Spectroscopy x-ray emission

X-ray emission can be induced in different ways. We have already mentioned the use of energetic electrons. Alternatively, heavier charged particles can be used. It is also possible to create an inner shell vacancy by irradiating the sample with X-ray radiation. We are then deahng with an iimer shell photoelectric effect. The chaiacteristic radiation following X-ray absorption is referred to as X-ray fluorescence following the terminology in the optical 

NaCl d = 0.56 mn) or LiF d = 0.4nm) are frequently used as crystals. The X-rays bundles are collimated using systems of thin metal plates (tliickness 50jj.m) arranged parallel to each other at small separations (0.5mm). In this way the divergence can be limited to one degree or less. 

An example of an X-ray fluorescence spectrum of an alloy sample is shown in Fig. 5.5. 

A Geiger counter, a proportional counter or a scintillation counter can be used for X-ray detection. The two former t5T es are gas-filled. The incoming X-rays cause the formation of ions which are then detected. Such detectors are mostly used for long-wavelength radiation (A 0.2mu). For X-rays of shorter wavelengths scintillation counters are used, in winch X-ray-induced light flashes in sodium iodide crystals are detected by a photomultiplier tube (Sect. 6.3). Imaging X-ray detectors am also available as discussed below in 

Fluorescence spectrometers are widely used in the metal industry. Frequently, parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard sample that is contained in the sample holder. In this way the intensity of the X-ray tube can be monitored. Frequently, a measurement is terminated when a preset number of comits for the reference sample has been obtained. The corresponding number of counts from the other detectors can then be directly used for a pai allel assessment of the elemental composition of the sample. With a sequential spectrometer, a number of selected elements are measured sequentially by turning the crystal and the detector to preset positions. With computer steering the measurement process is automatic. This type of instrument is well suited for varying types of analysis, whereas parallel spectrometers are more suited to continuous control operation of, for example, a steel mill in near-real time. 

In x-ray emission spectroscopy, the energy lost by relaxation of an outer electron (binding energy E,) into a hole created in a more tightly bound orbital (binding energy , ) is emitted as an x-ray photon of energy hv. Thus the frequency v of the emitted x-radiation is  

X-ray emission spectra are subject to an electric-dipole selection rule that requires that the orbital angular momentum quantum number (/) changes only one unit during the transition hence  

Permitted x-rays ( diagram lines ) will only be generated by the transitions s p, p r s or d, d p or f,f -d or g. Although this is essentially an atomic selection rule, it can be applied to transitions involving molecular orbitals in the valence band. Thus, x-ray spectra can provide valuable information about the atomic contributions to molecular orbitals. X-ray emission peaks are classified according to the orbital (or shell —K, L, M, etc.) in which the initial vacancy was created, a familiar system summarized in Fig. 2.7. 

for example, the final-state molecular orbital is composed of s and p orbitals from atoms X and Y, and the initial vacancy is an s orbital on atom X (written Xs to distinguish it from other s orbitals on X), then the above integral becomes  

The first term corresponds to a forbidden transition s - s on atom X and is hence zero. The second term is an allowed transition on X, and the third and fourth terms are crossover transitions from orbitals on Y to the vacancy on X. Examination of the magnitudes of valence orbitals of in the region of shows that they will be very small, so that the third and fourth terms above can be ignored. Thus we have  

Fluorescence spectrometers are widely used in the metal industry. Frequently parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard 

It might be expected that the intensity of a spectral line from a sample would be directly proportional to the amount of the corresponding element in the sample. In practice the intensity can deviate considerably from the expected linear relation due to absorption in the matrix material and multiple scattering processes. However, it is possible to correct for such effects and very reliable quantitative analyses can be performed. X-ray fluorescence measurements on alloys have an elemental sensitivity of about 10 ppm (ppm parts per million, 1 10 ). The typical penetration depth of the radiation in the metal is about 1 /zm and thus, primarily, the surface is analysed. X-ray emission techniques have been discussed in [5.3,4]. 

