SiO2 and Al2O3 valence band density of states from self-deconvolution of L23VV Auger spectrum

The self-deconvolution of L₂₃VV Auger spectra of SiO₂ and Al₂O₃ has been carried out. The transition density functions obtained are compared with the local density of states (LDOS) of the valence band near the surface, as given by other techniques (XPS, UPS, XES) and also by theory. A fair agreement in the number and peak positions of valence band is produced. These compounds with MgO constitute an oxide series of increasing ionicity and the effects of initial hole localization in the transition density function are discussed.     

Relationship between the auger parameter and the energy gap

In this communication we show that for different semiconductors which do not contain transition metal ions, the shift in the extra-atomic relaxation energy for the metal ion with the core hole-as measured by the Wagner Auger parameter shift with respect to the metallic state-is roughly linearly correlated to the energy gap with a slope approximately equal to −1. The relationship, useful from a practical point of view, can be rationalized using the qualitative concepts of local and non-local screening developed by Veal and Paulikas and the direct relation between the band gap and anion electronegativity.

The core-hole screening for the semiconductors that follow the correlation can be described by the non-local screening mechanism. In this case, the polarized ligands transfer electronic charge to the unfilled s-p metal orbitals which are pulled down to the energy level of the local anion valence band by the core hole. Semiconductors containing transition metal ions, for which local screening mechanisms can occur by transferring the charge from the ligands to the localized d electronic levels, do not follow the correlation. This is due to the fact that the Auger parameter of the cation in the compound is similar to that of the atom in the metallic state. This result indicates that the relaxation energy in the transition metal compounds can be similar to the screening energy in the metallic state.     

N23-core losses and autoionization emissions of clean and oxygen exposed zirconium

The electron energy loss spectra associated with N₂₃-excitation and the low energy N₂₃VV Auger emission have been studied for both the clean and oxygen exposed zirconium. In the high energy side of the N₂₃VV Auger spectrum, autoionization emission of electrons of the valence band due to the decay of 4p electrons excited to states ≈9eV above the Fermi level has been identified. The excitation process can be also observed in the electron energy loss spectra. This is the first time that an autoionization feature is observed in a electron excited Auger spectrum of a 4d transition metal.     

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.     

The YbNi interface studied with photoemission spectroscopy

The development of the ytterbium valence band region was followed with Synchrotron radiation induced Photoemission Spectroscopy (SPS) by interdiffusion of Yb into a Ni (110) single crystal in order to identify the valence conditions of Yb in the bulk and on the surface. During this process, also the width of the Ni L₃VV Auger transition was investigated with X-ray induced Photoemission Spectroscopy (XPS), reflecting the electron donation of Yb to the Ni valence band. By comparison between theory and experiment, strong multiplet splittings were found to take place in the 4d and 5p core level spectra of Yb due to the promotion of one 4f electron to the valence band by reaction with Ni. The 5p level is demonstrated to resonate strongly at hν=181 as a consequence of the 4d–4f giant resonance.     

Observation of selection rules and restricted valence band self convolution for the KVV Auger transition of alkali graphite intercalation compounds

The Auger spectrum of the KVV transition in alkali graphite intercalation compounds (AGIC's) gives unambiguous evidence that the electron states from the alkali metals in the valence band of the compound form a narrow structure in the KVV Auger spectrum which corresponds to the self convolution of these states and that they are not convoluted with the whole valence band. This observation shows that strong matrix element effects or selection rules for KVV Auger transitions have to be taken into account.     

Correlation effects in Auger spectra of Ni and Cu nanoclusters

Results of experimental research of exciton-like two-hole states in nanoclusters of narrow-band metals (Ni, Cu) on surface of high-oriented pyrolitic graphite by X-ray photoelectron and Auger electron spectroscopy are presented. It was found that the evolution of the electronic structure in Ni nanoclusters with the decreasing of their sizes can lead to appearance of long-living two-hole states in the valence band. One-particle and two-particle density of states are analyzed, and the Auger-electron spectra confirming the presence of the bound and localized states are obtained.     

