Auger decay/electrons

A popular electron-based teclmique is Auger electron spectroscopy (AES), which is described in section Bl.25.2.2. In AES, a 3-5 keV electron beam is used to knock out iimer-shell, or core, electrons from atoms in the near-surface region of the material. Core holes are unstable, and are soon filled by either fluorescence or Auger decay. In the Auger... 

Figure 3.25 The probability Q, to create a core hole in a level with binding energy E, with a primary electron of energy Ev maximizes for EfE, 2-3 (left). Auger decay is the preferred mode of dcexcitation in light elements, while X-ray fluorescence becomes more important for heavier elements... 

In kinetic emission, at higher kinetic energy above a certain threshold energy the impact of an ion can cause the emission of an electron from an inner shell. The core-ionized atom may subsequently decay by an Auger decay, which leads to the emission of another electron. 

The interaction of an electron with an atom gives rise to two types of X-rays characteristic emission lines and bremsstrahlung. The atom emits element-characteristic X-rays when the incident electron ejects a bound electron from an atomic orbital. The core-ionized atom is highly unstable and has two possibilities for decay X-ray fluorescence and Auger decay. The first is the basis for electron microprobe analysis, and the second is the basis of Auger electron spectroscopy, discussed in Chapter 3. 

These features of lines of various spectra (X-ray, emission, photoelectron, Auger) are determined by the same reason, therefore they are discussed together. Let us briefly consider various factors of line broadening, as well as the dependence of natural line width and fluorescence yield, characterizing the relative role of radiative and Auger decay of a state with vacancy, on nuclear charge, and on one- and many-electron quantum numbers. 

Figure 6.1 Schematic representation of one of the channels of the Is-1 Ne+ Auger decay one of the valence electrons (2s) is filling the core vacancy while another one (2p) is ejected into continuum. The same final state results also from the 2p —> Is recombination and 2s ionization (not shown here). The former ( direct ) and the latter ( exchange ) contributions interfere due to electron indistinguishability.

The above analysis suggests that the spectacular effect of the neighboring charge on the single-channel Mg 2p Auger decay has to do with the polarizable Mg 3s orbital that is involved both in the recombination and in the ionization parts of the two-electron transition. Let us consider now a more general situation, in which a polarizable orbital is involved only in the ejection of the Auger electron. An example of such a transition is readily provided by 2s-ionized Mg. Indeed, 2s ionization leads to the process in which... 

Table 6.5 Fano-ADC total Auger decay widths (in meV) for different electronic states of doubly ionized CH4, H2O, and NH3 molecules...

Interatomic Coulombic decay (ICD) is an electronic decay process that is particularly important for those inner-shell or inner-subshell vacancies that are not energetic enough to give rise to Auger decay. Typical examples include inner-valence-ionized states of rare gas atoms. In isolated systems, such vacancy states are bound to decay radiatively on the nanosecond timescale. A rather different scenario is realized whenever such a low-energy inner-shell-ionized species is let to interact with an environment, for example, in a cluster. In such a case, the existence of the doubly ionized states with positive charges residing on two different cluster units leads to an interatomic (or intermolecular) decay process in which the recombination part of the two-electron transition takes part on one unit, whereas the ionization occurs on another one. ICD [73-75] is mediated by electronic correlation between two atoms (or molecules). In clusters of various sizes and compositions, ICD occurs on the timescale from hundreds of femtoseconds [18] down to several femtoseconds [76-79]. 

Figure 1.3 Illustration of the two classes of two-electron processes caused by photoionization using magnesium as an example, using, on the left the model-picture of Fig. 1.1 and on the right an energy-level diagram (not to scale) (a) direct double photoionization in the outer 3s shell (b) 2p inner-shell photoionization with subsequent Auger decay where one 3s electron jumps down to fill the 2p hole and the other 3s electron is ejected into the continuum (Auger electron). The wavy line represents the incident photon (which is often omitted in such representations) electrons and holes are shown as filled and open circles, respectively arrows indicate the movements of electrons continuum electrons are classified according to their kinetic energy e.

Within the two-step model for photoionization and subsequent Auger decay, the kinetic energy of Auger electrons for normal (diagram) transitions comes from the energy difference of the ion states before (subscript i) and after (subscript f) the Auger decay, i.e.,... 

With rn( only the total decay rate or, equivalently, the total level width of an inner-shell hole-state has been considered so far. In general, the system has different decay branches. In many cases these branches can be classified as radiative (fluorescence) or non-radiative (Auger or autoionizing) transitions, and even further, by specifying within each group individual decay branches to different final ionic states. (Combinations of radiative and non-radiative transitions are also possible in which a photon is emitted and simultaneously an electron is excited/ ejected. These processes are termed radiative Auger decay (see [Abe75]).) As a result, the total transition rate Pnr and, hence, the total level width is composed of sums over partial values ... 

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