Auger electron-emission yields from

Auger Electron-emission Yields from Metal Surfaces67... 

Figure 22. Auger electron emission and X-ray fluorescence yields for K-shell electron vacancies as a function of atomic number. (From Ref. 69.)...

Quantitation is usually achieved by comparing the X-ray yields from the sample with yields obtained from standards. The ease with which measurements can be interpreted quantitatively depends on the sample. As illustrated in Fig. 7.7, the volume that is activated by the 10-100 keV electron beam has the shape of a pear with typical dimensions of a few pm. As a consequence, X-rays formed in the interior may be absorbed on their way out, and may stimulate the emission of photoelectrons, Auger electrons and again X-rays. The latter process, secondary fluorescence, can lead to an overestimate of the concentrations. For example, if the specimen is a bulk Fe-Ni alloy, Ni Ka radiation is adsorbed by iron and causes... 

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

Whenever an atomic electron is removed and the vacancy left behind is filled by an electron from a higher orbit, there is a competition between the emission of Auger electrons and fluorescent radiation. The number of X-rays emitted per vacancy in a given shell is the fluorescent yield. The fluorescent yield increases with atomic number. 

The XSW field established inside the crystal and above the crystal surface induces photoelectron emission from atoms within the field. The excited atoms (ions), in turn, emit characteristic fluorescence X-rays and Auger electrons. In the dipole approximation, the photoelectric effect cross section is proportional to the E-field intensity at the center of the atom. (It is necessary to consider higher-order multi-pole terms in the photoelectric cross-section under special conditions, as discussed by Fischer et al. (1998) and Schreiber et al. (2001). For this review, we will assume the dipole approximation.) Therefore, with the XSW intensity from Equation (6), the normalized X-ray fluorescence yield is defined as... 

X-ray emission and Auger electron ejection are competitive processes, especially in low-Z elements. As the Z value of the material becomes higher, X-ray emission becomes the more likely process. This is reflected in tabulations of fluorescence yield, which is the ratio of X-ray transition to Auger electron plus X-ray transitions this value increases with Z from 0.01 at fluorine to 0.97 at and above polonium (Firestone 1996). 

Equation (2.17) gives the rate of ionization of the K shell of the ith element. Following ionization, the vacancies in the K shell are filled by electron transitions from other shells with lower binding energies. In some of these transitions Auger electrons are emitted. In the remaining transitions characteristic K x-rays are emitted. The K-shell fluorescence yield shell ionizations that are followed by emission of characteristic K-series x-rays. Therefore the rate of emission of K x-rays from the ith element is... 

The primary purpose of bombarding a specimen with protons or heavier ions in PIXE is to eject bound electrons from the K or L atomic shells. The ejection can also be achieved by means of electrons or photons with energy in the 1-100 keV range. The vacancies will de-excite within 10 s with the emission of characteristic radiation or Auger electrons or both the probability of the radiative relaxation is the fluorescence yield ro. For low-Z elements (Z<30) and for Kvacancies, the production of Auger electrons is more probable than emission of characteristic radiation in the case of Lshell vacancies, this is always the case. Nevertheless, for elements with Z < 20, protons are more efficient than photons or electrons for producing characteristic X-ray. For heavier elements, photon-induced X-ray emission (i.e.. X-ray fluorescence, XRF) is more efficient. 

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