Core electrons ejection

Core electron ejection normally yields only one primary final state (aside from shake-up and shake-off states). However, if there are unpaired valence electrons, more than one final state can be formed because exchange interaction affects the spin-up and spin-down electrons differently. If a core s electron is ejected, two final states are formed. If a core electron of higher angular momentum, such as a 2p electron, is ejected, a large number of multiplet states can result. In this case it is difficult to resolve the separate states, and the usual effect of unpaired valence electrons is... 

As the X-ray energy is increased beyond that required for promotion of the core electron, ejection of core electrons into the continuum occurs. This ejected electron propagates from the Mn center until it encounters another atom from which it can be back-scattered. The interference of back-scattered waves with propagating waves leads to an interference pattern that is manifested as an oscillation in the X-ray absorption pattern. Fourier transformation of this oscillating spectrum from the frequency domain to the distance domain gives a new spectrum whose abscissa contains information on the distance between the target atom (i.e., the Mn center) and the back-scattering atoms. This second technique is called Extended X-ray Absorption Fine Structure, EXAFS, and has been the only spectroscopic tech-... 

Based upon the results achieved as well as physical arguments it is generally apparent that the relaxation experienced by a metal (M ) following core electron ejection exceeds that of the same metallic element in the form of one of its oxides M+ Eq. (2), that is,... 

ESCA analysis impUes measurement of the binding energies ( b) of the core electrons ejected by bombardment of the surface with a monoenergetic beam of X-rays. is given by the following equation ... 

We have seen above how X-ray photons may eject an electron from the core orbitals of an atom, whether it is free or part of a molecule. So far, in all aspects of valence theory of molecules that we have considered, the core electrons have been assumed to be in orbitals which are unchanged from the AOs of the corresponding atoms. XPS demonstrates that this is almost, but not quite, true. 

In XRF, as in AES, the ejection of the core electron from the atom A to produce the ion A, as illustrated in Figure 8.21, may be by an electron beam of appropriate energy or by X-rays. Much of the early work in XRF employed an electron beam but nowadays an X-ray source is used almost exclusively. 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

Xps is based on the photoelectric effect when an incident x-ray causes ejection of an electron from a surface atom. Figure 7 shows a schematic of the process for a hypothetical surface atom. In this process, an incident x-ray photon of energy hv impinges on the surface atom causing ejection of an electron, usually from a core electron energy level. This primary photoelectron is detected in xps. 

The earlier evaluation of the core radius was in terms of Bohr s impulse condition (see Sect. 2.3.3) at (relatively) high energies. This gives the core radius as 30 A at a particle energy of 10 MeV/amu. For much lower energies, this relation is unrealistic, since electrons ejected in glancing collisions penetrate... 

The essence of the XSW technique now lies in the effect these modulations have on the photoelectric cross-section of a target atom a distance c above the mure surface. The incident X-rays can eject a core electron from the atom so generating a vacancy and resulting in the emission of a fluorescent X-ray photon The probability of an incident photon ejecting the core electron, the photoelectric cross-section, is directly proportional to the electric field experienced by the atom Hcncc. the fluorescence yield, T(0.for an atom or ion distribution A (z) a distance above the mirror surface can be written... 

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. 

As illustrated in Fig. 7.15, the electromagnetic radiation measured in an XRF experiment is the result of one or more valence electrons filling the vacancy created by an initial photoionization where a core electron was ejected upon absorption of x-ray photons. The quantity of radiation from a certain level will be dependent on the relative efficiency of the radiationless and radiative deactivation processes, with this relative efficiency being denoted at the fluorescent yield. The fluorescent yield is defined as the number of x-ray photons emitted within a given series divided by the number of vacancies formed in the associated level within the same time period. 

Although not capable of the micrometer-sized lateral resolutions available with the aforementioned techniques, the surface spectroscopy, electron spectroscopy for chemical analysis (ESCA), also deserves mention. The ESCA experiment involves the use of X-rays rather than electrons to eject core electrons (photoelectrons), and it has comparable surface specificity and sensitivity to that of Auger electron spectroscopy (AES) (25, 26, 29). The principal advantage of ESCA relative to AES is that small... 

The kinetic energies of these ejected electrons originating within the first 30 A of the sample surface are measured by XPS. In XPS, a sample is bombarded by a beam of x-rays with energy hv and core electrons are ejected with a kinetic energy k that overcomes the binding energy E, and the work function (cf)). These core electrons are called the x-ray photoelectrons. The energy equation is expressed as follows ... 

ESCA involves the measurement of binding energies of electrons ejected by interactions of a molecule with a monoenergetic beam of soft X-rays. For a variety of reasons the most commonly employed X-ray sources are Al and MgKol>2 with corresponding photon energies of 1486.6 eV and 1253.7 eV respectively. In principle all electrons, from the core to the valence levels can be studied and in this respect the technique differs from UV photoelectron spectroscopy (UPS) in which only the lower energy valence levels can be studied. The basic processes involved in ESCA are shown in Fig. 1. 

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