Auger and other characteristic energies in secondary electron spectra from Al surfaces

The energy spectra of secondary electrons back-scattered from clean, oxygen covered, and Cu covered Al surfaces have been determined. The data support the previous suggestion that Auger electrons can experience both characteristic energy loss and absorption phenomena. From the experimental results it was not possible to determine whether densities of states of electrons in the valence band affected the Al L_2,3 VV Auger spectrum. This portion of the spectrum was greatly changed by oxygen absorption on the Al surface, but little affected by less than a monolayer of Cu. Conversely, characteristic loss spectra were less sensitive to oxygen on the surface, but were highly sensitive to the presence of copper at even less than monolayer coverage. A correlation between characteristic loss and “true” secondary spectra from clean surfaces was established and possible reasons for the correlation are discussed.     

Effect of the competition between relaxation and core hole decay on the Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Ni metal

The valence hole created in Ni metal either by the L2-L3V Coster–Kronig (CK) transition or by the L3V shakeup/off becomes screened out prior to the L3-hole decay. We denote the atomic shell Lx (x = 2, 3) by LX. The metastable two-hole L3V state relaxes to the fully relaxed single L3-hole state before the L3-hole decays. Thus, the coincidence L2-L3(V)-VV(V) Auger-electron spectrum resembles closely the coincidence L3-VV Auger-electron spectrum. The final state of the CK transition preceded Auger transition is a two-hole state rather than a three-hole state. The four-hole satellite about 8 eV below the L3-VV main line in the singles (non-coincidence) Auger-electron spectrum is partly due to the L3VV-VVVV transition and the L2-L3VV-VVVV transition. The valence holes created either by the L2-L3VV transition or by the L3VV double shakeup/off remain localized during the L3-hole decay. The L3-hole lifetime widths of Fe, Co and Ni metals are determined from the APECS spectra. The agreement between experiment and theory (the independent-particle approximation) is poor.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

The valence band of free K clusters studied by photoelectron and Auger spectroscopies

The valence states of free neutral potassium clusters produced by a gas aggregation source were probed by synchrotron radiation based photoelectron spectroscopy. The first ionization energy (IE) of the clusters was determined to be 10% larger than the work function of bulk potassium. Using electrostatic concepts and the IE, the mean size of the clusters was estimated to be ≈2000 atoms. Further information about the valence band was provided by investigation of the Auger process initiated by the ionization of the 3p level with a subsequent emission of an Auger electron from the valence band (M_2,3VV). Plasmon satellites were observed in Auger spectra of free metal clusters.     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

Electron excited Auger line shape study of ion-beam-mixed Pd–Au alloys

The electronic structure of the ion-beam-mixed Pd–Au alloys have been studied using valence band spectra of XPS and electron excited CVV–Auger spectra. To show the relationship between the electronic structure changes and the Auger spectral line shape, the data of the self-convolution of the partially weighted valence band spectra was compared with the Auger spectra of Pd–Au alloys. The Pd–Au alloy is one of the systems which both atomic and band-like contributions are evident in the Auger spectral line shape. Since the self-convolution of PDOS’s relates to the band-like part of Auger spectra, in Pd–Au alloys, the band-like structure in the Auger line shape can be classified by the self-convolution of the partially weighted valence band spectra. Finally, we found that the increase in peak size at ∼80 eV with the increase in Pd content is due to the band-like contribution in the Au N_6,7VV Auger line shape.     

Solid-state and atomic features in the valence-band Auger spectra of copper,silver, and gold

High-resolution measurements of the L₃VV, M₄₅VV, and N₆₇VV Auger spectra are reported for copper, silver, and gold, respectively. Qualitative trends in the spectra and experimentally determined values of U_eff/2W (U_eff is the effective energy required to excite two holes on the same atom; W is the d-electron bandwidth) are shown to be consistent with recent predictions by Sawatzky and Cini concerning the relative importance of atomiclike and bandlike features in Auger spectra.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

Cluster growth of Cu on graphite: XPS,Auger and electron energy loss studies

Auger, XPS and EELS techniques have been used to investigate the core levels, the d-valence band and the electronic transitions of different UHV deposited Cu clusters on graphite. The decreasing of the Cu particle size produces core levels and valence band shifts towards higher binding energies. Lower extra-atomic screening of the conduction electrons near the excited atom and shift of the d-band towards the isolated atom levels are claimed to explain these effects. The EELS results suggest that, for smallest clusters, no structural change but only a lattice parameter contraction of the f.c.c. cage occur.     

