X-ray photon, generation

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

Fig. 7.3. Typical raw data showing the low flux image obtained from detection of individual X-ray photons by a CCD detector. The image shows a 512 x 512 pixel region of the CCD. Each X-ray photon generates a charge that is proportional to its energy...

Figure 7.2 A pictorial representation of the processes involved in EXAFS spectroscopy. The incoming X-ray photon generates a photoelectron from the core level with higher energy. The electron wave of the photoelectron (solid line circles) is...

When an electron beam strikes a block of copper, x-rays with a frequency of 1.2 X lO1 Hz are emitted. How much energy is emitted at this wavelength by (a) an excited copper atom when it generates an x-ray photon (b) 2.00 mol of excited copper atoms (c) 2.00 g of copper atoms ... 

It is possible to extend the line scanning method to two-dimensional scanning. In its simplest form, the display is made bright every time an X-ray photon is counted, thus generating a image of bright dots. The dot density provides a qualitative measure of the concentration of the element of interest. 

The essence of the XSW technique now lies in the effect these modulations have on the photoelectric cross-section of a target atom a distance c above the mure surface. The incident X-rays can eject a core electron from the atom so generating a vacancy and resulting in the emission of a fluorescent X-ray photon The probability of an incident photon ejecting the core electron, the photoelectric cross-section, is directly proportional to the electric field experienced by the atom Hcncc. the fluorescence yield, T(0.for an atom or ion distribution A (z) a distance above the mirror surface can be written... 

Clearly visible in the spectrum are the Ka and the K[3 components of X-ray emission generated by fast electron propagation in the substrate of the Ti foil target. Also visible in the spectrum is a small feature at 2.7 keV due to a small number of events in which a Ka photon from an excited atom of the Si substrate of the CCD array escapes from the detector sensitive area. 

Fluorescence excitation techniques provide a more sensitive detection system in which fluorescent X-ray photons (a fraction of the ionized absorbing atoms relax by emission of a fluorescent X-ray photon) are counted as the photon energy is scanned. The signal generated is proportional to the absorption coefficient, p, of the absorbing atom. 

When a sample maintained in a high vacuum is irradiated with soft X-rays, photoionization occurs, and the kinetic energy of the ejected photoelectrons is measured. Output data and information related to (he number of electrons that arc detected as a function of energy are generated. Interaction of the soft X-ray photon with sample surface results in ionization from the core and valence electron energy levels of the surface elements. 

According to the published yields,25 one y-photon with the energy 1 Me V absorbed in water will generate 27,000 electrons, and one lOKeV X-ray photon (1.25 A) will... 

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