Theoretical investigation of the structure of X-ray photoelectron spectra of crystals

In the one-electron approximation general relations are obtained which determine the X-ray photoemission intensity from mono- and poly-crystalline solids. To describe the excited electron states plane waves and orthogonalized plane waves have been used. It is shown that consideration of orthogonalization terms is important when photoionization cross-sections of valence electrons are to be calculated. Results are discussed using the examples of transition metals, noble metals, semiconductors and dielectrics possessing the diamond and zinc-blende structure. The spectra from poly-crystals and single crystals of transition metals can differ markedly, but in the case of dielectrics the difference is small. Calculated X-ray photoelectron spectra of crystals are in good agreement with experimental data. To obtain detailed information of the spectrum structure, especially when transition metals are considered, the many-electron effects must be taken into account.     

Theoretical study of X-ray photoelectron spectra of mono- and polycrystalline samples

Recently Sixl and Sanwald¹ reported on measurements of fluctuation conductivity in very thin superconducting Al films. We reinterpret their data and find good agreement with the Aslamazov-Larkin and Maki-Thompson theories with a thickness dependent pairbreaking parameter.     

Theoretical study on transport properties of a BN co-doped SiC nanotube

We investigate the electronic transport properties of silicon carbide nanotubes (SiCNT) in presence of both boron (B) and nitrogen (N) impurities. The results show that co-doping BN impurities suppresses the important negative differential resistance (NDR) property. NDR suppression is attributed to the introduction of new electronic states near the Fermi level followed by weak orbital localization. BN co-doping results in exponential current-voltage (I-V) characteristics which is in contrast to linear I-V characteristics for individual boron and nitrogen doped SiCNTs. HOMO has no contribution from B impurity, whereas, LUMO has contribution from N impurity at low and high bias.     

X-ray photoelectron spectra of hexavalent iron

Binding energies of Fe 3p and Fe 2p electrons for K₂FeO₄ were measured and compared with those for γ-FeOOH, α-Fe₂O₃, and for metallic iron.     

X-ray photoelectron spectra of cobalt compounds

The X-ray photoelectron spectra of a variety of cobalt(II) and cobalt(III) complexes have been investigated. Intense satellite lines were observed for the 2p, 3s and 3p peaks in the case of the high spin cobalt(II) compounds, but not for low spin cobalt(III) complexes. The satellites of the 2p levels are best explained as arising from shake-up processes, whereas those of the 3s and 3p levels are thought to arise largely from multiplet (exchange) splitting of the levels. Multiplet splitting of the 2p level is small and responsible for an increase in the doublet separation of the 2p_1/2, 2p_3/2 spin-orbit levels of the high spin cobalt(II) compounds. The chemical shifts for cobalt differ for the 2p, 3s and 3p levels of the high spin cobalt(II) compounds. Those of the 2p and 3p levels of diamagnetic cobalt(III) and low spin cobalt(II) complexes are equal. The difference in the case of the high spin cobalt(II) compounds is thought to be due to the presence of unpaired 3d electrons.     

Theoretical study of X-ray photoelectron spectrum of germanium valence electrons

The density of states and X-ray photoelectron spectra of germanium valence electrons have been calculated. It is shown that to describe the shape of the photoelectron energy distribution curve of the greatest importance is precise allowance for the particularities of energy band structure and the ratio between excitation probabilities of s- and p-electrons. This is possible if the valence electron states are described with a linear combination of atomic orbitals while the excited electron states are described with the orthogonalized plane wave.     

Relative intensities in X-ray photoelectron spectra

An experimental determination of the relative intensities of X-ray photoelectron lines corresponding to the inner levels of elements with Z ? 20, and calculations of the total photo-ionization cross-sections for all shells of these elements with the Hartree—Fock—Slater potential are reported. The experimental and theoretical values agree well for the 1s levels while marked discrepancies are revealed for the 2p levels. The theoretical values of the cross sections for the atomic valence levels are used to calculate the relative intensities of molecular levels in CF₄, BF₄^?, BeF₄^2?, LiF, NO₃^?, CO₃^2?, CO, N₂, CO₂, H₂O and C₄H₅N. The results of the calculations agree satisfactorily with experiment.     

X-ray photoelectron spectra and structure of iron polynuclear complexes

Iron trimethylacetate complexes with different ligands have been investigated by X-ray photoelectron spectroscopy (XPS). The Fe 2p, Fe 3p, C 1s, O 1s, and N 1s spectra obtained at successive replacement of the Fe-O coordination bonds with Fe-N bonds have been analyzed. A satellite component is found in the spectra of iron trimethylacetate complexes, which is indicative of the high-spin state of Fe^II and Fe^III iron atoms. The XPS data made it possible to determine the degree of covalence of the metal-ligand bonding and reveal two nonequivalent states for iron atoms.     

