Promotion of electrons

Figure 9.8(a) shows how the conduction band C and the empty valence band V are not separated in a conductor whereas Figure 9.8(c) shows that they are well separated in an insulator. The situation in a semiconductor, shown in Figure 9.8(b), is that the band gap, between the conduction and valence bands, is sufficiently small that promotion of electrons into the conduction band is possible by heating the material. For a semiconductor the Fermi energy E, such that at T= 0 K all levels with E < are filled, lies between the bands as shown. 

The promotion of electrons will occur if, overall, it leads to a lowering of energy by permitting the formation of more bonds. Hybrid orbitals are constructed on an atom to reproduce the electron arrangement characteristic of the experimentally determined shape of a molecule. 

Bonding in solids may be described in terms of bands of molecular orbitals. In metals, the conduction bands are incompletely filled orbitals that allow electrons to flow. In insulators, the valence bands are full and the large band gap prevents the promotion of electrons to empty orbitals. 

The rise in activation energy on films occurred at a Ag content slightly higher than the 60% commonly expected from (2-band theory. Various reasons why an exact correspondence should not be expected were discussed, e.g., the possibility of d-s promotion of electrons in Ag and absorption of hydrogen in the Pd-rich alloys. In the case of the Pd-Ag wires, most of the increase in activation energy occurred beyond 80% Ag. The authors of the latter work demonstrated a correlation between the experi-... 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

Ge like Sn shows the ft effect, namely promotion of electron energy levels in an oxygen atom once removed. The effect is apparent in ethers, i.e. alkoxygermanes as well as acylgermanes. Voltammetry by RDE shows considerable cathodic shifts in the oxidation... 

Inorganic compound adsorptions, particularly those that are colored, involve the promotion of electrons in the d orbitals. Absorption can occur in both isolated compounds and inorganic moieties with organic ligands. 

Although such instruments as described earlier are available, they are not typically used in soil analysis. Today, samples are most often aspirated into a flame or torch to cause the promotion of electrons in elements, and the diagnostic wavelengths are detected and quantified by photomultipliers. Modern spectrometers are different because of the use of many different ways of heating samples and the range of wavelengths available. Today, because of increased sensitivity of instrumentation and detectors, more of the spectrum is available for this type of analysis. Thus, wavelengths from 200 to 900 nm can be used for the analysis of the elements that are present. 

What about molecules and complex ions Absorption of light by molecules and complex ions results in the promotion of electrons to higher energy states in the same way as in atoms. However, it is more complicated because molecules and complex ions have energy states that atoms do not. 

Though the core expansion leads to the appropriate fit, it may not be the proper explanation for the scale factor discrepancy. Hansen et al. (1987) note that the expansion of the core would lead to a decrease of 7.5 eV in the kinetic energy of the core electrons, at variance with the HF band structure calculations of Dovesi et al. (1982), which show the decrease to be only about 1.5 eV. An alternative interpretation by von Barth and Pedroza (1985) is based on the condition of orthogonality of the core and valence wave functions. The orthogonality requirement introduces a core-like cusp in the s-like valence states, but not in the p-states. Because of the promotion of electrons from s - p in Be metal, the high-order form factor for the crystal must be lower than that for the free atom. It is this effect that can be mimicked by the apparent core expansion. 

FIGURE 4.9 Promotion of electrons from the valence band to the conduction band by light. 

In certain solids such as titanium dioxide or cadmium sulfide, the energy of the band gap corresponds to that of light (visible, ultraviolet, or infrared), with the result that the solid, when illuminated, may become electrically conducting or acquire potent chemical redox characteristics because of the promotion of electrons to the conduction band (which is normally unoccupied). These properties have obvious practical significance and are considered at length in Chapter 19. 

UV-Vis spectroscopy involves the promotion of electrons from bonding or non-bonding orbitals to anti-bonding orbitals. 

There is, in fact, no reason for us not to accept configurations that involve a considerable amount of promotion of electrons. Figure 11-8... 

Two points of view are applicable to these species, as they also are to the isoelectronic noble gas fluorides (1) a valence bond approach with promotion of electrons to d orbitals and (2) three-center, four-electron bonds. The same arguments, pro and con. apply as given previously, so they will not be repeated here. Independent of die alternative approaches via VB or MO theory, all are agreed that Madelung energy ( ionic character ) is very important in stabilizing both the polyhalide tons and the polyhalogens.27... 

The use of Gillespie s VSEPR theory has allowed the rationalization of these as well as several other structures of noble gas compounds (Fig. 17.3). One of the signal successes of this approach was the early prediction that XeF6 was nonoctahedral (see Chapter 6). The most serious objection lo it is the required promotion of electrons. This has been estimated lobe about 1000 kJ mol 1 (10 eV) or more for xenon, a large amount of energy. Furthermore. J oibit.ils tend lo be diffuse and their importance in nonmetal chemistry is a matter of some controversy (sec Chapter 18). 

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