K-shell

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... 

X-Rays. If an x-ray is emitted, it has an energy, AE, equal to the difference in the binding energies of the two atomic shells, E — Ej. If the original hole is in the K shell, the x-ray is called a K x-ray if the hole is in the L shell it is an L x-ray. Because the hole can be filled by an electron from any of the several outer shells, x-ray spectra contain a large number of discrete lines. 

Auger Electrons. The fraction of the holes in an atomic shell that do not result in the emission of an x-ray produce Auger electrons. In this process a hole in the 4h shell is filled by an electron from theyth shell, and the available energy is transferred to a kth shell electron, which in turn is ejected from the atom with a kinetic energy = E — Ej —. Usually, the most intense Auger electron lines are those from holes in the K shell and involve two... 

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 —. ... 

Figure 8.1.5 (Berty et al 1989) shows the effect of step size on integration results at a 513 K shell temperature with boiling water. This is the...

Details of oxygen K shell in NiO, illustrating NES and EXELFS oscillations and the measurement of the integrated edge intensity used for quantitative concentration determination. 

The surface to be analyzed is irradiated with soft X-ray photons. When a photon of energy hv interacts with an electron in a level X with the binding energy Eg (Eg is the energy E of the K-shell in Pig. 2.1), the entire photon energy is transferred to the electron, with the result that a photoelectron is ejected with the kinetic energy... 

Fig. 2.18. Electron-impact ionization cross-section for the Ni K shell, as a function of reduced electron energy U [2.128] U = Ep/Ek, where Ep is the primary electron energy and E

The cross-section for electron impact ionization has already been mentioned in Sect. 2.2.2.2 in connection with electron sources, and a variety of experimental and theoretical cross-sections have been shown in Fig. 2.18 for the particular case of the K-shell of nickel. The expression for the cross-section derived by Casnati et al. [2.128] gives reasonably good agreement with experiment the earlier expression of Gry-zinski [2.131] is also useful. 

Fig. 4.19. Allowed electronic transitions to the K shell and corresponding X-ray lines after ionization of an atom (two forbidden transitions are also shown as dashed lines).

In these approximations for the K series the value 1 is subtracted from the atomic number Z to correct for the screening of the nuclei by the remaining K-shell electron. For the L series the screening effect of the two K-shell electrons and the seven remaining L-shell electrons must be taken into consideration by subtracting 7.4. 

Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

The question now is, what role do the K, L, M,. . . electrons play in generating the K, L, M,. . . series The answer is not obviously predictable from a knowledge of visible or ultraviolet spectra. Neither hydrogen nor helium has a K series, although each has K electrons. Why Because the K series is generated only when the K shell contains a hole that is filled by an electron that leaves one of the outer (L, M,. . . ) shells or the generation of the K series requires (1) the absence of a K electron, (2) the presence of an outer-shell electron whose transition to the K shell is permitted by the selection rules. This picture explains why—no matter what the method of excitation—all K lines have the same excitation threshold so that all K lines appear together if they appear at all. 

To illustrate the concept of fluorescence yield, we turn again to the K spectrum. Assume that an element is irradiated with an x-ray line energetic enough to excite the K spectrum. If the irradiation is continued, a steady state will soon be reached in which the rate at which holes are produced in the K shell (i.e., the rate at which atoms in the K state are produced) is just balanced by the combined rates of the various processes causing such holes to disappear. Let n1, n2,. . . , % be the individual rates rii at which the filling of holes leads to the production of the i lines in the K spectrum. The fluorescence yield, for this simple case is... 

If certain quanta suitable for the excitation of a line are absorbed without photon emission, a radiationless transition is likely. This transition is known as the Auger effect,39 and it may be thought to involve an absorption by the atom of the photon produced when the hole in the K shell is filled by an electron from one of the external shells such as the L shell. The absorption of this photon results in the ejection of a second electron from one of the shells to leave a doubly charged residue of what had been a normal atom. The atom in this condition is described by naming the two states in which the electron holes are to be found e.g., the atom is in the LL or LM or LN state. An atom in such a state is, of course, vastly different from the usual divalent cation. 

---