly typical for different microorganisms. With the objective of using FT-IR spectroscopy for discrimination between diverse microbial species and strains on a routine basis, a wide range of chemometrics techniques need to be applied. Still a major issue in using FT-IR for successful bacteria characterization is the method for spectra pre-processing. We analyzed different spectra pre-processing methods and their impact on the reduction of spectral variability and on the increase of robustness of chemometrics models. Different types of the Enterococcus faecium bacterial strain were classified according to chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis (PFGE). Samples were collected from human patients. Collected FT-IR spectra were used to verify if the same classification was obtained. In order to further optimize bacteria classification we investigated whether a selected combination of the most discriminative spectral regions could improve results. Two different variable selection methods (genetic algorithms (GAs) and bootstrapping) were investigated and their relative merit for bacteria classification is reported by comparing with results obtained using the entire spectra. Discriminant partial least-squares (Di-PLS) models based on corrected spectra showed improved predictive ability up to 40% when compared to equivalent models using the entire spectral range. The uncertainty in estimating scores was reduced by about 50% when compared to models with all wavelengths. Spectral ranges with relevant chemical information for Enterococcus faecium bacteria discrimination were outlined.     

Local heating of a medium in the nanovicinity of the <Superscript>57</Superscript>Co nucleus that underwent radioactive decay to <Superscript>57</Superscript>Fe

Process of formation of the Auger blob (nanosized cloud of two or three hundred of ion–electron pairs) around a ⁵⁷Co radioactive atom after E-capture followed by the emission of a cascade of Auger electrons by the daughter atom ⁵⁷Fe has been considered. The effect of local temperature elevation in the Auger blob caused by energy emission during ionization slowing down of Auger electrons and the secondary electrons generated by them is discussed. Some consequences of this effect have been noted.     

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s-->sigma* resonance: experiment and theory

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.     

Radiationless decay in the region of the 2t2g and 4eg resonances in SF6

The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.     

A density functional theory study of shake-up satellites in photoemission of carbon fullerenes and nanotubes

Carbon 1s shake-up spectra of fullerenes C(60), C(70), and C(82) and single-walled carbon nanotubes (SWCNTs) of (5,5), (6,5), and (7,6) have been investigated by using equivalent core hole Kohn-Sham density functional theory approach, in which only one-electron transition between molecular orbitals within core-hole potential is considered. The calculated spectra are generally in good agreement with results of equivalent core-hole time-dependent density functional theory calculations and available experiments, and reliable assignments for the complicated shake-up spectra of such large systems are provided. Calculations have also been performed for endohedral metallofullerene Gd@C(82) to demonstrate the possible use of shake-up processes to identify the charge transfer between the metal ion and the carbon cage. It is found that the exciton binding energy of all systems under investigation is around 0.5 eV.     

Summary of ISO/TC201 technical report: ISO/TR 18392: 2005—Surface chemical analysis—X‐ray photoelectron spectroscopy—Procedures for determining backgrounds

ISO Technical Report 18392 provides the guidance for determining backgrounds in X‐ray photoelectron spectra. The methods of background determination described in this report are applicable for the quantitative evaluation of the spectra of photoelectrons and Auger electrons excited by X‐rays from solid surfaces and surface nanostructures. Copyright © 2006 John Wiley & Sons, Ltd.     

Report on the 42nd IUVSTA workshop ‘Electron scattering in solids: from fundamental concepts to practical applications’

A summary is given of the workshop entitled ‘Electron Scattering in Solids: from fundamental concepts to practical applications,’ which was held in Debrecen, Hungary, on July 4–8, 2004, under the sponsorship of the International Union of Vacuum Science, Technique, and Applications (IUVSTA). This workshop was held to review the present status and level of understanding of electron‐scattering processes in solids, to identify issues of key importance (hot topics) in the light of the most recent scientific results, and to stimulate discussions leading to a deeper understanding and new solutions of current problems. This report contains summaries of presentations and discussions in sessions on elastic scattering of electrons by atoms and solids, inelastic scattering of electrons in solids, modeling of electron transport in solids and applications, and software. The principal areas of application include Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy, elastic‐peak electron spectroscopy (EPES), reflection electron energy‐loss spectroscopy (REELS), secondary‐electron microscopy, electron‐probe microanalysis (EPMA), and the use of coincidence techniques in electron‐scattering experiments. A major focus of the workshop was determination of the inelastic mean free path of electrons for various surface spectroscopies, particularly corrections for surface and core‐hole effects. Published in 2005 by John Wiley & Sons, Ltd.