X-ray reflection and diffraction

X-rays have a large penetration depth. In order to obtain a sufficiently intense signal from the surface layer the incident beam is applied under a very small angle (see exercise 8.4). Typical vertical irradiation angles a are 0.1°, which leads to penetration depths of 5 nm. With a wavelength of a few A (often the Cu-Ka line with A = 1.54 A, is used) the X-rays are sensitive enough to analyze monolayers. Also thicker layers can be analyzed. Widely used X-ray techniques are X-ray reflection (XR) and diffraction (XD), which provide different information on thin films [585,599,601], 

X-ray reflection (a = a, j3 = 0). The intensity of the directly reflected beam is measured for different angles a (typically up to 5°). The experiments give information about the film thickness, the electron density distribution in electrons per A3 perpendicular (normal) to the liquid surface, and the roughness of the surface [602,603], To understand this we begin by discussing how film thickness is obtained. 

If you have a thin film on top of a water surface, the incident X-ray beam is split into two beams one being reflected from the film surface, the other being reflected from the film-water 

A is the wavelength of the X-rays, n is the order of the minimum (n = 1,2,3.), and d is the film thickness. Bragg s law implies that the higher the angle at which you find a minimum or maximum the smaller is the corresponding film thickness for a given wavelength. 

X-rays interact with the electrons of the atoms in a material. Therefore, a necessary condition to resolve the additional film on the water surface is that the electron density of the film and the underlying liquid differ sufficiently. Additionally, for a = a there is no information on the horizontal component. Thus, the reflected intensity is an electron density image along the normal from air to water which is modified by a present monolayer film. 

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Ocko BM, Wang J, Davenport A, Isaacs H (1990) In situ x-ray reflectivity and diffraction studies of the Au(OOl) reconstruction in an electrochemical cell. Phys Rev Lett 65 1466... 

J. Als-Nielsen, K. Kjaer, X-ray reflectivity and diffraction studies of liquid surfaces and... 

The analysis of x-ray diffraction data is divided into three parts. The first of these is the geometrical analysis, where one measures the exact spatial distribution of x-ray reflections and uses these to compute the size and shape of a unit cell. The second phase entails a study of the intensities of the various reflections, using this information to determine the atomic distribution within the unit cell. Finally, one looks at the x-ray diagram to deduce qualitative information about the quality of the crystal or the degree of order within the solid. This latter analysis may permit the adoption of certain assumptions that may aid in the solving of the crystalline structure. 

We then performed in situ X-ray reflectivity and grazing incidence diffraction (GID) measurements to study the thermal stability of these samples. Figure 9.13 shows the reflectivity (a) with the first order DIP Bragg peak (b) for different temperature steps. 

Weygand M, Wetzer B, Pum D, Sleytr UB, Cuvilher N, Kjaer K, Howes PB, Losche M (1999) Bacterial S-layer protein coupling to Lipids X-ray reflectivity and grazing-incidence diffraction studies. 

Figure 9. Side views of the sample regions of the experimental setups used on station 7.3.3 at the ALS for combined X-ray fluorescence and diffraction in transmission and reflection mode. The white wedge to the left of each frame represents the focused beam (width not to scale) coming from the vertical-focus mirror on the left. The large black object is the diffraction detector. The cylindrical object coming in from the upper left is the Si detector used for fluorescence mapping.

See also Asbestos. Color Measurement. Forensic Sciences Thin-Layer Chromatography. Gas Chromatography Pyrolysis Mass Spectrometry Fourier Transform Infrared Spectroscopy. Microscopy Applications Forensic. Spectrophotometry Diode Array. Textiles Natural Synthetic. X-Ray Absorption and Diffraction X-Ray Diffraction - Powder. X-Ray Fluorescence and Emission X-Ray Fluorescence Theory Energy Dispersive X-Ray Fluorescence Total Reflection X-Ray Fluorescence. 

See alsa Air Analysis Outdoor Air. Cement. Ceramics. Fluorescence Quantitative Anaiysis. Fourier Transform Techniques. Infrared Spectroscopy Near-Infrared. Microscopy Techniques X-Ray Microscopy. Particie Size Anaiysis. Pharmaceuticrai Anaiysis Drug Purity Determination. Quaiitative Anaiysis. Stmcturai Eiucidation. Thermai Anaiysis Overview. X-Ray Absorption and Diffraction Overview X-Ray Absorption X-Ray Diffraction - Singie Crystai. X-Ray Fiuores-cence and Emission X-Ray Fiuorescence Theory Waveiength Dispersive X-Ray Fiuorescence Energy Dispersive X-Ray Fiuorescence Totai Reflection X-Ray Fluorescence Particle-Induced X-Ray Emission. 

Table 9.1 Relation between solution concentration and film thickness as determined by ellipsometry, X-ray reflectivity and specular X-ray diffraction (see text)...

The actual structure at a vapor-liquid interface can be probed with x-rays. Rice and co-workers [72,73,117] use x-ray reflection to determine the composition perpendicular to the surface and grazing incidence x-ray diffraction to study the transverse structure of an interface. In a study of bismuth gallium mixtures. 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

In recent years, high-resolution x-ray diffraction has become a powerful method for studying layered strnctnres, films, interfaces, and surfaces. X-ray reflectivity involves the measurement of the angnlar dependence of the intensity of the x-ray beam reflected by planar interfaces. If there are multiple interfaces, interference between the reflected x-rays at the interfaces prodnces a series of minima and maxima, which allow determination of the thickness of the film. More detailed information about the film can be obtained by fitting the reflectivity curve to a model of the electron density profile. Usually, x-ray reflectivity scans are performed with a synchrotron light source. As with ellipsometry, x-ray reflectivity provides good vertical resolution [14,20] but poor lateral resolution, which is limited by the size of the probing beam, usually several tens of micrometers. 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

FIGURE 27.9 (a) Voltammetry curve for the UPD of TI on Au(l 11) in 0.1 M HCIO4 containing ImMTlBr. Sweep rate 20mV/s. The in-plane and surface normal structural models are deduced from the surface X-ray diffraction measurements and X-ray reflectance. The empty circles are Br and the filled circles are Tl. (b) Potential-dependent diffraction intensities at the indicated positions for the three coadsorbed phases. (From Wang et al., 1998, with permission from Elsevier.)... 

As a result of compelling three-dimensional models and remarkably high levels of precision, it is often assumed that structural elucidation by single crystal X-ray diffraction is the ultimate structural proof. Spatial information in the form of several thousands of X-ray reflection intensities are used to solve the position of a few dozen atoms so that the solution of a structure by X-ray diffraction methods is highly overdetermined, with a statistically significant precision up to a few picometers. With precise atomic positions, structural parameters in the form of bond distances, bond... 

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