Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the Mo M_4,5NN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the Mo N_2,3VV Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molydenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.     

Some aspects of auger electron spectra of 3d transition metal oxides

Auger electron spectra of the transition metals Cr, Mn, Fe, Co and Ni as well as their oxides have been investigated in the energy range between 0–100 eV. In each case of the clean metal surface the observed spectrum consists essentially of one Auger line identified asM _2,3 VV transition. After oxidation a line doublet is observed revealing two transitions instead of one. Additional new Auger peaks appear in the low energy range between 0–30 eV. The “splitting” of the Auger line can be explained as resulting from aM _2,3 V _dV_d and aM _2,3 V _pV_p transition. The latter is characteristic for the compound and can in a simple way be interpreted as a cross transition.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Giant core-exciton effects on Si(111) 7 × 7 surfaces

We present the first direct experimental evidence for a large surface influenced core-exciton effect on silicon. The Si(111) 7 × 7 L_2,3 absorption edge has been studied with core-level electron energy loss (ELS) and X-ray photoemission spectroscopy (XPS). An excitonic shift of ～1–2 eV have been found for transitions from Si(2p) to empty surface states.     

Benford’s law predicted digit distribution of aggregated income taxes: the surprising conformity of Italian cities and regions

Core-level photoemission spectra of the Fabre salts with X = SbF₆ and PF₆ were taken using hard X-rays from PETRA III, Hamburg. In these salts TMTTF layers show a significant stack dimerization with a charge transfer of 1e per dimer to the anion SbF₆ or PF₆. At room temperature and slightly below the core-level spectra exhibit single lines, characteristic for a well-screened metallic state. At reduced temperatures progressive charge localization sets in, followed by a 2nd order phase transition into a charge-ordered ground state. In both salts groups of new core-level signals occur, shifted towards lower kinetic energies. This is indicative of a reduced transverse-conductivity across the anion layers, visible as layer-dependent charge depletion for both samples. The surface potential was traced via shifts of core-level signals of an adsorbate. A well-defined potential could be established by a conducting cap layer of 5 nm aluminum which appears “transparent” due to the large probing depth of HAXPES (8–10 nm). At the transition into the charge-ordered phase the fluorine 1s line of (TMTTF)₂SbF₆ shifts by 2.8 eV to higher binding energy. This is a spectroscopic fingerprint of the loss of inversion symmetry accompanied by a cooperative shift of the SbF₆ anions towards the more positively charged TMTTF donors. This shift does not occur for the X = PF₆ compound, most likely due to smaller charge disproportion or due to the presence of charge disorder.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

A qualitative and quantitative study of the oxides of aluminum and silicon using AES and XPS

Experimental AES and XPS results from powdered α-Al₂O₃, SiO₂, Al₂Si₂O₇·2H₂O and an aluminosilicate glass are presented. The AES kinetic energy and the XPS binding energy values of core-level states and the peak shapes of X-ray induced O KLL Auger transitions are presented as qualitative reference data for the purpose of aiding in the identification of unknown aluminum oxide/silicon oxide samples. The AES peak-to-peak heights and the XPS peak areas of core-level states are presented as quantitative data and are used in the calculation of empirical relative sensitivity factor values. In addition, the sensitivity factor values are given parameters according to a single variable, f₈, defined as the ratio of the volume density of the oxygen atoms to the sum of the volume densities of the aluminum and silicon atoms.     

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.     

High resolution lmm auger electron spectra of some first row transition elements

Auger spectra have been recorded from elements of the first transition series using a hemispherical analyser. Highly resolved LMM spectra were obtained showing for the first time the composite nature of these peaks for many of the elements studied. The recorded spectra show a general similarity for the elements Sc → Zn but interesting differences emerge. At the beginning of the transition period the L₃ based transitions have the relative intensities L₃ M_2,3, M_2,3 > L₃ M_2,3, M_4,5<> L₃ M_4,5, M_4,5 whereas towards the end of the series the order L₃ M_4,5, M_4,5 > L₃ M_2,3, M_4,5<> L₃ M_2,3, M_2,3 is observed. Pronounced chemical shifts have been observed upon oxidation. The spectra are interpreted in terms of an L -S coupling scheme and the fine structure discussed in terms of effects produced by multiplet splitting.     

Comparative photoemission study of Bi2(Sr,Ca)3Cu2O8+y and Bi1.7Pb0.3Sr2Ca2Cu3O10+y

The electronic structure of Bi-based 2223 single phase superconductor has been studied by ultraviolet and X-ray photoemission. By comparison with that of 2212 phase superconductor, we find a higher density of states nearE _F for 2223 phase. From analysis of the Cu 2p core-level spectroscopy, we obtain a relatively smaller charge transfer energy between copper and oxygen as well as the Coulomb repulsion energy on Cu site for 2223 phase. We relate these changes to the increase of hole concentration from 2212 to 2223 phase. The experimental results support the viewpoint that the transition temperature should be correlated with the density of states atE _F .     

Chemical states of GeTe thin-film during structural phase-change by annealing in ultra-high vacuum

The chemical states of GeTe thin film are investigated using high-resolution X-ray photoelectron spectroscopy (HRXPS) with synchrotron radiation, during amorphous to crystalline structural phase transition. As the temperature increases from 250 to 400 °C, we observe the rock-salt crystalline structure and phase with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Spin-orbit splitting of the Ge 3d core-level spectrum clearly appears after annealing at 400 °C for 5 min. However, the binding energy of the Ge 3d_5/2 core-level peak of 29.8 eV does not change in the amorphous to crystalline structural phase transition. In the case of the Te 4d core-level, change in binding energy and peak shapes is also negligible. We assume that the Te atom is fixed at a site between the amorphous and crystalline phases. Although the structural environment of the Ge atoms changes during the structural phase transition, the chemical environment does not.     

