Binding energy of electron

Consequently one of the key experimental observations of electrochemical promotion obtains a firm theoretical quantum mechanical confirmation The binding energy of electron acceptors (such as O) decreases (increases) with increasing (decreasing) work function in a linear fashion and this is primarily due to repulsive (attractive) dipole-dipole interactions between O and coadsorbed negative (positive) ionically bonded species. These interactions are primarily through the vacuum and to a lesser extent through the metal . 

In the photoelectric effect, energy absorbed from photons provides information about the binding energies of electrons to metal surfaces. When light interacts with free atoms, the interaction reveals information about electrons bound to individual atoms. 

Figure 3.19 UPS spectra of adsorbed species reveal the binding energies of electrons in the orbitals of the adsorbate. The densities of states at the top are those of the d-band of the metal and an adsorbate level when they have no interaction. Upon adsorption, the adsorbate level has broadened and shifted to lower energy (see also the Appendix). Note the attenuation of the d-band signal and the work function increase caused by adsorption.

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. 

Absorption of X-rays of the energies corresponding to binding energies of electrons in atoms by electronic shells as a rule leads to photoexcitation of an atom and to the photoeffect. The microscopic quantity describing absorption of X-rays is the so-called effective absorption cross-section om, characterizing the absorption of X-rays of frequency a> by a single atom, and is defined as... 

Fig. 1.11 Photoelectron spectroscopy, (a) Schematic illustration of apparatus, comprising radiation source, sample, electron energy analyser and detector, all in a vacuum chamber, (b) Spectrum obtained from solid CdO, using X-rays of photon energy 1284 eV. (c) Interpretation of peaks in spectrum. The zero of energy in this scale corresponds to electrons with just sufficient energy to leave the solid positive values are the kinetic energies of emitted electrons, negative values correspond to the binding energies of electrons in the solid.

Figure 8.24 Term diagram for XPS, AES, and EDX. The vacuum energy Evac defines the zero point of the energy scale. The binding energy of electrons Eb, the Fermi energy Ef and the kinetic energy of free electrons Ekin are indicated.

The matrix element is understood to be on-the-energy-shelF, i.e., the energy e of the photoelectron has to be calculated according to equ. (1.29a). Due to the different binding energies of electrons ejected from different shells of the atom, it is therefore possible to restrict the calculation of the matrix element to the selected process in the present example to photoionization in the Is shell only. As a consequence, the matrix element factorizes into two contributions, a matrix element for the two electrons in the Is shell where one electron takes part in the photon interaction, and an overlap matrix element for the other electrons which do not take part in the photon interaction (passive electrons). The overlap matrix element is given by... 

The correction for the differences in binding energies of electrons (which of course are present both in the hydrogen atoms and in the atom being formed) is a very small one that need not concern us here. 

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