Photoelectric peak

The resolution of Nal (Tl) spectrometers depends on the energy of the radiation. At energies of about 10 MeV when the response in the crystal is mainly due to pair production, the resolution is about 10%. A slight difficulty in interpretation arises from the presence of subsidiary peaks due to escape of one or both of the pair annihilation quanta. At lower energies the pair lines are also found but the Compton distribution becomes more important and the pair peaks are superimposed on this. At energies below the pair threshold there are pulses from the Compton recoil electrons and from photoelectrons and as the quantum energy decreases the photoelectric peak becomes more important but the resolution decreases, as E, until it is only of the order of 20 to 30% for 50keV quanta. 

The shape of the photoelectric peak is therefore determined by the density function of the above normal distribution (i.e., by a Gaussian curve), the mode/expected value (MAX) and halfwidth (FWHM) ofwhich are... 

The above formulas show that this simple stochastic model not only explains the shape of the photoelectric peak, but also predicts that the smaller is the average energy spent on the production of one charge carrier/excited state in a detector, the better is its energy resolution. 

It has been assumed above that there are no statistical effects at later stages of pulse formation that could change the shape of the photoelectric peak. However, such effects do exist. Consider the scintillation detectors, for instance, which waste much of the potential of energy resolution hidden in the primeval shape of the photoelectric peak. 

X-Ray photoelectric ionization is believed to take place in a time interval of about 10-18 s. Therefore separate XPS peaks are possible for atoms if the lifetime of the asymmetric electronic state is greater than about 10 18 s, whether or not the atoms are structurally equivalent. We may represent the ground state of a localized mixed valence compound (involving two metal atoms differing in oxidation state by one unit) by the following formula, where the dot represents the extra valence electron M—M. The two possible XPS transitions can then be represented as follows, where the asterisk indicates core ionization,... 

In this circumstance, it is more reliable to measure the absolute integrated intensity of the substrate peak and compare this with the integrated intensity from an equivalent crystal of the substrate material on which no layer has been grown. In the angular position for diffraction from the substrate, the layer will not diffract and the substrate peak intensity will be simply reduced by normal photoelectric absorption. For a symmetric reflection, it is easy to see that the integrated intensity 1 of the substrate peak with the layer of thickness t present is related to the integrated intensity of the bare substrate / o by... 

In order to analyze the intensity of the various photoelectron peaks, it is necessary to know their associated transition probabilities, or photoelectric cross-sections. 

Cadmium sulfide particulate films, generated in thicknesses of 300 50 A at arachidic acid (AA) monolayer interfaces, have been characterized in situ by STM under potentiostatic control [644], Electrical contact was made between the tip of the STM, acting as the working electrode (WE), which was in contact with the CdS particulate film floating on aqueous 0.30 M NaCl, and the reference (RE) and counter (CE) electrodes, placed in the subphase (Fig. 112) [644]. A well-defined single-reduction wave at about — 1.15 V was observed. Prolonged exposure to room light shifted the reduction peak to — 0.85 V. Electrical and photoelectrical characterizations have also been performed on Ti-foil-supported, 5000-A-thick CdS particulate films in an electrochemical cell (Fig. 113) [644]. The Ti foil was used as the WE, while the RE and CE were placed into 0.50 M... 

The XPS mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft Mg or Al anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize die core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their BE. The resulting XP spectrum is a characteristic set of peaks for a specific element, with BE as the abscissa and counts per unit time as... 

One important form of spectroscopy has developed directly from the photoelectric experiments described above. By making the electrons pass between two slits, with a deflecting electric field in between, their complete energy distribution can be measured, rather than just the maximum emitted energy as in Fig. 1.10. This is known as photoelectron spectroscopy, and may be performed on gases or solids, with radiation in the UV or X-ray ranges of the spectrum. Figure 1.11 shows the type of apparatus used, and the spectrum obtained from a sample of solid CdO. A series of peaks is found, and the interpretation based on Einstein s theory is also shown. 

The XPS Measurement. In an XPS spectrometer, the studied material is exposed inside a vacuum chamber to a flux of X-rays (energy 1 keV). The kinetic energy of the photoelectrons ejected from the sample is measured by an appropriate analyzer. This energy is directly related to the binding energy of the electrons inside the sample on a wide scan XPS spectrum, the unscattered electrons result in characteristic peaks their energies serve to identify the elements in the material (atomic composition), to characterize the molecular environment of these atoms (chemical analysis, see inset A of Figure 1), and, by the measurement of the photoelectric lines ratios, to reach some quantitative results. Such type of measurement from the core level peaks can usually be... 

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