Electron spectroscopic studies of clean and oxidized iron

X-ray photoelectron spectroscopy (XPS) and soft X-ray appearance potential spectroscopy (APS) together with Auger electron spectroscopy (AES) were used to study the electronic properties of clean and oxidized (Fe₃O₄) iron surfaces. The features arising from excitations of electrons from Fe 2p core levels are discussed consistently within the common one-electron picture (i.e. neglecting final state effects). For pure Fe the shape of the APS L₃ peak is evaluated taking into consideration the theoretical density of states above the Fermi level and is found to agree well with that observed. As a consequence it is shown that in this case the appearance potential is about 1 eV larger than the threshold energy for the excitation of a core electron to the Fermi level. Thus for $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons this quantity results to be 704.8 eV from both XPS and APS techniques. Successive oxidation at 500°C leads to an increase of the appearance potentials of the Fe 2p levels by only 0.5 eV, whereas the positions of the corresponding XPS peaks are shifted by as much as 3.5 eV. However this apparent disagreement can be eliminated by taking into account the above mentioned effect concerning the appearance potentials from pure Fe and the fact that the threshold energies (which determine the appearance potentials) of the XPS signals are shifted only by 1.7 eV. This example demonstrates that considerable care has to be taken in discussing “binding energies” or “chemical shifts” as derived from different electron spectroscopic techniques. The observed splitting of the MVV Auger transition of Fe at 47 eV upon oxidation is interpreted in terms of the qualitative features of the valence band structure of Fe₃O₄ and ascribed to the participation of a cross-transition between O 2p and Fe 3p states.     

The local electronic structure of PdO crystal and PdO catalyst supported on SiO2 and γ-Al2O3 from L3 and L1 x-ray absorption Pd edge in XANES spectra

We have studied the local electronic structure of PdO catalysts supported on characteristic inert and quasi-inert substrate from experiment on X-ray absorption near edge structure XANES using Synchrotron radiation. From the joint analysis of L₃ absorption edge and XPS core data in Pd and PdO we find that the white line in PdO XANES is an excitonic state with 0.8 eV binding energy. From the joint analysis of L₃ and L₁ edges the p-like and the d-like local unoccupied electronic states have been determined. The local structure of PdO catalysts is different from that of PdO crystal. Evidence for structural disorder in PdO catalysts and PdO-substrate interaction is reported.     

X-ray absorption spectra: K-edges of 3d transition metals,L-edges of 3d and 4d metals,and M-edges of palladium

The X-ray absorption spectra of the 3d and 4d transition metals have been calculated within the single-particle approximation by a new linearized augmented plane wave method. The spectra, calculated with sharp atomic and band-structure single-particle levels, have been convoluted with a Lorentzian broadening function whose width is the sum of that of the core hole and the excited electrons. Plots are shown for (i) the K-edge fine structures up to at least 100 eV above the edge for Ca, Ti, Cr, Co, Cu, and Zn, (ii) the L_{2, 3} white lines for Ca, Ti, Cr, Co, and Cu, (iii) the L₃ white lines for Sr, Zr, Nb, Ru, Rh, and Pd, and (iv) the M_{2, 3} and M_4,5 spectrum of Pd. Systematic features which depend on the crystal structure and the placement of the Fermi level with conduction band are briefly discussed.     

Low energy electron spectroscopy of Pt (100) and Pt (100)/CO

Fine structure in the n_vi, _VIIVV spectrum of clean Pt (100) has been observed, and interpreted as “band like” in origin rather than quasi-atomic. Differences in the dependence of the Auger yield on primary beam energy are observed between the N_VI, _VIIVV and O_IIIVV peaks, and are associated with anomalies in the dependence of the inner shell ionization crossection of the 4f level. Low energy electron loss spectra on the clean surface have been investigated at primary energies in the range 71–774 eV and at angles of incidence of the beam 0–60°. The results are related to high energy loss and optical data, and assignments are given for inter-band and plasmon losses. With approximately $\text{3}\text{4}$ of a monolayer of CO on the surface there is a prominent additional loss at around 13.5 eV, which is interpreted as a one electron transition from a σ state below the d band to available states several electron volts above the Fermi level.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

The unoccupied electronic structure characterization of hydrothermally grown ThO2 single crystals

Single crystals of thorium dioxide ThO₂, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L₃, M₃, M₄, and M₅ X‐ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n‐type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chlorine K X-ray absorption-edge structures of CIS-[Pt(NH3)2Cl2], trans-[Pt(NH3)2Cl2], trans-[Pd(NH3)2Cl2] and (NH4)2PdCl4