Photoemission study of the electronic and chemical structure of amorphous CuxY100-x films

Valence band and core level photoelectron spectra are reported for clean Cu_xY_100-x amorphous thin films grown in situ by sputtering of targets with different stoichiometry. The density of occupied states of these amorphous metallic alloys is shown not to be a simple linear superposition of the constituent densities of states. Experimental evidence shows overlap of Cu and Y d bands with significant electron transfer from Y to Cu atoms which induce large bonding shifts, narrowing and intensity modifications of the valence band. The same conclusion is reached from the Cu core level shift and asymmetry. An estimation of the effective Coulomb interaction on Cu sites from Auger results is attempted.     

Many-electron effects in the Auger-photoelectron coincidence spectroscopy spectra of the late 3d-transition metals

The valence hole created by the L2–L3 M45 Coster–Kronig (CK) transition may hop away from the ionized atomic site before the L3-hole decays. Then when the third (Auger) electron emitted by the L3-hole decay is measured in coincidence with the photoelectron emitted by the initial L2-level electron ionization, the coincidence spectrum becomes similar or identical to the singles spectrum of the secondary (Auger) electron emitted by the L3-hole decay as if it decayed as an initial single core hole. Thus the coincidence spectrum is essentially governed by the valence-hole dynamics of both the intermediate states and the final states of the L2–L3 (M45) CK-transition preceded Auger transition. In the present paper the Auger-photoelectron coincidence spectroscopy (APECS) spectra of Fe, Co, and Ni metals reported by C.P. Lund et al. (Phys. Rev. B55 (1997) 5455) are analyzed in light of the delocalization and localization of the valence hole(s) created by the CK transition or the CK-transition preceded Auger transition.     

Tracking the excitation dynamics in the Mn:Ge(111) metallic interface by resonant electron spectroscopy

Resonant photoemission from the valence band of a (√3 × √3)R30° reconstructed Mn:Ge(111) metallic interface has been carefully analyzed with the aim to track the transition from resonant Raman to normal Auger emission. The transition energy has been compared with the Mn 2p binding energy, as well as with the Mn L(3) absorption edge energy. Close similarities emerge with respect to the case of elemental Mn thin films, suggesting that the excitation dynamics is dominated by the electronic properties of Mn 3d states, in spite of the bonding with Ge atoms. The switching from the resonant Raman Auger (RRAS) to the normal Auger regime is found about 2 eV below the Mn L(3) absorption edge. A change of the lineshape due to the transition from an overall N - 1 electron final state (RRAS channel) to an N - 2 electron final state (normal Auger channel) is evidenced by the analysis of the experimental data, which also allowed the ratio to be tracked between charge delocalization and core-hole time scales as the photon energy is tuned across the Mn L(3) edge.     

On the Auger-photoelectron coincidence spectroscopy spectra of Cu(100), Cu metal and CuOx/Cu surface

The M₃–VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Cu(100) and the L₃–VV APECS spectra of Cu metal and CuOx/Cu surface are analyzed in detail. The narrowing and energy shift of the photoelectron line in the M₃–VV APECS spectrum is well predicted by the present theory. The spectrum shows the presence of the M₂–M₃(V)–VV(V) decay in which a hole in the 4s band hops away prior to the decay of M₃ hole. The analysis of the L₃ photoelectron spectra of Cu metal measured in coincidence with the ³F or ¹G Auger line raises a question concerning the presence of two different core–hole states upon the L₃ level ionization recently proposed by Thurgate and Jiang [Surf. Sci. 466 (2000) L807]. The analysis of the L₃–VV APECS spectrum of CuOx/Cu shows that the final-state charge–transfer interaction plays an important role in CuO.