The X-ray fluorescence spectrum

X-ray fluorescence of an isolated atom results from a two-step process. 

The second step is the stabilization of the ionized atom. It corresponds to the re-emission of all, or part, of the energy acquired during excitation. Almost instantaneous (in 10 s), an electron from an outer orbit of the atom jumps in to occupy the vacancy. Since outer shell electrons are more energetic than inner shell electrons, the relocated electron has an excess of energy that is expended as an X-ray fluorescence photon. In this way, the atom returns to its ground state very quickly. 

Cascade electronic rearrangements are observed for heavy atoms, in contrast to lighter elements whose electrons are distributed over a smaller number of ground states. 

This reorganization produces photons of fluorescence. If is the energy of the initial electron and 2 the energy of the electron which comes to fill the created hole, a radiative transition ( a fluorescent photon ) will occur, with a probability between 0 and 1, and characterized by a frequency a, such that  

The interpretation given above is simplified since fluorescence is not the only process by which the atom can lose its excess energy. When a primary X-ray strikes a sample, it can be absorbed or scattered through the material. Other phenomena occur such as Rayleigh scattering (elastic scattering) and the Compton effect (inelastic scattering with release of Compton electrons). 

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Compton effect. A small fraction of the primary X-ray excitation beam is scattered in the form of radiation whose wavelength depends on the angle of observation. This radiation is superimposed on the X-ray fluorescence spectrum. The shift in angstroms between the two wavelengths (excitation and Compton) is given by ... 

In the X-ray fluorescence spectrum of tin, as in those of other elements, transitions such as 3d l.s and 4<7 l.v, which are forbidden by the selection rules, may be observed very weakly due to perturbations by neighbouring atoms. 

The X-ray fluorescence spectrum is then registered on a photographic film or by Geiger, proportional or scintillation counters, semiconductor detectors, etc. 

When an element is irradiated by a beam of X-rays or electrons, a secondary X-ray radiation is emitted. The X-ray spectral fines of any element arise from transitions between energy levels in the inner electron shells of the atoms, and their frequencies are characteristic of the element concerned. The X-ray fluorescent spectrum of a sample can be used for quantitative chemical analysis, since the frequencies of secondary emissions are largely independent of the state of chemical combination of the element concerned and their intensities are proportional to the quantity of the element present. 

The example of a X emission X-ray fluorescence spectrum of a solid sample of tin, shown in Figure 8.30, shows four prominent transitions. The method of labelling the transitions is, unfortunately, non-systematic but those in the lower-energy group are labelled a and the... 

Collection of a spectrum relies on the ability of the detector to recognise the energy of each of the emitted photons. This particular detector, installed close to the sample to collect part of the X-ray fluorescence, is either a proportional gas counter (neon)... 

Figure 15. Fluorescence-detected (m situ) x-ray absorption spectrum for an underpotentially deposited (UPD) monolayer of copper on a gold (111) electrode with the plane of polarization of the x-ray beam being perpendicular (A) or parallel (B) to the plane of the electrode. Inset Edge region of the x-ray absorption spectrum for a copper UPD monolayer before (C) and after (D) stripping. (From Abruna, H. D., White, J. H., et al., J. Phys. Chem. 92, 7045 (1988), with permission.)...

Figure 12.6 Radioactive source Fe. Emission spectrum obtained by placing a source in the sample compartment of an energy dispersive spectrometer. The signal corresponds to the X-ray fluorescence of Mn, that is the nucleus arising from Fe decomposition. On this spectrum, the resolution at 5.9 keV, measured at mid-height (FWHM) is around 150 eV.

Figure 36. Lead levels in bone can be measured in vivo using XRF spectroscopy, (a) y-rays or X-rays are used (source) to eject either L-shell electrons (L-XRF) or K-shell electrons (K-XRF) from lead in bone when outer-shell electrons fill this vacancy, photons are released (fluorescence) and are monitored by the detector (10, 523). A typical X-ray fluorescence spectrum [(b), e.g., of a 112 pg Pbg phantom ) provides the number of counts observed as a function of photon energy. Emissions characteristic of lead occur at 72.8 keV (PbKa2), 75.0 keV (PbKoti), and 84.9 keV (Pb Kpi) (436, 523). Measurements on actual samples are correlated with those obtained from standard phantoms made of plaster-of-paris and doped with a known amount of lead to obtain bone lead concentrations in micrograms of Pb per gram (pg Pbg bone). The bone lead levels obtained by this method correlate extremely well with independent measurements of BLL (c). [Parts (a) and (c) adapted from (524). Part ( ) adapted from (436).]...