The K absorption-edge spectra of the ligand chlorine ion in square-planar complex compounds cis- and trans-[Pt(NH₃)₂Cl₂], trans-[Pd(NH₃)₂Cl₂], and (NH₄)₂PdCl₄ are reported and discussed in connection with the chlorine K absorption spectra of K₂PtCl₄ and K₂PdCl₄, reported previously. The observed chemical shift of a white line at the absorption threshold is interpreted in terms of the difference of the ligand-field splitting of electronic states for metal ions. The white line is attributed to the electronic transition from the Cl^? ls level to the lowest unoccupied antibonding molecular orbital (MO), which is specified by a $\text{MO}\text{b}{}_{\mathrm{1g}}\text{(σ}{}^1\text{)}$ in the square-planar complex with D_4h symmetry. The other absorption structures are regarded as continuum “shape resonances” of the outgoing electron trapped by the cage of the surrounding atoms. The effect of geometrical isomerism is found in the chlorine K absorption spectra of cis- and trans-[Pt(NH₃)₂Cl₂].     

Delayed onset of 4d photoemission relative to the giant 4d photoabsorption of La

For the giant 4d photoabsorption of La, both the total photoabsorption spectrum and the N_4.5-derived Auger emission intensity spectrum increase significantly above hν ? 112 eV, with spectral peaks at hν = 118 and 119 eV, respectively. However, the predominant 4d photoemission partial cross section shows a delayed onset of ～ 4 eV, with a peak at hν = 121 eV, while the 5s, 5p, and 5d partial cross sections all show a strong resonant enhancement at lower energies, with spectral peaks at hν = 116.6 eV. These results are compared with a recent many-body calculation for Ce. The photon energy dependence of the La 4d_{$\text{5}\text{2}$}/4d_{$\text{3}\text{2}$} photo-emission branching ratio is consistent with a “final-state model.”     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Adsorbate-induced surface resonances observed in photoemission from a (2 × 2)R45° sulphur layer on Pd(100)

Angular-resolved photoemission spectra from a $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ sulphur layer on Pd(100) reveal peaks independent of photon energy, which show strong dispersion in the electron energy range 3…9 eV relative to the vacuum level. The appearance of these levels can be correlated with absolute gaps in the projected bulk band structure of palladium. We interpret these features, which are also observed in secondary electron emission, as adsorbate-induced surface resonances above E_F. The sulphur 3p-derived levels below E_F show strong dispersion effects in the [110] but not in the [100] crystal azimuth. The projected band structure also shows that $\text{sd}$ hybridisation gives rise to an absolute gap at approximately this energy in the [110] $\text{(}\text{ΓX}\text{)}$ direction.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Electrical conductivity and defect structure of Ni-doped and “CO-reduced” Ni-doped SrTiO3 single crystals

The electrical conductivities of Ni-doped and “CO-reduced” Ni-doped SrTiO₃ single crystals were measured at temperatures 700–1200°C and P_o2's of 10^?7–10^?1 atm. Plots of log σ vs $\text{1}\text{T}$ at constant P_o2's were found to be linear, and the activation energies appeared to be 0.92 eV for Ni-doped SrTiO₃ and 0.50 eV for “CO-reduced” Ni-doped SrTiO₃ single crystals, respectively. Plots of log σ vs log P_o2 at constant temperature were found to be linear with an average slope of ?$\text{1}\text{4}$ for SrTiO₃:Ni and of ?$\text{1}\text{6}$ for “CO-reduced” SrTiO₃:Ni single crystals, respectively. The electrical conductivity dependencies on P_o2 indicate that a triply ionized titanium interstitial and an oxygen vacancy model are applicable to Nidoped and “CO-reduced” Ni-doped SrTiO₃ single crystals, respectively. The small polaron conduction was suggested on “CO-reduced” Ni-doped SrTiO₃ single crystal from the temperature dependence of conductivity.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

An IR study to optimize the absorption coefficient of cadmium stannate

Cadmium stannate (3Cd₂SnO₄/CdSnO₃) has been shown to exhibit broad band IR absorption over the wavelength range 3–12 μm. The intensity and position of the band maximum arise from “free” electrons present in the conduction band of the semiconductor. To enhance the absorption coefficient, we have studied the electron-donating properties of antimony dopant and the lattice defects arising from high temperature, vacuum and hydrogen exposure treatments. Our results demonstrate that appropriate dopant and treatment conditions generate cadmium stannate having an absorption coefficient value of greater than $\text{3000}\text{cm}{}^2\text{g}$. In support of these findings, X-ray diffraction analysis has identified phase changes that occur in conjunction with variations in optical properties.     

