Grazing Incidence X-Ray Absorption Spectroscopy of Near Surface Regions: Possibilities and Limitations

The application of the grazing incidence X-ray absorption spectroscopy (GIXAS) to the investigations of near surface regions of layer systems is discussed. Starting with transmission standards, GIXAS spectra for different detection modes were calculated as functions of the grazing angle, layer composition and surface/interface roughness. For systems consisting of only one layer that participates to the XAFS signal, the fine structructure is a linear superposition of the fine structures in the real and the imaginary part of the energy dependent refractive index. The surface roughness iess than about 2 nm does not significantely reduce the surface sensitivity of the method. An example of near surface structure determination for an oxidized metal surface is given

An Ex Situ Reflection Mode XANES Study of Underpotentially Deposited Copper Monolayers

Reflection mode XANES was performed ex situ on a copper monolayer underpotentially deposited onto a polycrystalline gold electrode. According to calculations based on reference substances, the thickness of the adsorbate layer was 0.21 nm and its oxidation state was close to +1. The ex situ spectra were found to be very similar to the previously published in situ data indicating that the adsorbate layer on the electrode surface does not significantly change after the removal from the electrolyte. The results showed that the reflection mode XAS is well suited for the study of adsorbate systems

Near surface structure determination using X-ray reflection absorption spectroscopy

Near surface layers structure data as r, N, σ, ΔE0 and known phases and amplitudes as well as thickness were used for the evaluation of X-ray reflection spectra. With these data, the energy dependent index of refraction n = 1 − δ − iβ for each layer can be calculated. The application of the Fresnel theory to an assumed layer structure yields the reflectivity as a function of the energy. As an example, oxidized copper surface is discussed

An Ex Situ Reflection Mode XANES Study of Underpotentially Deposited Copper Monolayers

Reflection mode XANES was performed ex situ on a copper monolayer underpotentially deposited onto a polycrystalline gold electrode. According to calculations based on reference substances, the thickness of the adsorbate layer was 0.21 nm and its oxidation state was close to +1. The ex situ spectra were found to be very similar to the previously published in situ data indicating that the adsorbate layer on the electrode surface does not significantly change after the removal from the electrolyte. The results showed that the reflection mode XAS is well suited for the study of adsorbate systems

X-ray reflectivity fine structure from homogeneous materials in the hard-energy range

To a good approximation, the X-ray reflectivity fine structure from homogeneous materials was found to be, in the hard-energy range, a linear superposition of the fine structures of the real and imaginary parts of the refractive index. As a consequence, a simple formula for the extraction of the absorption fine structure (EXAFS) from the reflectivity data is given. For a certain glancing angle, the Kramers-Kronig transform of the reflectivity fine structure is, to a constant factor, approximately equal to EXAFS. The magnitudes of the Fourier transform of the reflectivity fine structure and of EXAFS have for all glancing angles approximately the same shape

Grazing Incidence X-Ray Absorption Spectroscopy of Near Surface Regions: Possibilities and Limitations

The application of the grazing incidence X-ray absorption spectroscopy (GIXAS) to the investigations of near surface regions of layer systems is discussed. Starting with transmission standards, GIXAS spectra for different detection modes were calculated as functions of the grazing angle, layer composition and surface/interface roughness. For systems consisting of only one layer that participates to the XAFS signal, the fine structructure is a linear superposition of the fine structures in the real and the imaginary part of the energy dependent refractive index. The surface roughness iess than about 2 nm does not significantely reduce the surface sensitivity of the method. An example of near surface structure determination for an oxidized metal surface is given

Reflection XAFS from a Au/In composite film

An Au/In bilayer was annaled in air at 350 °C. The surface layer structure of this composite film was studied with the grazing incidence X-ray absorption fine structure technique. Comparison of the specimen and standard spectra shows that indium diffuses through the gold film and forms an oxide film on the gold surface. The short range order structure of this oxide film is found to be very similar to that formed on heated metallic indium

X-ray-reflectivity fine structure and EXAFS

A simple and fast method is given for the extraction of extended x-ray-absorption fine structure from x-ray-reflectivity spectra. The method is based on the description of the x-ray-reflectivity fine structure as a linear superposition of the fine structures in the real and in the imaginary part of the refractive index. It is applicable at all glancing angles, i.e., above as well as below the critical angle. Besides the smooth part of the refractive index, no other additional information is necessary. As an example, results obtained with calculated nickel data are presented

In situ examination of anodic silver oxide films by EXAFS in the reflection mode

Thin sputter-deposited silver films were oxidized anodically in 1 M NaOH. The structural modifications of the silver electrode surface during the course of a complete oxidation/reduction cycle were studied in situ with the grazing incidence X-ray absorption fine structure technique. The results show that the short-range order structure of the anodic oxide films is similar to that of crystalline Ag2O powder at 0.6 V while it resembles the structure of crystalline AgO at 0.85 V (SHE). Besides a significantly reduced reflectivity, the metal surface revealed a more disordered near-order structure after a completed oxidation/reduction cycle

An X-ray absorption fine structure study of the initial stages of the anodic oxidation of silver

The grazing incidence X-ray absorption fine structure technique was used for the in situ investigation of the initial stages of the anodic oxidation of silver electrodes in 1M NaOH. The reflectivity fine structure of the oxide covered electrode was measured for several potentials corresponding to different oxide thicknesses. The comparison of the experimental data with those of model calculations enabled the extraction of short-range order structural information about the anodic overlayer. Considerable changes of the reflectivity fine structure data are observed for silver(I) oxides formed at different potentials. In addition, the potentiostatic oxide layer formation as well as the electrochemical reduction causes significant changes of the X-ray reflectivity of the silver working electrode. Even the reduction of thin oxide layers of about 2–5 nm thickness causes a microroughening of the electrode surface