Observation of opposite 4f-surface core level binding energy shifts for W(111) and Ta(111)

Surface 4f core level binding energy shifts have been measured in photoemission from W(111) and Ta(111). The surface shift was found to change sign across the row of 5d-metals: for the topmost layer of Ta(111) a +0.40 eV shift toward higher binding energy is found, whereas for W(111) the shift is -0.43 eV toward lower binding energy. The shifts are shown to be dependent on surface crystallography. Chemical shifts are determined for saturation coverage of hydrogen.     

Strong influence of defects on the electronic structure of Pt adatoms and clusters on graphite

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt₈ clusters. Surface diffusion is strongly reduced by depositing Pt and Pt₈ into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.     

Reflectometric study of surface states and oxygen adsorption on clean Si(100) and (110) surfaces

External differential reflection measurements were carried out on clean Si(100) and (110) surfaces in the photon energy range of 1.0 to 3.0 eV at 300 and 80 K. The results for Si(100) at 300 K showed two peaks in the joint density of states curve, which sharpened at 80 K. One peak at 3.0 ± 0.2 eV can be attributed to optical transitions from a filled surface states band near the top of the valence band to empty bulk conduction band levels. The other peak at 1.60 ± 0.05 eV may be attributed to transitions to an empty surface states band in the energy gap. This result favours the asymmetric dimer model for the Si(100) surface. For the (110) surface at 300 K only one peak was found at 3.0 ± 0.2 eV. At 80 K the peak height diminished by a factor of two. Oxygen adsorption in the submonolayer region on the clean Si(100) surface appeared to proceed in a similar way as on the Si(111) 7 × 7 surface. For the Si(110) surface the kinetics of the adsorption process at 80 K deviated clearly. The binding state of oxygen on this surface at 80 K appeared to be different from that on the same surface at 300 K.     

The reactions of oxygen and water with the rare-earth metals terbium to lutetium studied by x-ray photoelectron spectroscopy

The X-ray excited oxygen 1s photoelectron spectra of water adsorbed on clean evaporated films of the heavy rare-earth metals, terbium to lutetium, is characterised by two peaks: (1) at 531.0 ± 0.5 eV binding energy, and (2) at 533.0 ± 0.5 eV binding energy, assigned respectively to oxide and hydroxide species. Variation of the relative intensities of these peaks with exposure to water leads to the postulate that the oxidation mechanism is island growth with a layer of hydroxide at the surface of the oxide island. At low temperatures, adsorption of water gives two additional peaks: (3) at 534.5 ± 0.5 eV, and (4) at 535.7 ± 0.2 eV, assigned respectively to chemisorbed and condensed water. On adsorption of small amounts of dry oxygen the O 1s spectra exhibit solely peak (1), whose intensity increases with further oxygen treatment to reach a steady value after ～40 L exposure. The kinetics of reaction with oxygen follow a logarithmic relation, once correction is made for the effect of escape-depth on peak intensity. However, ytterbium, with a closed 4f shell in the metallic state, exhibits oxidation characteristics different from the other rare-earth metals; its oxygen 1s intensity increases linearly with exposure, and the steady plateau level is reached sharply rather than asymptotically. Island growth with a limited number of nucleation sites may explain this behaviour.     

Coverage and adsorption-site dependence of core-level binding energies for tin and lead on Al(111) and Ni(111)

Thin layers (0.2–10 monolayers) of Pb and Sn were prepared on Al(111) and Ni(111) surfaces and characterized by means of LEED, AES, UPS and work-function measurements. The binding energy of the shallow Sn 4d and Pb 5d core-levels was investigated with respect to coverage and adsorption-site-dependent changes. On Al(111) the Sn and Pb monolayers exhibit ordered, two-domain, aligned but not in registry structures. For these layers core-level binding energies were found identical to those of the bulk metals. On Ni(111), Pb gives rise to a 3 × 3 structure, followed then by a 4 × 4 structure at higher coverages. The Pb 5d core-level binding energies shift continuously to higher values. A final shift of 0.42 eV is reached after about 2 monolayers. Sn on Ni(111) exhibits two welll separated peaks lying at 23.70 and 23.97 eV for the $\text{4d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ line. These two lines can be correlated with two different adsorption sites which have to be assumed for the $\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ and the (2 × 2) structure found at different coverages. The binding energy shifts are discussed in a model based on a Born-Haber cycle.     