X-ray and UV photoelectron spectra of the metal sesquioxides

He(I,II) and MgKα valence region photoelectron spectra of polycrystalline Ti₂O₃, V₂O₃, Cr₂O₃, Mn₂O₃, Fe₂O₃ and Rh₂O₃ have been recorded. The method of sample preparation employed gave rise to UV-PE spectra which are significantly different from those previously reported. While clear changes are observed in the spectra accompanying the semiconductor-to-metal transition of V₂O₃ at 150 K, there is no apparent modification of the Ti₂O₃ spectra accompanying its second order transition at 500 K. The remaining first row transition metal sesquioxides exhibit spectra indicative of localised 3d electrons.     

Theoretical study of the photoelectron spectra of gaseous Cu3Cl3

Ab initio calculations have been performed to interpret the photoelectron spectrum of gaseous cuprous chloride, Cu₃Cl³. Density functional calculations revealed Cu₃Cl₃ to be a planar cyclic D_3h molecule. Koopmans' theorem and two-hole/one-particle calculations with canonical Hartree-Fock orbitals were used to interpret the vertical ionization energies. These were compared with similar calculations using B3LYP Kohn-Sham orbitals. The results confirm the claim by Casida that Kohn-Sham orbitals mimic Dyson orbitals.     

Surface sensitivity and angular dependence of X-ray photoelectron spectra

By using an ultra-high vacuum X-ray photoelectron spectrometer designed specifically for surface studies, the surface sensitivity (ratio of surface to volume signal) of the technique was found to be enhanced by approximately one order of magnitude at low angles of electron emission measured from the surface plane. While the $\text{Mo(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) peak intensity from a clean polycrystalline target of molybdenum was independent of angle, the $\text{Cs(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) intensity from an adsorbed cesium layer on molybdenum increased and the $\text{Mo(3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$) signal from the molybdenum substrate decreased as the angle of emission was decreased. A model is presented to account for the angular dependence and it is proposed that the angular dependence of spectral intensities can be used to differentiate surface from bulk species.     

X-ray photoelectron spectra of halogens in coordination compounds

X-ray photoelectron data on C12p energies for more than 200 coordination compounds of transition metals have been analysed. The effects of the central atom, other ligands, and the geometrical location of the chlorine atom on the C12p energy have been considered and certain regularities have been observed. The data on Br3d and F1s lines in coordination compounds have also been considered.     

Electronic structure of bismuth ferrite and hematite single crystals: X-ray photoelectron study and calculation

The X-ray photoelectron spectra of the inner levels and valence bands of the surfaces of BiFeO₃ and Fe₂O₃ single crystals have been measured using X-ray photoelectron spectroscopy on an ESCALAB 250 X-ray photoelectron microprobe with monochromatization of X-ray radiation of the AlK _α line. The ab initio calculations have been performed by the full-potential pseudopotential method in terms of the density functional theory (Quantum-Espresso program package). The band structure and the total and partial densities of states have been calculated for the BiFeO₃ and Fe₂O₃ systems. A comparison of the theoretical and experimental results has demonstrated good agreement between theory and experiment.     

X-ray photoelectron spectra of some Ni mono- and polynuclear complexes

The core-level photoelectron spectra for Ni(bpy)₂(OOCCMe₃)₂, Ni₂(H₂O)bpy₂(OOCCMe₃)₄, Nibpy(OOCCMe₃)₂, Ni(C₁₀H₄(CH₃)₂N(SO₂C₆H₄(CH₃), Ni₂(2,4-Lut)₂(OOCCMe₃)₄, and (2,4-Lut is 2,4-lutidine) are reported and discussed, and the role of ligands in their appearance is determined. X-ray irradiation of some complexes was found to induce removal of some pyridyl rings and oligomerization both at room and liquid nitrogen temperature. The dependence of composition on temperature was used to find correlation between ligand surrounding and the spectra.     

Satellite phenomena in the X-ray photoelectron spectra of lanthanide trihydroxides

Using the direct CI method perturbation corrections to the Koopmans ionization energies have been obtained up to order 10 for the molecules CN^? and H₂O. The results are compared with those from the fully converged CI and B_k-type calculations. The convergence of the perturbation series is found to be quite slow although substantial improvement is possible using a variation-perturbation method.