Strongly Z-dependent Auger and photoelectron diffraction in MgO(001)

We have measured angular distributions of Mg KL_2,3L_2,3 Auger and O 1s core-level photoemission intensities at high angular resolution for MgO(001). We find substantial differences in the two types of scans over many regions of k space, despite the very similar local structure of Mg and O in the rocksalt lattice. These diffraction scans are quantitatively predicted by spherical-wave, single-scattering theory, assuming an s wave for the Mg KL_2,3L_2,3 Auger transition. The differences that are observed between Mg KL_2,3L_2,3 Auger and O 1s diffraction scans are shown to be due purely to differences in the elastic scattering phase shifts for Mg and O atoms in the solid.     

SrTiO3(100) step sites as catalytic centers for H2O dissociation

Features arising from surface Sr(Ti) atoms on the SrO(TiO₂) terraces of SrTiO₃ (100) have been resolved in core-level photoemission spectra, which provides new insights into the surface electronic structure and reactivity of transition metal oxides. The surface Sr 3d (Ti3s) features lie to lower (higher) initial energy of the bulk-derived peaks by ca. 1.0 eV (1.7 eV), being consistent with the expected enhancement of covalent bonding in the TiO₂-terrace surface. Step-sites, which connect the two types of terrace, are found to act as catalytic centers for H₂O dissociation.     

A LEED-AES study of the initial stages of oxidation of Fe (001)

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H₂O at 10^?6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M_2,3M_4,5M_4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.     

Growth and properties of oxide films on Zn(0001)

The initial stages of the formation of ZnO films on cleavage planes of Zn single crystals have been studied. The growth rate is found to be linear in pressure and independent of temperature over the range from 77 to 425 K. The growth law is consistent with the rate limiting step being the adsorption of oxygen rather than ion transport through the oxide layer. Both the atomic and electronic structure have been monitored during oxygen exposure. The LEED patterns and intensity-voltage data indicate that at low temperatures the oxide is either amrophous or a fine grained polycrystal while above room temperature it is single crystal ZnO epitaxially oriented on Zn(0001). The changes during oxygen exposure of the Zn M₂₃M₄₅M₄₅, M₂₃M₄₅V, M₂₃VV and corresponding M₁ Auger spectra have been studied in some detail. The details of the spectra are identified through comparison with calculated and measured band structures and with previous observations of LMM transitions. The relaxation energy associated with the two d-band holes in the final state (i.e., δE(M₄₅M₄₅)] is found to be ～9 eV in good agreement with a recent calculation. The corresponding relaxation energy for the oxide is ～13.5 eV. The development of the electron energy loss spectra with oxygen exposure has been followed for a range of primary energies. A growth model which is consistent with the LEED observations, Auger peak heights and lineshapes and energy loss spectra is that the oxide grows heterogeneously on the Zn surface with no distinguishable precursor adsorbed state. However, contrary to previous models, it is shown that the surface is completely covered by oxide at a mean thickness of 2–3 monolayers. LEED, Auger and energy loss data for the two polar cleavage faces of ZnO are presented for comparison with those from the oxide overlayer.     

Ab initio study of rumpled relaxation and core-level shift of barium titanate surface

The rumpled relaxation and the core-level shift of full-relaxed BaTiO₃ (0 0 1) surface have been investigated by first-principles calculation. Based on the work function and the electric-field gradient, the right size of vacuum and the slab have been evaluated. The large displacements of ions deviated from their crystalline sites to lead to the formation of the surface rumples have been found. Some fully occupied surface oxygen p states at the top M point of the valance band and the empty surface titanium d states at the edge of the bulk conduction band are observed on the TiO₂-terminated surface. In contrast, on the BaO-terminated surface, two different core levels of the Ba 5p states shifted about 1.29 eV are induced by the bulk perovskite Ba atoms and the relaxation of surface Ba atoms, respectively. Our calculations are consistent with the experimental data.     

Si(111)解理面氧吸附后Si L2,3VV俄歇谱形的分析

对Si(111)解理清洁表面及其在氧吸附后测得的SiL_2,3VV俄歇谱进行数字积分、背底扣除及退自卷积后,得到了解理清洁表面在氧吸附前后的部分跃迁态密度。分析这两者之间的差别,结合别人的实验及理论计算结果表明:氧同时以分子形式及原子形式进行吸附,在吸附过程中,还同时形成氧化硅的价态。 关键词：     

Metallization of the Ge(111) surface at high-temperature probed by energy-loss and Auger spectroscopies

We present new electron energy-loss spectroscopy (EELS) and Auger (AES) experiments aimed to study the structural transition of the Ge(111) surface taking place at high temperatures. Our advanced high-temperature set-up allowed us to collect accurate EELS spectra near the M_2,3 excitation edges and AES MMV and MVV spectra, corresponding to different probing depths ranging from 4 to 10 Å. The metallization of the surface has been clearly detected by the shift of the M_2,3 edge and of the MMV, MVV Auger energies. A detailed study of the transition has been performed using a fine temperature step under thermal equilibrium conditions. The AES and EELS experiments show that a sudden semiconductor-metal transition takes place at about 1000 K involving mainly the topmost layers. Deeper layers within 10 Å are also involved in the metallization process (in a range of 10 above 1010 K) and a smooth change in the topmost layers is also observed at higher temperatures up to 1070 K. These transitions are not fully reversible upon cooling (down to 870 K). Structural and electronic characteristics of the surface transition are discussed in light of available models.     

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.