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X-ray emission and absorption techniques

X-ray absorption fine structure (XAFS) spectroscopy is the measurement of the effective linear mass absorption coefficient of a material as a function of excitation energy around a specific absorption edge. With instrumentation enabling highly resolved measurements modulations in the edge structure can be observed, which are caused by the chemical environment of the element of interest. Two kinds of analysis have to be distinguished: X-ray absorption near edge spectroscopy (XANES or NEXAFS - near edge X-ray absorption fine structure) and Extended X-ray absorption fine structure (EXAFS).

X-ray fluorescence spectroscopy (XRF)

X-ray fluorescence spectroscopy (XRF) enables non-destructive elemental analysis for a broad variety of samples and elements (Na - U). Due to the profound knowledge of the fundamental interactions of X-rays with matter, quantitative interpretation is possible and can be used used for the analysis of the composition as well as for example the layer thicknesses of unknown samples.

·        Micro-XRF and confocal techniques

Due to the improvement of X-ray optics and new quantification procedures Micro-XRF which uses a polycapillary lens in the excitation channel has recently developed into a well established, non-destructive analytical method for the investigation of elemental distribution in a sample.

When using a second optic in the detection channel in a confocal arrangement three-dimensionally resolved analysis is rendered feasible. We have developed and used 3D Micro-XRF as well as 3D Micro-XANES in different application fields in the past years. The lateral and depth resolution of these techniques lie in the range of 10 – 50 µm depending on the used optics.

·        Grazing incidence and grazing exit  X-ray fluorescence spectroscopy (GI-/ GEXRF)

In a GIXRF geometry the penetration depth of the incoming beam and hence the information depth is tuned by varying the angle of incidence of an excitation beam with low divergence. The element specific fluorescence radiation from the specimen is detected by an energy dispersive solid state detector. Hence it is also possible to gain the depth distribution of the elements of f.e. a layered system. Here, the depth resolution lies in the nm- to µm-regime depending on the used energy region. Similarly, depth profiles are obtained with GEXRF by varying the exit beam of the fluorescence radiation and using a detection system with high angle resolution.

If combined GI- or GEXRF with NEXAFS, information about the chemical state of an element of interest in a certain depth becomes accessible.

·        Total reflection and standing waves

If the surface of the specimen has a low roughness (rms) of just a few nanometres it is possible to gain total reflection if the angle of incidence is below the critical angle of external total reflection. In this case the incident beam does not penetrate the specimen. On the other hand a standing wave field is formed above the surface by the interference of the incoming and the reflected beam. If tuning the angle over the critical angle, the standing wave field penetrates the surface, above the surface it gets less pronounced because of the then only partially reflected beam.

·        X-ray emission spectroscopy (XES)

The energies of X-ray emission lines are sensitive to the chemical environment of the probed atom. When detecting these lines with high energy resolution, this chemical shift can be used to probe the chemical state of specific atoms.

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