Ab initio cluster model approach to the chemisorption of NH3 on Pt(111)

The chemisorption of NH₃ on Pt(111) has been studied using several Pt₁₀ clusters that model different adsorption sites of the Pt(111) surface. Ab initio methods have been used to obtain a theoretical estimate of several spectroscopic features that can be directly compared to experiment. The comparison includes the variation of the difference between the 3a₁ and 1e levels, the vibrational frequency shifts and the order of stability on different surface sites. Chemisorption at the on-top site is predicted to be favoured, the calculated interaction energy appears to be quite close to the experimental estimate, and it is suggested that NH₃ chemisorbs molecularly, in an N-down orientation, with an equilibrium geometry representing a small distortion from the gas-phase molecular geometry and no azimuthal preference, in good agreement with ESDIAD experiments. Constrained space orbital variation (CSOV) analysis of the interaction has also been performed using a Hartree–Fock wave function. This analysis shows that the leading bonding mechanisms are substrate polarisation and charge transfer from ammonia to the surface.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

Hydrogen-induced reconstruction of the W(100) surface,studied by surface core level spectroscopy

We report W(4?) surface core level shifts which yield new information on the energetics of the W(100) (1 × 1) → C(2 × 2)H phase transition. At small hydrogen coverages we find two co-existing surface core levels from atoms on normal lattice sites and from atoms in reconstructed domains. These surface levels are shifted to smaller binding energy (toward E_F) by 0.35 eV and 0.13 eV relative to the bulk level, respectively. The most stable configuration is obtained at a fractional coverage θ_H ? 0.2, at which all surface atoms are shown to be paired with neighboring atoms in the surface plane.     

CHaracterization of CO binding sites on Rh{111} and Rh{331} surfaces by XPS and LEED: Comparison to EELS results

Changes in the nature of the binding site of chemisorbed CO on the Rh{111} and Rh{331} single crystal surfaces during adsorption and desorption have been monitored by X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Two bonding states of molecular CO have been identified from the O 1s photoemission line. These states are assigned as atop and bridge-bonded species and are observed to be coverage and temperature dependent. On both surfaces atop sites are populated first and at higher CO coverages bridge sites are filled. On Rh{111} the bridge sites are filled at a CO coverage of θ_CO ～ 0.50 and their presence is correlated with a change in the LEED pattern. The presence of the step atoms on the Rh{331} surface markedly influenced the sequential filling of binding sites in comparison to that observed on the Rh{111} surface. A comparison of our data to previous Electron Energy Loss Spectroscopy (EELS) work on Rh{111} is in remarkable quantitative agreement with EELS peak heights.     

Defect induced surface chemistry: A comparison of the adsorption and thermal decomposition of C2H4 on Rh{111} and Rh{331}

The adsorption and thermal decomposition of C₂H₂ on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C₂H₄ exposure exhibit a shift in binding energy (E_b) from 283.5 eV at 1 L C₂H₄ exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s E_b value and full width at half maximum as a function of surface temperature after a 10 L exposure of C₂H₄ at 300 K reveals that a species consistent with a C₂H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CH_ads or C_ads surface fragment is formed and increases in concentration at the expense of the C₂H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C₂H₄ adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Surface effects in Dy,DySb and DyTe observed by photoelectron spectroscopy

Cold condensed (100 K) Dy-films, as well as DySb and DyTe were studied by photoelectron spectroscopy using HeII and HeII^* excitation sources. The spectra were analyzed using the relative intensities and energy splittings of the individual 4f-multiplets from intermediate coupling calculations for bulk and surface multiplet components. At the surface of Dy the 4f ⁸ final state components are found to be shifted to higher binding energy by 0.60±0.05 eV. Their larger linewidth as compared to the bulk lines indicates the presence of lowcoordinated sites at the low-temperature surface. The 4f-surface shift for trivalent DySb and DyTe is found to be only 0.2 eV and 0.4eV, respectively, as a consequence of a chemical shift of 1.1 eV towards higher binding energy as compared to the pure metal. The Dy atoms at the surface of DyTe are found to be stabilized by only 0.2 eV in their trivalent state. Finally, a careful oxidation study of the cold-condensed Dy-films reveals that no manybody satellite has to be invoked to explain the appearance of a 6 eV peak in the spectra.     

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.     

