Photoelectric process

Figure 5.1 The X-ray emission and Auger processes (Pollard and Heron 1996 37). An inner shell vacancy is created in the K shell by the photoelectric process (emitted photoelectron not shown), (a) shows the X-ray emission process, where an L shell electron drops down to fill the vacancy, and the excess energy (EK - EL) is carried away as an X-ray photon. In (b), an L shell electron drops down, but the excess energy is carried away by an Auger electron emitted from the M shell, with kinetic energy approximately equal to EK - EL — EM. Reproduced by permission of the Royal Society of Chemistry.

During the photoelectric process the photoconductor undergoes a series of voltage changes ... 

We may regard the commercial polymers which have broad technical application separately. The photosensitivity of such materials, as a rule, is caused by impurities and dopants. The main reason for studying their photoelectrical processes is the clearance of the stabilization problems. 

Since there will be a vacancy or hole where the captured electron was previously located, an electron from another energy level will fill the hole resulting in the emission of an X-ray and the creation of another hole. This second hole will be filled by another electron resulting in an X-ray of different energy, and so on. Thus, several characteristic X-rays can be emitted. It should be said that instead of X-ray emission, an internal photoelectric process can result in the emission of... 

Multiplet Splitting. Interaction of a core vacancy resulting from the photoelectric process with unpaired electrons in a valence shell induces multiplet splitting in the lines corresponding to the emitted electron. Thus, the 3s level in the transition metals exhibit relatively simple splittings, often of several eV, specific for each chemical state (1 ). The 2p levels are split into multiple lines, and the effect of their convolution is to widen the apparent split of the doublet (14). Frost, et al (15) tabulate the 2p splitting for cobalt compounds, which varies in the range 14.6 to 16.1 eV. There should be similar variability... 

Since solving the Schrodinger equation for systems investigated in XPS is not possible, several approximations have been applied to theoretically describe the photoelectric process and to calculate binding energies. It is far beyond the scope of this chapter to present these models instead the reader is referred to the corresponding literature [1, 2]. 

In traversing through matter. X-rays are attenuated by coherent (Rayleigh) and incoherent (Compton)scattering and are absorbed by the photoelectric process (6, 7). X-rays of energy below 100 keV are mainly absorbed by the photoelectric process with a cross section (i.e., the probability for absorption) proportional to (6), where E is the X-ray energy and Z... 

Where doses have been given originally in roentgens (r), they have been converted by assuming that the material in question behaves like water, which absorbs 55.2 x 10 2 ev/gm from a radiation field which supplies 1 r to air. This assumption is adequate for the energies of X-and y-rays commonly used which interact principally by the Compton process (and by the photoelectric process at the lower-energy end of the... 

The organic gas molecules, when ionized, lose their energy by dissociation rather than by photoelectric processes. Thus, the number of photoelectrons. 

XPS spectra also exhibit some minor peaks due to a number of other processes. Small peaks called X-ray satellite peaks appear at lower binding energies due to the non-monochromatic nature of the X-ray source. X-ray ghost peaks occur due to other elements present in the X-ray source Cu lines in spectra are an example of these ghost peaks. Copper is the base material used in the X-ray anode, so Cu lines may appear in the spectra of samples that contain no Cu. A complex photoelectric process results in... 

The observed UPS spectrum is the representation of the filled states of a molecular cation resulting from the photoelectric process modified by the relaxation (polarization). The width of the peaks is presumably due to inhomogeneity of the film since the organic film is normally amorphous with random disorder. Therefore, the center of the HOMO peak corresponds to the HOMO energy of the most populous molecular cation, with the emitted photoelectron at infinity (beyond vacuum level). Conceptually, a similar analysis applies to the IPES spectrum. Induced by the injected electron, the relaxation (polarization) of the surrounding medium makes the IPES spectrum the representation of the relaxed anion instead of the neutral state of the... 

An x-ray of energy E enters the detector through the thin entrance window and interacts with the gas inside via the photoelectric process to produce a cloud of ionization. The average number n of ions produced is given by... 

With the normal operating reverse bias of approximately 1000 V, the diode is depleted of the remaining free charge carriers, and it becomes a solid-state ionization chamber. X-ray photons enter the detector through the front contact and interact primarily by the photoelectric process to produce a cloud of ionization in the form of electron-hole pairs. On the average the number of electron-hole pairs produced n is proportional to the photon energy E ... 

Figure 1 Relationship of the mass attenuation coefficient for the photoelectric process versus photon energy. The lower inset shows in greater detail the change in the attenuation coefficient at the K absorption edge for lead. (Reproduced with permission from Johns HE and Cunningham JR (1983) The Physics of Radiology. Courtesy of Charles C. Thomas Publisher, Ltd., Springfield, Illinois.)...

The host material should not give rise to interfering X rays, and Compton scattering and photoelectric processes should be insignificant... 

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