Surface Extended X-ray Absorption Fine Structure Study Research Articles

Structure and local order in Co magnetic thin films on Au(111): A surface EXAFS study

The structure of cobalt thin films on an Au(111) surface has been studied using x-ray absorption spectroscopy at the cobalt K edge. The polarization dependence of x-ray absorption near-edge spectra and extended x-ray absorption fine structure (EXAFS) spectra evidences a hexagonal stacking for films thicker than 4 ML. For all the thicknesses, the analysis of the first-nearest-neighbor shell shows that this hexagonal structure is very close to that of the Co hcp bulk one: the cobalt does not grow in a coherent epitaxy on the Au(111) surface. This incoherent epitaxy leads to a wide radial distribution of the Co-Au bonds at the interface: this effect and the contribution of Au atoms to the EXAFS signal are discussed. The weak strains inside the Co magnetic thin films allow us to neglect the contribution of the magnetoelastic anisotropy to the perpendicular-magnetic anisotropy existing in this system.

Bond length and adsorbate vibrations of (2 x 3)N/Cu(110): A SEXAFS study.

Temperature- and angle-dependent surface-extended x-ray-absorption fine-structure (SEXAFS) measurements on the system (2X3)N/Cu(110) are reported. Three inequivalent adsorption sites of the N atoms ...

The structure of sodium intercalated 1T and 2H polytypes of TaS2: a photoemission and SEXAFS study

Na deposited 1T and 2H polytypes of TaS2 have been investigated by means of surface extended X-ray absorption fine-structure (SEXAFS) and photoemission studies. The photoemission spectra show that at room temperature Na mainly intercalates for both polytypes with no reactive component. The intercalated sodium atoms are positioned in the van der Waals gap in a sixfold coordinated site due to the SEXAFS analysis. The Na-S nearest neighbour distances are 2.95 ± 0.05 Å for 1T and 2.85 ± 0.05 Å for 2H polytype corresponding to an increase of the gap width by 1.5 ± 0.06 and 1.3 ± 0.06 Å, respectively. With increasing amount of deposited Na, a fraction of sodium atoms remains on the surface. The adsorbed Na atoms occupy a threefold hollow site on the surface with a Na-S bond length of 2.91 ± 0.05 Å for 1T and 2.80 ± 0.05 Å for 2H polytype.

The local adsorption geometry of oxygen on Pt(210): a SEXAFS study

Polarisation-dependent surface-extended X-ray-absorption fine-structure (SEXAFS) experiments have been performed on the Pt(210)-(5 × 2)rect-O system. The analysis shows that single 3-fold coordinated adsorption sites and mixtures of different 3-fold sites with a nearest-neighbour OPt bond length of R = 1.79 ± 0.05 A ̊ on both microfacets — (110) and most likely (310) — can reproduce the results. A mixture of different sites consisting of 3-fold coordinated sites on (110) and high-symmetry sites of other than 3-fold coordination on (310) must be excluded.

Structure of the quasihexagonal close-packed (4 × 4) Na layer on Al(111): a SEXAFS study

The Al(111)-(4 × 4)-Na structure formed by adsorption of Na on Al(111) at low temperature is studied by surface extended X-ray absorption fine structure (SEXAFS). Analysis of the SEXAFS measurements shows that the adsorbed layer is quasihexagonal close-packed and slightly compressed, with a nearest-neighbour Na-Na bond length of 3.70 ± 0.03 Å. Na atoms are adsorbed in 3 different adsorption sites with roughly equal Na-Al bond lengths of 2.8 Å but different perpendicular distances from the surface. Thus the Na layer is not strictly planar.

Unusual chemisorption structure of Na on Al(111): a SEXAFS study

The adsorption of Na on Al(111) has been studied by surface extended X-ray absorption fine structure (SEXAFS) in the coverage range of θNa = 0.16-0.7 monolayer at room temperature. For θNa = 0.16–0.33, Na atoms occupy an unusual 6-fold coordinate site formed by displacing every third (θNa = 0.33) Al atom in the first layer of the substrate. The NaAl bond length is 3.31 Å, independent of coverage, consistent with metallic bonding. These results have important implications for models of alkali metal adsorption.

NEXAFS and SEXAFS studies of chemisorbed molecules

Near-edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) spectroscopies and their application to the determination of the adsorption geometry and bonding of low-Z molecules on surfaces are discussed. NEXAFS is characterized by intramolecular resonances and probes the internal structure of the molecule (intramolecular bond lengths and possibly bond angles) as well as its orientation relative to the surface. SEXAFS provides information about the adsorption site and the molecule-substrate distances. After demonstrating the full power of SEXAFS in the analysis of oxygen adsorption on Cu(110) and on Ag(110) an example is given of a complete structure determination for the formate species (HCO2) on Cu(110) using NEXAFS and SEXAFS.