Width, x-ray lines

X-ray line-width measurements, while a Rich Seifert Debyeflex unit was used for the other powder patterns. A North American Philips back-reflection camera was employed for the precision determination of lattice constants. The uncertainty in the lattice constants varies between 0-001 and 0-0025 A. Anatase samples were heated in quartz tubes placed in a vertical tubular furnace fitted with a thermo-regulator. The fraction of anatase in a mixture was determined by the equation,... 

The initial anneal to 350° showed a very marked narrowing of the half width of the line. Further heat treatment to 550° C. showed a small decrease in half width. Above this temperature, where crystal growth sets in, the two became indistinguishable, indicating that crystal growth was the most important factor in the determination of the x-ray line width when heated above this temperature. 

All spectra were obtained under the same conditions Data were taken with the specimen normal at 80° to the axis of the analyzer input lens (grazing electron exit angle). The analyzed spot on the specimen was, then, 6 mm by 5 mm. The acceptance angle of the input lens was 6°. The analyzer was a Vacuum Generators ESCALab Mk II operated with 0.5 eV resolution. Data were taken at 0.1 eV steps. The actual width of the Cls line from virgin PTFE was 1.6 eV as a result of the x-ray line width and differential charging of the specimen. 

X-ray PES or XPS has been used to study molecular core levels, and information from the core electron ionization can be used to evaluate orbital overlap and charge potential contributions to electron binding energies of valence orbitals. XPS resolution is lower than that of UPS due to broader X-ray photon line widths and shorter lifetimes of core electrons. The most commonly used X-ray sources involve MgATo (1,253.6 eV) and ALAa (l,486.6eV). In XPS, a monochromator is used to remove the background continua and unwanted lines from the X-ray sources to increase the X-ray purity and reduce the X-ray line width. A line width of about 0.2 eV has been achieved for the MgKa and AlATo sources. The electron analyzer of an XPS spectrometer can be similar to that in a UPS spectrometer. Therefore, UPS and XPS spectra can be taken on the same instrument if both sources are available. 

Eig. 15. Micro densitometer tracings of x-ray line exposures with line widths of (a) 10 and (b) 1000 p.m. 

Table 2.2 lists the energies and line-widths of the characteristic X-ray lines from a few possible candidate materials. In practice Mg Ka and A1 Ka are the two used universally because of their line energy and width and their simple use as anode material. 

Tab. 2.2. Energies and line-widths of some characteristic low-energy X-ray lines.

There are several considerations that go into selecting an X-ray line to excite XPS spectra. Included are the energy of the X-rays and the width of the line. If the energy is too low, the number of photoelectron lines that will be excited will be too small for general use. If the line width is too large, the resolution in the XPS spectrum will also be too small. Therefore, it is useful to consider the processes involved in X-ray generation. 

Another consideration is the natural line width and satellite structure of the x-ray line used. Titanium (TiKa=4510.9 eV) has seen limited use (12) for non-destructive depth profiling, but the observed spectra are complicated by the TiKa satellite structure and the large natural line width of 2.0 eV (13). 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

Finally, the width of x-ray lines can be used as a measure of the degree... 

Work on the structure of crystals and fibers was not the only way in which Mark made use of x-rays. With several collaborators, he reported the results of a number of significant investigations of the physics of x-rays in 1926 and 1927. With Ehrenberg he reported studies of the index of refraction of x-rays, and with Leo Szilard studies verifying the linear polarization of x-rays scattered from electrons at 90. An investigation of the width of x-ray lines was carried out by Mark and Ehrenberg, and Mark and Kallmann reported work on the properties of Compton-scattered x-radiation and on the theory of the dispersion and scattering of x-rays. 

As we have seen in Chapter 1, we need something near a plane wave in order to see the finest details of the specimen stracture. A single-axis diffractometer utilises a beam that is very far from a plane wave. Thus, single-crystal rocking curves are broadened due to the beam divergence, and the spectral width of the characteristic X-ray lines. 

TEM and differential X-ray line broadening (expressed by the ratio of the width at half height of the 104 relative to that of the 110 reflection) indicate that the thickness of the platy Al-hematite crystals decreases as Al/(Fe-t Al) increases (Schwertmann et al., 1977 Barron et al., 1984). It is this change in morphology, rather than the structural Al, which governs the IR spectra, in particular the shape factor and the absorp-... 

Xps requires a source that can provide a single x-ray line reasonably narrow in energy. The absolute energy requirement for this x-ray line is that it must be energetic enough to generate photoelectrons from core levels of a majority of the elements with reasonable resolution. Of the many possible x-ray sources, those which best meet this requirement are the A1 Ka x-ray line at 1486.6 eV and the Mg Ka T) x-ray line at 1253.6 eV. These lines have full-widths-at-half-maxima (FWHM) that are ca 0.85 and 0.7 eV, respectively. 

FIGURE 18 Average crystallite size measurement by X-ray line broadening. The width of characteristic X-ray lines decreases markedly as cerium dioxide powder is sintered. The crystallites grow from an initial size of 50 to 400 A after heating in air for several hours. 

X-ray photoelectron spectroscopy (XPS) uses X-rays generated by allowing an intense beam of electrons to strike an appropriate target material. The inherent width of X-ray lines is greater than the line sources of ultraviolet photoelectron spectroscopy (UPS), but diffraction from a spherically bent crystal can reduce the line width to around 0.25 eV. 

Fig. 18 The width of X-ray lines as a function of particle dimension. (From Ref. l)...

This method yields about 2.5 g of feroxyhyte with broadened X-ray lines (Fig. 7-1) whose widths follow the order 100, 110 < 101 < 102 indicating... 

X-ray diffraction patterns of the lead-substituted lead ruthenates support the conclusion that they are pyrochlores of cubic symmetry. It must be noted, however, that since these materials were prepared at relatively low temperatures, the peaks in their x-ray spectra were broadened. X-ray line broadening was significant [half-height peak width at 50-60 (20) was equal to 0.5-0.8 (20)] thus any subtle evidence of distortion to lower symmetries would be difficult to... 

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