X-ray monochromatic

X-ray spectrometer An apparatus used in the X-ray study of crystals in which a fine beam of monochromatic X-rays impinges at a measured angle on the face of a crystal mounted in its path, and in which the intensity of the X-rays diffracted in various directions by the crystal is measured with an ionization chamber mounted on an arm of the spectrometer table, or is recorded photographically. 

A much better way would be to use phase contrast, rather than attenuation contrast, since the phase change, due to changes in index of refraction, can be up to 1000 times larger than the change in amplitude. However, phase contrast techniques require the disposal of monochromatic X-ray sources, such as synchrotrons, combined with special optics, such as double crystal monochromatics and interferometers [2]. Recently [3] it has been shown that one can also obtain phase contrast by using a polychromatic X-ray source provided the source size and detector resolution are small enough to maintain sufficient spatial coherence. 

Figure 8.17 A short, barely resolved, vibrational progression in the v- vibration of CHj in the carbon Is X-ray photoelecton spectrum of methane obtained with a monochromatized X-ray source. (Reproduced, with permission, from Gelius, U., Svensson, S., Siegbahn, H., Basilier, E., Faxalv, A. and Siegbahn, K., Chem. Phys. Lett, 28, 1, 1974)...

If monochromatic X-rays are used as the ionizing radiation the experimental technique is very similar to that for XPS (Section 8.1.1) except that it is the kinetic energy of the Auger electrons which is to be measured. Alternatively, a monochromatic electron beam may be used to eject an electron. The energy E of an electron in such a beam is given by... 

Step 2. The computer opens a shutter, bathing the crystal in a monochromatic x-ray beam. The computer rotates the crystal for about one minute and the rotation diffraction image is stored on the detector and then read into the computer memory. When the operator examines the image and is confident that the sample is indeed a single crystal, the experiment can proceed. 

For a narrow, parallel monochromatic x-ray beam incident perpendicularly upon an absorber of unit area, and of uniform thickness and composition as shown in Figure 1-4, Beer s Law has been found to hold, and may be written... 

Fig. 1-5. Transmittance as a function of the thickness of absorber. The two curves for the transmittance of monochromatic x-rays are pure exponentials. The curve for the transmittance of mixed x-rays is the sum of two exponential curves. The experimental arrangement is shown in Fig. 1-4.

Equation 1-5 was written for a sample containing a single element upon which monochromatic x-rays are incident. In so far as x-ray absorption is an atomic property, the mass absorption coefficients for other samples are additive functions of the weight-fractions of the elements, free or combined, that are present that is,... 

Equation 1-7 may be considered always valid for absorptiometry with monochromatic x-rays, and usually valid for absorptiometry with polychromatic x-rays also. Difficulties sometimes arise with polychromatic beams. 

The setup in Figure 1-7 becomes an effective generator of nearly monochromatic x-rays when various elements inserted in the sample position are irradiated by x-rays of energy sufficient to excite the characteristic spectra that is, when by x-ray excitation the characteristic... 

The preceding quotation serves admirably as a brief historical introduction to x-ray diffraction. This field of research has proved enormously fruitful in yielding information about crystal structure, and in providing a means of obtaining monochromatic x-rays and of measuring their wavelength. The determination of crystal structure, though important to analytical chemistry, is outside the scope of this book.31... 

We say then that a crystal is satisfactory for purposes of chemical analysis if the beam it reflects is monochromatic within the limits established by the collimating system. As theory shows,15 some broadening is to be expected on Bragg reflection even from perfect crystals, but this broadening is so small (not over 0.001°) that we need not consider it. Relatively few crystals, notably some diamonds and calcites, approach perfection. Sodium chloride, more useful in x-ray spectrog-raphy, broadens monochromatic x-rays appreciably, but the. total broadening can be smaller than 0.30°,16 the collimator a perture. See Figure 4-9. 

For the purposes of analytical chemistry, four kinds of monochromatic beams need to be considered. (The quotation marks are to remind the reader that the beams under discussion are not always truly monochromatic.) Three kinds of beams—those produced by Bragg reflection (4.9), filtered beams (4.6), beams in which characteristic lines predominate over a background that can be neglected— will be discussed later ( 6.2). The fourth kind of beam contains monochromatic x-rays that are a by-product of our atomic age and that promise to grow in importance they are given off by radioactive isotopes. These x-rays must not be confused with 7-rays (11.1), which also originate from radioactive atoms but which differ from x-rays because the transformation that leads to radiation involves the nucleus. 

As indicated above, absorption effects can generally be predicted and satisfactorily estimated when only monochromatic x-rays are involved. When a polychromatic beam is used for excitation, the filtering of the beam by the sample complicates the situation and modifies the absorption effects. Even then, fairly reliable estimates of the effects can be made. 

Calcium, determination by x-ray emission spectrography, 222, 328 in biological samples, determination by monochromatic x-ray absorption-edge method, 299-301 in biological sections, determination by x-ray absorption-edge method, 293, 294... 

Films on stainless steel, analysis by x-ray emission spectrography, 230 Film thickness, determination, 146-159 by attenuation of monochromatic x-rays from substrate, 149-152 by attenuation of unresolved beam from substrate, 147-149 by x-ray diffraction techniques, 147 intercomparison of three methods used in, 158... 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

Crystals produce different diffraction patterns when subjected to bombardment of monochromatic X-ray sources and thereby provide unequivocal identification of crystalline materials. 

X-ray diffraction occurs in the elastic scattering of X-ray photons by atoms in a periodic lattice. The scattered monochromatic X-rays that are in phase give constructive interference. Figure 4.4 illustrates how diffraction of X-rays by crystal planes allows one to derive lattice spacings by using the Bragg relation ... 

A crystal therefore acts as a three-dimensional diffraction grating for these x-rays, and three equations (the Laue equations) must be satisfied if there is to be constructive interference of these monochromatic x-rays. 

X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect. When a sample is irradiated with monochromatic X-rays, such as the K lines of Mg (1253.6eV) or Al (1486.6 eV), core-level electrons from the inner shells of atoms in the sample will be ejected from the sample to the surrounding vacuum. The kinetic energy, Er, of the emitted photoelectron is given by... 

ESCA analyses were performed nsing a Physical Electronics PHI 5600 ci spectrometer eqnipped with an aluminum monochromatic x-ray sonrce. Hydrogen reduction at 75°C and 150°C was performed in situ in a reaction chamber attached to the main ESCA analytical chamber. Gas mixtnre of 80/20 N2/H2 was nsed for the hydrogen rednction. Charge correction was performed shifting the C-(C,H) peak in C Is spectra to 284.8 eV. PHI MultiPak software version 6.0A was nsed for data analysis. 

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