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THE ELEMENT S FORMS OF PRESENCE DETERMINATION BY X-RAY EMISSION SPECTROSCOPY... 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Ion beam spectrochemical analysis Auger emission spectroscopy Scanning electron microscopy (SEM) Electron microprobe (EMPA) Particle-induced X-ray emission spectroscopy (PIXE)... 

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

Analytical electron microscopy by x-ray emission spectroscopy can be extremely useful as a qualitative analysis tool, e.g. to determine which elements are present in lOnm diameter areas of the specimen. However, the greatest impact of AEM comes from quantitative chemical profiles across minute regions or features in the specimen, information that usually cannot be obtained by other means. 

Bismuth Molybdates. Bismuth molybdates are used as selective oxidation catalysts. Several phases containing Bi and/or Mo may be mixed together to obtain desired catalytic properties. While selected area electron diffraction patterns can identify individual crystalline particles, diffraction techniques usually require considerable time for developing film and analyzing patterns. X-ray emission spectroscopy in the AEM can identify individual phases containing two detectable elements within a few minutes while the operator is at the microscope. 

Recently, methods for quantitatively determining the chemical element composition of solid materials by x-ray emission spectroscopy using the electron microprobe have become available. A significant advantage of the electron microprobe, compared with methods for bulk analysis. Is Its capability for rapid analysis of many different mlcron-slze areas of a solid sample. Thus, In a relatively short time, we can obtain several hundred quantitative analyses of the chemical element content of a solid sample. These analyses usually will be different because sample homogeneity Is absent on the micron level. Thus, each chemical analysis Is a linear sum of the chemical elements In the subset of minerals present at that location. Generally, we expect the number of minerals present In a mlcron-slze spot to be less than the total number of minerals In the bulk sample. 

Examples of conventional instrumentation used for electron-excited X-ray emission spectroscopy and Auger electron spectrometry are shown in Figures 2 and 3 respectively. Details concerning the instrumentation may be found elsewhere (25-29). 

In the following, we will discuss a number of different adsorption systems that have been studied in particular using X-ray emission spectroscopy and valence band photoelectron spectroscopy coupled with DFT calculations. The systems are presented with a goal to obtain an overview of different interactions of adsorbates on surfaces. The main focus will be on bonding to transition metal surfaces, which is of relevance in many different applications in catalysis and electrochemistry. We have classified the interactions into five different groups with decreasing adsorption bond strength (1) radical chemisorption with a broken electron pair that is directly accessible for bond formation (2) interactions with unsaturated it electrons in diatomic molecules (3) interactions with unsaturated it electrons in hydrocarbons ... 

Since the mid-1960s, a variety of analytical chemistry techniques have been used to characterize obsidian sources and artifacts for provenance research (4, 32-36). The most common of these methods include optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), particle-induced X-ray emission spectroscopy (PIXE), inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray fluorescence spectroscopy (XRF), and neutron activation analysis (NAA). When selecting a method of analysis for obsidian, one must consider accuracy, precision, cost, promptness of results, existence of comparative data, and availability. Most of the above-mentioned techniques are capable of determining a number of elements, but some of the methods are more labor-intensive, more destructive, and less precise than others. The two methods with the longest and most successful histoty of success for obsidian provenance research are XRF and NAA. 

Species distribution studies have shown that trace element (e.g. metals) concentrations in soils and sediments vary with physical location (e.g. depth below bed surface) and with particle size. In these speciation studies the total element content of each fraction was determined using a suitable trace element procedure, for example, solid sample analysis by X-ray emission spectroscopy or neutron activation analysis, or alternatively by dissolution of sample and analysis by ICPOES, AAS or ASV. The type of sample fraction analysed can vary, and a few... 

Lancaster KM, Roemelt M, Ettenhuber P, et al. X-ray emission spectroscopy evidences a central carbon in the nitrogenase iron-molybdenum cofactor. Science. 2011 334 974-7. 

This paper describes chemical analyses at points across individual zeolite crystals in the size range 0.1-2.0pm. The technique employed was x-ray emission spectroscopy in the scanning transmission electron microscope (STEM). Two ZSM-5 preparations were made with Si Al ratios about 10 and 40. Many particles were examined carefully to detect chemical segregation. To check the analysis procedure, particles of NaA zeolite were examined as a control. 

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