The X-ray absorption spectrum can be detected by monitoring the X-ray absorption directly, or by monitoring the X-ray fluorescence, or by monitoring electron yield. For dilute biological specimens the X-ray fluorescence provides by far the best sensitivity, and essentially all modern measurements of metalloproteins employ X-ray fluorescence detection of some kind. 

The greatest disadvantage of the scheme is the difficulty it causes in obtaining broadband excitation. Even if the x-ray tube can be mechanically moved to irradiate the specimen directly, its output is too intense to be used in direct excitation. Most high-power tubes are unable to operate stably in the microampere (/xA) current range that is required for direct excitation with energy-dispersive spectrometers mounted close to the specimen. The alternative is to replace the secondary fluorescer with an efficient scatterer such as carbon or some form of hydrocarbon. This scatters the x-ray tube spectrum onto the specimen. Unfortu-... 

The x-ray fluorescence spectrometer consists of three main parts the excitation source, the specimen presentation apparatus, and the x-ray spectrometer. The function of the excitation source is to excite the characteristic x-rays in the specimen via the x-ray fluorescence process. The specimen presentation apparatus holds the specimen in a precisely defined position during analysis and provides for introduction and removal of the specimen from the excitation position. The x-ray spectrometer is responsible for separating and counting the x-rays of various wavelengths or energies emitted by the specimen. In this book the term x-ray spectrometer denotes the collection of components used to disperse, detect, count, and display the spectrum of x-ray photons emitted by the specimen. When referring to the entire instrument, including excitation source, sample presentation apparatus, and x-ray spectrometer, the term x-ray fluorescence spectrometer will be used. In this latter sense the term x-ray fluorescence analyzer is sometimes encountered in the literature. 

The x-ray emission spectrum is obtained by bombardment of the sample with electrons (or other x-rays) with sufficient energy to excite the characteristic x-ray emission from the P atom. When the excitation is by higher energy x-rays, the technique is usually known as x-ray fluorescence analysis (XRF). Identification is specific, and the method requires only small quantities of sample which are recoverable. 

The photoelectric effect is followed by the deexcitation of the atom via X-ray transitions or by the emission of a secondary electron (Auger effect, see later in Sect. 8.5.2). As above the K edge absorption dominates, one can collect a complete X-ray spectrum from the subsequent electron transitions. The energy and intensity of the X-rays are characteristic of the composition of the emitter. Thus, y sources can be used to induce photoelectric effect and subsequent X-ray transitions in order to make qualitative and quantitative analyses of samples, which is called the X-ray fluorescence method. 

Fig.5.5. X-ray fluorescence spectrum of chrome-nickel plating on a silver-copper base. The recording was taken with a wavelength-dispersive spectrometer [5.2]... 

Accordingly, acquisition of the full X-ray fluorescence spectrum at each pixel is especially interesting for cathodes material containing multiple transition metals or... 

There are many situations in which the X-ray absorption spectrum is most easily measured (indirectly) by monitoring the fluorescence produced following absorption of X-rays. One measures variations in the fluorescence intensity of a particular atomic species as the energy of incident photons is varied over an absorption edge of a selected element. This fluorescence excitation spectrum , distinct from the fluorescence spectrum, provides an indirect measurement of the X-ray absorption coefficient, albeit one that is subject to several well-known instrumental effects. If care is taken in sample preparation, systematic errors can be minimized. Fluorescence detection is the method of choice for dilute systems, because it provides an improved signal-to-noise ratio. 

A material is said to fluoresce if it emits radiation as the result of absorbing higher energy radiation fi om some remote source. Each element has a characteristic X-ray fluorescence spectrum that is essentially independent of the composition of the material that is, the characteristic lead X-ray fluorescence spectrum is the same for red lead pigments as for white lead pigments. 

Figure 8.29 X-ray fluorescence transitions forming (a) a K emission spectrum and (b) an L emission spectrum. The energy levels are not drawn to scale...

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