Delocalized valence-hole states

Irrespective of some still unresolved details of the primary process that leads to generation of electron-hole pairs one can determine the effective absorption coefficient, called a], that determines the photoelectric geminate pair yield in the low field limit, i.e. at fields where (j)o is constant. The result is shown in fig.7 for DCH. has a singularity close to the energy where electroreflectance measurements of Sebastian and Weiser (24) locate the onset of the transition between delocalized valence and conduction band states of the chain. The feature near 3.5 eV is due to charge transfer from the carbazole substituent to the polymer chain (49). 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

FIGURE 3. (a) Comparison of the He(I) PE spectra of the >3d-symmetric molecules ethane and disilane, which exhibit Jahn/Teller splitting (J/T) of their M (e) states with dominant contributions ( ) to positive hole delocalization, based on their quantitative radical cation state assignment (see text), (b) United Atom correlation for the isoelectronic 18-electron species from Ar to SiFLj including the iso(valence)electronic 10-electron molecule CH4 with average atomic ionization energies... 

Studies have demonstrated that the electronic relaxation depends upon the location of the core hole site and the configuration of the core hole excited state. The important feature is that the core hole is localized on a specific atom and this localization is projected onto the valence electrons in the decay process. Molecular Auger spectra thus present a view of molecular electronic structure from the perspective of particular atoms in a molecule. The spectra therefore can serve to identify particular molecules and functional groups, to distinguish between localized and delocalized bonding, and to measure orbital atomic populations for various atoms in a molecule (Rye and Houston 1984). This localization and the projection onto the valence... 

When one assumes that the electrons or holes generated as a result of redox reactions, such as Eqs. (6.16) or (6.17), end up delocalized (i.e., in the conduction or valence bands, see Chap. 7), the implicit assumption is that the cations were only stable in one oxidation state (e.g., A1 or Mg). For oxides in which the cations can exist in more than one oxidation state, such as the transition metal ions, an alternate possibility exists. 

The electronic structure of a semiconductor (note Ti02 and ZnO are the compounds most commonly used) is characterized by a filled valence band and an empty conduction band. A photon with energy that matches or exceeds the bandgap energy can promote an electron from the valence band to the delocalized state of the conduction band (ecb ) leaving a hole (hyb ) behind ... 

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