Extended X-ray Absorption Fine Structure Spectroscopy of Surfaces and Thin Films: Local Structure, Dynamic and Magnetic Properties
Published in 1999
Other
Undefined
158 pages
978-3-89574-341-2 (ISBN)
Description
This work reports temperature- and angular-dependent extended X-ray absorption fine structure spectroscopy (EXAFS) measurements, which were carried out on surfaces and thin 3d transition metal films. lt was possible to improve the EXAFS analysis so that electronic information from the interstitial region between atoms could be achieved in addition to the local structure and dynamics.
The reconstruction of copper and nickel single crystal surfaces upon chemisorption of carbon, nitrogen and oxygen atoms was investigated in this work. The local structure and dynamics were analyzed by surface EXAFS (SEXAFS) measurements at the K-edges of the light adsorbates. A strong anisotropy of thermal adsorbate-substrate vibrations in- and out-of-plane was found to be characteristic of the reconstructed surfaces. A measure of the reconstruction "strength" was given by analyzing this anisotropy. By describing the temperature dependence of the SEXAFS data in terms of anharmonic effective pair Potentials for the investigated bonds, the trend of a larger anharmonicity was found to correspond with the strength of the reconstruction. lt was shown that the neglect of the anisotropy of thermal vibrations and the anharmonicity of the effective pair Potentials leads to erroneous results in the SEXAFS analysis.
By investigating low frequency oscillations in the SEXAFS spectra the backscattering at interstitial charge densities, which was theoretically predicted in 1978, was identified unambiguously for the first time. The use of low-Z atoms chemisorbed on metal surfaces allowed the identification of the low frequency oscillations as truly being due to backscattering and not to multi-electron excitations. A deeper understanding of the socalled atomic EXAFS (AXAFS) was achieved by calculations using a multiple-scattering approach implemented in the ab initio FEFF code. The adaption of the program to surface systems allowed us to explain the phase of the AXAFS oscillations in terms of charge transfer from the metal atoms to the light adsorbate.
The magnetic EXAFS (MEXAFS) measurements for polycrystalline and epitaxially grown Fe and Co films were carried out at the L3,2-edges by investigating the helicity-dependent X-ray absorption coefficient. The detailed analysis of the experimental data in comparison to ab initio calculations showed that the overlap of the edges does not hinder the analysis. The comparison of the normal EXAFS with the MEXAFS made it possible to analyze the local crystallographic structure separately from the local magnetic structure. The first temperature-dependent MEXAFS investigation, carried out within the framework of this thesis, showed that the MEXAFS signal is influenced by thermal vibrations in addition to local spin fluctuations. The separation of single- and multiplescattering contributions in the ab t'nt'tio calculations revealed an enhancement of multiplescattering contributions for the MEXAFS as compared to the normal EXAFS.
The reconstruction of copper and nickel single crystal surfaces upon chemisorption of carbon, nitrogen and oxygen atoms was investigated in this work. The local structure and dynamics were analyzed by surface EXAFS (SEXAFS) measurements at the K-edges of the light adsorbates. A strong anisotropy of thermal adsorbate-substrate vibrations in- and out-of-plane was found to be characteristic of the reconstructed surfaces. A measure of the reconstruction "strength" was given by analyzing this anisotropy. By describing the temperature dependence of the SEXAFS data in terms of anharmonic effective pair Potentials for the investigated bonds, the trend of a larger anharmonicity was found to correspond with the strength of the reconstruction. lt was shown that the neglect of the anisotropy of thermal vibrations and the anharmonicity of the effective pair Potentials leads to erroneous results in the SEXAFS analysis.
By investigating low frequency oscillations in the SEXAFS spectra the backscattering at interstitial charge densities, which was theoretically predicted in 1978, was identified unambiguously for the first time. The use of low-Z atoms chemisorbed on metal surfaces allowed the identification of the low frequency oscillations as truly being due to backscattering and not to multi-electron excitations. A deeper understanding of the socalled atomic EXAFS (AXAFS) was achieved by calculations using a multiple-scattering approach implemented in the ab initio FEFF code. The adaption of the program to surface systems allowed us to explain the phase of the AXAFS oscillations in terms of charge transfer from the metal atoms to the light adsorbate.
The magnetic EXAFS (MEXAFS) measurements for polycrystalline and epitaxially grown Fe and Co films were carried out at the L3,2-edges by investigating the helicity-dependent X-ray absorption coefficient. The detailed analysis of the experimental data in comparison to ab initio calculations showed that the overlap of the edges does not hinder the analysis. The comparison of the normal EXAFS with the MEXAFS made it possible to analyze the local crystallographic structure separately from the local magnetic structure. The first temperature-dependent MEXAFS investigation, carried out within the framework of this thesis, showed that the MEXAFS signal is influenced by thermal vibrations in addition to local spin fluctuations. The separation of single- and multiplescattering contributions in the ab t'nt'tio calculations revealed an enhancement of multiplescattering contributions for the MEXAFS as compared to the normal EXAFS.