Empty electronic states in the sodium tungsten bronzes: a study by inverse photoemission spectroscopy

Empty electronic states in the sodium tungsten bronze $\text{Na}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{WO}{}_3$ have been studied by inverse photoemission spectroscopy using a novel narrow-bandpass photon detector. The empty density of states in the t_2g(W:5d) band peaks about 3 eV above the Fermi energy, implying an overall t_2g bandwidth greater than found in recent bandstructure calculations.     

Electronic states near fermi level in superconductors La2 − x Sr x CuO4 by means of x-ray absorption spectra near Cu-K and La-L edges

The electronic states of La_{2? x} Sr _x CuO₄ with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu²⁺ ion and the L-edges of La³⁺ ion. It is found that characteristic white lines occurring near L _II and L _III edges of La³⁺ ion show a slight energy shift depending on substituted Sr²⁺ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La³⁺ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78?K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d?⁹ L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La_{2? x} Sr _x CuO₄ are related to the 5dπδ and 3d?⁹ L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La³⁺ with Sr²⁺ ions.     

New hydrogen-like states due to Ni in ZnS

In addition to the narrow zero-phonon line at the photoionisation band edge at 2.4377 eV reported in absorption measurements of Ni-doped ZnS, another two sharp lines at 2.4268 and 2.4325 eV in the electroabsorption spectrum have been detected for the first time. At the bottom of the photoinization band a new structure has also been observed, which consists of a zero-phonon line at 3.3897 eV and its LO-phonon sidebands. The zero-phonon lines in the photoionization band are attributed to transitions into hydrogen-like states of a hole, loosely bound to a Ni¹⁺ ion which is either in the ground ${}^2\text{T}{}_2$ or high-lying excited ${}^2\text{E}$ states.     

Interpretation of the x-ray photoemission spectra of cobalt oxides and cobalt oxide surfaces

CoO and Co₃O₄ have been studied by high-resolution X-ray photoemission. The characteristic binding energies in the Co 2p_{$\text{3}\text{2}$}, 2p_{$\text{1}\text{2}$}, and 3s regions, their band shapes and widths, the associated shake-up structure, the O(1s) and O(2s) BE's, and the valence band spectra have been examined. The two oxides are readily distinguished from their spectra though it is shown that the O(1s) BE's are identical at 529.50 ± 0.14 eV. Argon and oxygen ion sputtered surfaces were examined to establish the integrity of the oxides. A higher BE O(1s) component (530.7–531.6 eV), the intensity and BE of which vary with the treatments mentioned above, corresponds to non-stoichiometric surface oxygen. The results are discussed with respect to the electronic structures of the oxides and the often conflicting earlier studies of these oxides.     

Angular distributions of auger electron emission: Clean and gas-adsorbed polycrystalline Ni surfaces

The excitation angle (β) and emission angle (θ) dependences of the Ni M₂,₃VV (61 eV) and Ni L₃VV (850 eV) Auger emissions from clean polycrystalline Ni surfaces, and the S L₂, ₃ M₂, ₃M₂, ₃ (150 eV) Auger emission from S-adsorbed poly-Ni surfaces have been investigated. In the case of Ni (61 eV) and S Auger emissions, the β-dependence shows the $\text{1}\text{cos β}$ distribution, while a significant deviation from $\text{1}\text{cos β}$ is observed for Ni (850 eV) Auger emission. The cosθ distribution and the intermediate between isotropic and cosθ distributions are observed for Ni (61 eV), and for Ni (850 eV) and S Auger emissions, respectively. Those results have been found to be in fairly good agreement with the calculations based on the simple continuum model without consideration of the diffraction effect and the inherent anisotropic emission.     

High-resolution photoemission study of adatom bonding effects: Cl (√2 × √2) R 45° on Cu (100)

We have investigated photoemission from an ordered $\text{(}\text{2}\text{×}\text{2}\text{) R 45°}$ Cl-overlayer on Cu (100) at good angular (Δθ ? 1°) and energy (ΔE = 100 meV) resolution. We observe two prominent adsorbate- induced features at 1.95 eV (FWHM ≈ 100 meV) and 5.4 eV (0.65 eV) below E_F. By determination of their angular momentum character, their spatial orientation and their energy dispersion, they are identified as “antibonding” and “bonding” resonances, respectively. Significant modification of the Cu d-band emission is observed and interpreted in terms of substrate orbitals which are involved in bonding.