Synchrotron sources

Tonner et al. have taken scanning XPS microscopies at the Advanced Light Source Synchrotron Radiation Center of Lawrence Berkeley National Laboratory [2.6]. They investigated a polished and sputter-cleaned surface of mineral ilmenite with the nominal composition FeTi03, and used the Fe 3p and Ti 3p lines for imaging. Using synchrotron radiation they demonstrated spatial resolution of approximately 0.25 p,m. 

The progress achieved is closely linked to the development of both powerful detectors and brilliant X-ray sources (synchrotron radiation, rotating anode). Such point-focus equipment has replaced older slit-focus equipment (Kratky camera, Rigaku-Denki camera) in many laboratories, and the next step of instrumental progress is already discernible. With the X-ray free electron laser (XFEL) it will become possible to study very fast processes like the structure relaxation of elastomers after the removal of mechanical load. 

The XAS spectrometer is similar to a UV-visible system in that it consists of a source, a monochromator, and a detector. The most favorable XAS source, synchrotron radiation, is tunable to different wavelengths of desirable high intensity. A laboratory instrument for analysis of solids and concentrated solutions may use a rotating anode source (further described in Section 3.3). The monochromator for X-ray radiation usually consists of silicon single crystals. The crystals can be rotated so that the wavelength ( i) of the X-rays produced depends of the angle of incidence (0) with a Bragg lattice plane of... 

Though cross section variation can be studied to a very limited extent using line sources, synchrotron radiation is far superior for investigations of this type. Synchrotron studies are used extensively on transition metal solids for deconvoluting valence bands into partial densities of states from the contributing AO. Studies on transition metal molecules have demonstrated the power of the technique for band assignment and the investigation of covalency. 

The X-rays enter the analysis chamber typically through a thin window made of 50-100nm thick membranes of Al, Si, or SijN4. Such windows have high transmission (e.g., the transmission of the 100 nm Al window varies from 70% to 90% for photon energies between 400 and 1000 eV) and prevent gases from entering the X-ray source (synchrotron beamline or anode), which needs to be operated in vacuum. The window must be placed as close as possible to the sample to minimize the absorption of X-rays by the gas phase inside the reaction chamber. 

Continuum microscope (Thermo Fischer) Source = synchrotron Spotlight microscope (Perkin Elmer) Source = globar... 

Useful X-ray photon beams are provided by classical sealed X-ray tubes, rotating anode generators and also high energy electron storage rings, named synchrotron sources. Synchrotron sources are in many cases preferred because they provide powerfiil, continuously tunable and well collimated X-ray beams that are particularly adequate for SAXS experiments. 

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