Absolute composition depth-profiles in surface segregation of PtRh alloys

The absolute composition depth profiles of the first 12 atomic layers of the (001) surface and the first 9 layers of the (111) surface of a PtRh alloy have been measured. As found in earlier studies, the surface is enriched with Pt, the sub-surface layer is depleted of Pt, and the Pt concentration oscillates toward the bulk value. The detailed oscillations are, however, very different for the two surfaces. Using a simple model for the metallic bonds, Monte Carlo simulations are performed to extract a set of bond energy differences of the constituent atom pairs from the experimental data.     

X-ray photoemission study of physically absorbed SF6

The physical adsorption of octahedral SF₆ on Ru(001) has been studied with X-ray photoelectron spectroscopy (XPS) in an attempt to see effects on the energy levels resulting from the conformation of the molecule on the surface. Near 80 K surface coverages up to a monolayer have been studied at various steady state pressures of SF₆. Kinetic studies, core level binding energies, and peak areas indicate that the surface species studied was a physically adsorbed monolayer of sf₆. The sticking coefficient of SF₆, at ? 80 K is approximately unity. Also, a multilayer structure was observed at the highest pressures of SF₆. The binding energy of the F(ls) peak for monolayer coverage is centered at 688.2 ± 0.2 eV relative to the Ru Fermi level. while the multilayer F(ls) peak is shifted more than 3.5 eV to higher binding energy. The F(ls) linewidth for one monolayer has a full width at half maximum of 1.75 ± 0.1 eV. The F(ls) linewidth of the multilayer peak narrows with increasing coverage. Its narrowest observed linewidth was 1.35 eV ± 0.1 eV or approximately the same as that found in the gas phase. One of the mechanisms which may account for the F(ls) linewidth with monolayer coverage is a difference in F(ls) binding energy between those F atoms in contact with the substrate and those further away. This may be due to the variation in chemical environment and relaxation effects as a function of distance from tlie substrate. A classical image force calculation including finite screening effects of the substrate indicates that there is a differential binding energy, ΔW. between the F ligands; ΔW = 0.85 ± 0.25 eV, for realistic ranges of adsorption distances from the substrate and screening lengths in the substrate. The observed broadening of the monolayer F(ls) level is consistent with a ΔW of 0.7 ± 0.1 eV, indicating the possible existence of such a mechanism. Adsorption of a monolayer of SF₆ onto the Ru covered with a monolayer of oxygen shifts the F(ls) peak to lower binding energy by 0.8 eV. Similar effects due to oxygen have been observed previously in the physical adsorption of Xe on W(111).     

Bi2Sr2CaCu2-xLixOy体系的光电子能谱研究

采用X射线光电子能谱对Bi₂Sr₂CaCu_2-xLi_xO_y体系在真空中获得的清洁表面进行了研究。结果表明:Li的掺入对Bi和Sr的化学键性质几乎没有影响,而Ca,Cu,O的化学键性质有较大的变化,其结合能随着Li含量x的增加向高能方向移动。同时对真空中获得的清洁表面和普通表面样品O1s进行了比较,结果发现真空中获得的清洁样品O1s有一个峰(528.4eV),而普通表面样品O1s有两个峰(528.4eV)和(531eV),这表明O1s的高能峰是由污染产生的。 关键词：     

Study of the initial stage of yttrium oxidation using characteristic electron-energy-loss spectroscopy

The characteristic electron-energy-loss (EEL) spectra of the pure surface of metallic yttrium and of this surface in the initial stages of oxidation are recorded. The energy of the primary electron beam E _p is 200–1000 eV. The spectra exhibit high-and low-frequency peaks. During oxidation, the positions of the basic peaks in the EEL spectra are significantly shifted. The peaks corresponding to the bulk energy loss shift toward higher energies upon oxidation. The peak corresponding to the low-frequency surface oscillations also shifts, but toward lower energies, and its intensity monotonically decreases with increasing oxygen dose. The differences between the spectra recorded at different E _p are explained as resulting from an increase in the electron escape depth with E _p .     

Adsorption and diffusion of OH on Mo modified Pt(111) surface: First-principles theory

A first-principles study of adsorption and diffusion of OH on Pt and PtMo(111) surfaces is described. It confirms that the dissociation of water is much easier on PtMo than on pure Pt. Furthermore, we also found that OH binds most strongly at Mo atop site with adsorption energy of −3.32 eV, which is ∼1 eV stronger than binding to the pure Pt(111) surface. OH is much more localized on the PtMo alloy surface than on pure Pt. Both the stranger bond and the higher localization of OH contribute to the enhanced fuel cell performance with PtMo electrodes compared to pure Pt.