Auger decay

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 Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

A core-ionized atom has two possibilities to lower its energy, namely Auger decay and X-ray fluorescence (described in more detail in Chapter 7). The Auger yields for processes following core hole creation in the K and L shell are sketched in Fig. 3.25 (right). Obviously, Auger processes are the dominant decay mode in light elements. 

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.

APPLICATIONS OF FANO-ADC THEORY TO AUGER DECAY IN THE MULTIPLY IONIZED SYSTEMS... 

Let us describe several recent applications of the Fano-ADC theory to Auger decay in multiply ionized systems. We shall start with the simplest problem of this kind - effect of a single neighboring charge on the rate of atomic Auger decay. Then, we shall explore the effect of a spectator vacancy residing... 

Figure 6.2 Auger decay width of (2p n) Mg+-H+ as a function of the Mg-proton distance, R.z is Mg-proton axis. Diamonds and solid line Fano-ADC(2)x calculation with atomic orbital basis centered both on Mg and on the proton circles and long-dashed line Fano-ADC(2)x calculation with atomic orbital basis centered only on Mg stars and short-dashed line Fano-ADC(2)x calculation for (2p 1) Mg+ alone, with atomic orbital basis centered both on Mg and at the distance R along the z-axis, showing the so-called basis set superposition error (BSSE) triangles and dashed-dotted line Fano-ADC(2)x calculation with atomic orbital basis centered on Mg only, with the 3s orbital of Mg being frozen at its shape at R = 6.5A. The inset shows the low-r part of the plot on logarithmic scale. See Ref. [35] for the details of the computation.

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 6.3 Fano-ADC total and partial Auger decay widths (in meV) for doubly ionized Ne atom...

---