EXAFS et EXAFS of surface

EXAFS (Extended X-ray Absorption Fine Structure) is a widely used technique for the structural characterization of bulk materials and thin films. The experiment consists in measuring the X-ray absorption coefficient of a mate rial as a function of the photon energy: When the photon energy reaches the binding energy of an electronic level of one of the constituting elements of the sample, the absorption coefficient abruptly increases (absorption th reshold) and then oscillates. These oscillations (EXAFS oscillations) are d ue to the scattering of the photoelectron (created by the photon absorption ) wave function by the surrounding neighbours of the excited atom. The scat tered part of the wave can return to the absorbing atom to interfere with t he outgoing wave. When changing the photons energy these interferences are either constructive or destructive, thus generating the oscillations of the absorption coefficient. The EXAFS is therefore directly correlated to the structure and the local order around the excited atom. EXAFS is therefore a n element-selective technique. It is a very local probe, which allows to st udy disordered or amorphous samples. In this paper, the classical EXAFS formula is derived for K-edges, with lin early polarized light, taking into account only one single scattering of th e wave function (single scattering approximation). In this demonstration, i t clearly appears that, for non-isotropic structures, the absorption spectr a depend on the polarization direction of the light, with respect to the cr ystallographic axes of the sample. This effect can be used to measure a pos sible distortion of the sample structure, e.g. in thin films epitaxied on s ingle crystal substrates. The spectra analysis procedure uses Fourier trans form to separate the contributions of the different neighbours shells to th e EXAFS signal. With the classical EXAFS formula, one can easily extract th e crystallographic parameters of the first nearest neighbour shell from the experimental data (number and type of first nearest neighbours, first near est neighbour distance, and width of the radial distribution function). The single scattering formula is no longer correct for the interpretation o f the distant shells contributions. It requires a multiple scattering analy sis of the spectra. In this formalism, all the possible scattering paths of the wave function are taken into account. Using computing codes, one can e xactly calculate the absorption coefficient near the threshold (XANES :X-ra y Absorption Near Edge Structure). The calculation is highly simplified by fixing a maximum number of scatterings for the photoelectron wave function. In this approximation, codes allow a complete calculation of the absorptio n spectra, both in the XANES and the EXAFS region. One of the main attractions of X-ray absorption techniques is their chemica l selectivity. One can choose the element whose environment is being probe by measuring the absorption spectra at a selected edge. This chemical selec tivity can be used for the study of adsorbates or thin films, deposited on a substrate. The spectra are recorded at one adsorbate's edge; the EXAFS os cillations then only probe the environment of the deposited atoms. This kin d of study requires suited detection modes. Once an atom has absorbed a pho ton, two decay processes are possible : the excited atom emits either an Au ger electron or a fluorescence photon. The intensities of these decay proce sses, as well as the total number of electrons emitted by the sample (total yield), are proportional to the absorption coefficient. Total yield procur es a quite good sensitivity to the surface and high count rates, whereas fl uorescence yield is more selective.