Auger processes

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

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 common example of an Auger process involves the ejection of a photoelectron, as shown in Figure 8.23, from the K shell (i.e. a lx electron), with energy E, which is not considered further. Following the ejection of a if electron it is common for an electron from the L shell, specifically from fhe (or 2s) orbifal, fo fill fhe vacancy releasing an amounf of energy... 

The Auger process described in this example is designated K-LiLn or, alternatively, KLiLii or, simply, KLL. Unfortunately, there is little standardization of the way in which the type of Auger process is indicated. 

Write down the configurations and derive the core states arising from Auger processes of the KLM type in krypton. 

L-subsheU electrons. For example, assume an initial hole in the K shell is filled by an electron from the subsheU and that the Auger process results in the ejection of an electron from the subsheU. The kinetic energy of the latter electron is then equal to F(K) — F(Lj) — E Ij2 ), and the electron is denoted as a KE E Auger electron. The probabUity of producing a KXY Auger electron from a hole in the K sheU is simply 1 —. ... 

The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of inner shell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from these inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. 

Fig. 2.1. Schematic diagram of electron emission processes in solids. Left side Auger process, right side photo-...

Chemical effects are quite commonly observed in Auger spectra, but are difficult to interpret compared with those in XPS, because additional core levels are involved in the Auger process. Some examples of the changes to be seen in the KLL spectrum of carbon in different chemical environments are given in Fig. 2.24 [2.130]. Such spectra are typical components of data matrices (see Sect. 2.1.4.2) derived from AES depth profiles (see below). 

The technique of INS is probably the least used of those described here, because of experimental difficulties, but it is also one of the physically most interesting. Ions of He" of a chosen low energy in the range 5-10 eV approach a metal surface and within an interaction distance of a fraction of a nanometer form ion-atom pairs with the nearest surface atoms. The excited quasi molecule so formed can de-excite by Auger neutralization. If unfilled levels in the ion fall outside the range of filled levels of the solid, as for He", an Auger process can occur in which an electron from the va-... 

Figure 4.6. Photoemission and the Auger process. Left An incident X-ray photon is absorbed and a photoelectron emitted. Measurement of its kinetic energy allows one to calculate the binding energy of the photoelectron. The atom becomes an unstable ion with a hole in one of the core levels. Right The excited ion relaxes by filling the core hole...

The binding energy, E, increases with atomic number Z, and this decreases the electron yield from the Auger process, so AES is most sensitive to elements with Z< 45. XPS provides higher sensitivity for heavier elements. 

Accompanying the photoemission process, electron reorganisation can result in the ejection of a photon (X-ray fluorescence) or internal electronic reorganisation leading to the ejection of a second electron. The latter is referred to as the Auger process and is the basis of Auger electron spectroscopy (AES). It was Harris at General Electric s laboratories at Schenectady, USA, who first realised that a conventional LEED experiment could be modified easily to... 

The physical basis of AES involves three basic steps, namely atomic ionization (by the removal of a core electron), electron emission (the Auger process), and analysis of the emitted Auger electrons. 

The Auger process is somewhat more complicated than that of X-ray photoemission (see Section 5.1.2). Let us firstly consider the energies of the various energy levels in an isolated, multi-electron atom (Figure 5.28). 

The Auger process is initiated by the creation of a core hole, which is typically carried out by exposing the sample to a beam of high energy electrons (with a typical... 

The lifetime of the core-ionized atom is measured from the moment it emits a photoelectron until it decays by Auger processes or X-ray fluorescence. As the number of decay possibilities for an ion with a core hole in a deep level (e.g. the 3s level) is greater than that for an ion with a core hole in a shallow level (e.g. the 3d level), a 3s peak is broader than a 3d peak. 

We have already met the Auger process at the beginning of this chapter (Fig. 3.2) as a way in which photo-ionized atoms relax to ions of lower energy. However,... 

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. 

---