Grazing incidence x-ray diffraction experiment

As a companion to this study. X-ray reflectivity (XR) and grazing-incidence X-ray diffraction experiments on similar lipid... 

Figure 24-8. Phase diagram deduced from 0-2 9 and grazing incidence X-ray diffraction experiments. This shows the domain of existence of the three micellar phases as a function of the CiiTABr/Si molar ratio and the aging time of the initial silica sol at40°C. (Besson, S., Gacoin, T., Ricolleau, C.,Jacquiod, C. and Boilot, J.-P. J. Mater. Chem. 2003 13 404-09, Reproduced by permission of The Royal Society of Chemistry)...

A quantitative comparison of our results with experiments should be possible for Van der Waals (non-polar) liquids such as PDMS (silicone oil) spreaded on various preprepared rough surfaces like etched glass, fused silica and mica. In a grazing incidence X-ray diffraction experiment the ratio of intensities scattered from the liquid and solid surfaces lL q)/Is q) is proportional to the ratio of the -component of the mean-squared height fluctuations of the two surfaces, (Ci( ))/(Cs( ))... 

The following results relative to Pd growth on Ni(llO) are the fmit of experiments combining many surface science techniques LEED, STM, AES, XPS, LEIS. It has been also the subject of recent investigations by grazing incidence X-ray diffraction, using synchrotron radiation facilities at ESRF (Grenoble), and by DFT theoretical calculations. 

Unique information about the unit cell in quasi-crystaUine monolayers can be obtained from X-ray °, neutron , heUum or low energy electron diffraction (LEED) data. In the grazing incidence X-ray diffraction (GIXD) experiment the beam is directed at the coated surface at a low angle and experiences total internal reflection from the metal support underneath the monolayer. The analysis of reflectivity and diffraction pattern of this reflected beam provides information about the molecular structure of the crystalline films, the thickness and refractive index of the layers and the roughness of the surface s . These experiments, however, require sophisticated and expensive equipment and are not therefore used routinely for monolayer characterization. 

In either case, however, the reflectivity is an average over the entire surface in the X-ray footprint area. By contrast, a grazing-incidence X-ray diffraction (GIXD) experiment will probe only domains with 2D-crystalline order (and for each Bragg rod there will be contributions only from those few domains of the 2D powder which are azimuthally oriented to diffract) while the rest of the sample will contribute to the background intensity only. In this respect, and in the nature of the structural information that they contribute, GIXD and XR are complementary. 

Let ns come back to water. At interfaces, Collins concept as well as what was written before on hydrophobic surfaces seem to be good approaches. In total, it turns out that specific ion effects are more pronounced at interfaces compared to bulk properties. Whereas powerful experimental techniques exist for several years for the characterisation of the molecular distribution of ions in water, see Chap. 6, experiments of comparable precision for interfacial ion specific properties only emerged in the last years, especially for the solution-air interfaces. The present state of the art of the most relevant of these techniques is summarised in Chaps. 4 and 5. Second harmonics and sum-frequency generation, and grazing incidence X-ray diffraction will more and more provide us with quantitative details of specific ion adsorption (and their concentrations) at interfaces and their profiles perpendicular to the surface. They help us to check the validity of molecular dynamics simulation results and will soon be the new experimental reference standards for all kinds of models and simulations. However, as always with newly emerging techniques, there are still some ambiguities in the interpretation of the measured signals. 

Illuminating the sample at grazing angles. The penetration depth of photons depends on the cosine of the incidence angle and, therefore, can be reduced by this procedure. Although such an approach has limited use, it has been successfully employed in a few instances, such as for x-ray diffraction experiments. 

Grazing incidence X-ray diflftaction (GIXRD) experiments were carried out at room temperature using a Seifert URD-6 diffractometer equipped with a DSA 6 attachment. CuKa radiation was used at 30 kV and 10 mA. Monochromatization of the beam was obtained by means of a nickel filter and a pulse-height analyzer. The angle of incidence was fixed to 0.5°, so that the X-rays penetration into the sample could be kept constant during measurements. Diffraction scans were collected for 20 values from 2° to 60° with a step of 0.1°. 

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