Atomic subshell photoionization cross sections and asymmetry parameters: 1 ⩽ Z ⩽ 103

The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.

https://iopscience.iop.org/article/10.1088/0034-4885/44/1/001/pdf

Valence fluctuation phenomena

http://w0.rz-berlin.mpg.de/hjfdb/pdf/110e.pdf

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

Extended X-ray absorption fine structure is used to study local order in silicate glasses around sodium and silicon atoms. It is demonstrated that modifying cations like sodium have well-defined short range order that is complementary rather than incidental to the glass forming silicate network.

Local structure of silicate glasses

Surface-sensitive photoelectron measurements reveal bulk-to-surface shifts of the Sm 4f/sup 6/ level which imply inhomogeneous valence mixing on the surface of Sm and SmB/sub 6/. The surface valence fraction is estimated to be the same for both materials. The measurements take advantage of a large resonant enhancement of 4f electron emission due to 4d ..-->.. 4f photon absorption, and detailed spectra showing this phenomena are presented. It is shown that the 4d hole in the 4d ..-->.. 4f absorption process stabilizes the 4f state by approx. 4 --7 eV. Exposure of Sm films to oxygen is found to eliminate, rather than increase, the emission from the surface 4f/sup 6/ state, showing that the 4f/sup 6/ state does not arise from oxygen contamination. Observed variations in Sm film spectra are described, including the finding in some films of an unexplained photoemission peak 2.4 eV below the Fermi level. SmB/sub 6/ also displays a broad band of Auger emission when a boron 1s core hole is created, and this is ascribed to electrons in the boron 2p bonding band. Various trends in 4d and 4f binding energies for Sm and SmB/sub 6/ are pointed out and discussed.

Surface mixed valence in Sm and SmB/sub 6/

We have investigated the oxidation of Al(111) surfaces by means of polarization-dependent surface extended-x-ray-absorption-fine-structure (EXAFS) measurements. The experiments were performed above the oxygen $K$ absorption edge (\ensuremath{\sim}535 eV) using the total electron-yield detection technique. The oxidation range from monolayer chemisorption corresponding to an ordered oxygen (1 \ifmmode\times\else\texttimes\fi{} 1) overlayer to the formation of a thick surface layer of amorphous oxide has been studied. For the chemisorbed monolayer (100-150-L ${\mathrm{O}}_{2}$) we find a strong polarization dependence of the EXAFS signal which allows the determination of the O-O intraoverlayer separation 2.90 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. The O-Al chemisorption bond length has been determined to be 1.79 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}. Within experimental error we find the same O-Al distance for the initial oxide-like phase which accompanies the chemisorbed phase. At higher exposures (1000-L ${\mathrm{O}}_{2}$) the vanishing low-energy-electron-diffraction pattern is accompanied by an increase in the O-Al bond length (1.88 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{}). This value is also observed for heavily oxidized surfaces. The O $K$ edge shows the formation of a strong "white-line"-like threshold resonance with increasing oxygen exposure. This resonance is attributed to transitions to localized final states in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-like molecular units. It is pointed out that detailed analysis of the absorption threshold may by itself provide information on the local structure of surface complexes. Models for the progressive oxidation process of Al(111) surfaces implied by our data are discussed.

Surface extended-x-ray-absorption-fine-structure study of oxygen interaction with Al(111) surfaces

Extended-x-ray-absorption-fine-structure studies of low- Z atoms in solids and on surfaces: Studies of Si 3 N 4 , Si O 2 , and oxygen on Si(111)

The valence-band structure of single crystals of titanium carbide (TiC) and titanium nitride (TiN) have been studied with angle-integrated photoelectron spectroscopy using ${\mathrm{He}}_{\mathrm{I}}$, ${\mathrm{He}}_{\mathrm{II}}$, and synchrotron radiation for excitation. The results are compared with the partial densities of states derived from an augmented-plane-wave band-structure calculation. Peak positions and widths agree fairly well, but the observed intensity modulations with photon energy cannot be interpreted by such a comparison.

Valence-band structure of TiC and TiN

The initial ordered chemisorption stage of oxygen on Al(111) has been investigated using polarization-dependent surface extended x-ray absorption fine-structure measurements above the O $K$ absorption edge. The O-Al bond length in the threefold hollow adsorption site has been determined to be 1.79\ifmmode\pm\else\textpm\fi{}0.05 \AA{}. A strong polarization dependence of the extended x-ray absorption fine-structure signal allows the determination of the O-O distance (2.90\ifmmode\pm\else\textpm\fi{}0.05 \AA{}) between the chemisorbed oxygen atoms.

L. I. Johansson

Papers

Surface mixed valence in Sm and SmB/sub 6/

Surface extended-x-ray-absorption-fine-structure study of oxygen interaction with Al(111) surfaces

Extended-x-ray-absorption-fine-structure studies of low- Z atoms in solids and on surfaces: Studies of Si 3 N 4 , Si O 2 , and oxygen on Si(111)

Valence-band structure of TiC and TiN

Bonding of oxygen on Al(111): A surface extended x-ray absorption fine-structure study