Electron vacancy

Evidence emerges from both the microcatalytic and spectroscopic studies for enhanced dissociative interaction of N,0 with preoxidised RhO,/CeO, materials exposed to helium-flush or vacuum-outgassing at T > 573 K. This is consistent with generation of reduced active-sites, such as electron-vacancy complexes, by such treatments. 

When a molecule in a solution absorbs light, an electron in the highest occupied molecular orbital (HOMO) is excited to the lowest unoccupied molecular orbital (LUMO) creating an electron vacancy, that is, a hole in the HOMO. The electron may be provided to a molecule in the solution to reduce it, whereas the hole in the HOMO may be provided to a molecule in the solution to oxidize it. This is similar to the reduction-oxidation process in the bulk semiconductor/electrolyte photoelectrolytic cell described earlier [13,17]. 

Upon absorption of light an electron from the HOMO of the adsorbed dye, D, is raised to the LUMO from where it is injected into the conduction band of the n-type semiconductor and transferred to a counter electrode where an oxidant, O, is reduced. From the reduced species, R, the electron is transferred to the HOMO of the adsorbed dye to fill the electron vacancy, so that after the overall photoelectro-chemical process the dye is in its original oxidation state. Vlachopoulos et al. (1987) have reported on Ti02 photoelectrodes that were sensitized to visible light with various dyes and that showed high quantum yields of interfacial electron transfer under visible irradiation. 

Figure 11.1 Promotion of an electron from the valence band to the conduction band on irradiation of a semiconductor. The electron vacancy left behind in the valence band is known as a hole (h+)...

The XANES region of the Pt Lm and Ln absorption edges can be used to determine the fractional d-electron occupancy of the Pt atoms in the catalyst sample by a so-called white line analysis. Figure 2 shows the XAS spectrum collected at both Pt Lm and Lii absorption edges of Na2Pt(OH)e. The sharp features at the absorption edges are called white lines after the white line observed in early photographic film based XAS measurements. Mansour and coworkers have shown that comparison of the white line intensities of a sample with those of a reference metal foil provides a measure of the fractional d-electron vacancy, f, of the absorber atoms in the sample. is defined as follows ... 

At P02 higher than we observe the formation of doubly ionized cationic vacancies and positive electronic vacancies (usually defined as electron holes by symbol h ) ... 

Semiconductor band-gap luminescence results from excited electrons recombining with electron vacancies, holes, across the band gap of the semiconductor material. Electrons can be excited across the band gap of a semiconductor by absorption of light, as in photoluminescence (PL), or injected by electrical bias, as in electroluminescence (EL). Both types of luminescence have been used in chemical sensing applications [1,3]. 

X-rays resulting from an electron transition tilling an electron vacancy in the innermost shell of an atom are known as K x-rays or K lines those from the L shell are known as L lines, and so on. 

The scintillation process in inorganic scintillators differs from that in organic scintillators. Consider the structure of an ionic crystal, as shown in Figure 18.19. When an energetic electron passes through the crystal, it may raise valence electrons from the valence band to the conduction band. The electron vacancy in the valence... 

A iV-electron vacancy (hole) in a shell may be denoted as nl N = ni4l+2-N (see aiso Chapters 9, 13 and 16). As we have seen in the second-quantization representation, symmetry between electrons and vacancies has deep meaning. Indeed, the electron annihilation operator at the same time is the vacancy creation operator and vice versa instead of particle representation hole (quasiparticle) representation may be used, etc. It is interesting to notice that the shift of energy of an electron A due to creation of a vacancy B l is approximately (usually with the accuracy of a few per cent) equal to the shift of the energy of an electron B due to creation of a vacancy A l, i.e. 

An electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) when a molecule in solution absorbs light. The excited electron in the LUMO may transfer to a neighboring molecule (oxidant) in solution, leading to the reduction of the oxidant, whereas the electronic hole (electron vacancy) in the HOMO may transfer to another neighboring molecule (reductant) in solution, resulting in the oxidation of the reductant. Quite similar photoinduced reduction-oxidation processes can occur at the semiconductor/solution (semiconductor/liquid) interface when a semiconductor in solution absorbs light. Fig. 4.1 schematically illustrates the... 

Let us turn our attention to the dominant recombination or deexcitation processes that follow the excitation of electrons from the inner shell or from the valence shell (Fig. 13). The first mode of deexcitation is the Auger process, which leads to further electron emission. The second mode of deexcitation may result in the emission of electromagnetic radiation and is commonly called X-ray fluorescence. In the Auger transition, the electron vacancy in an inner shell is filled by an electron from an outer band. The energy released by this transition is transferred to another electron in any... 

When a bond is broken and a free electron is removed, a hole or electron vacancy is created. When this vacancy is filled by an electron from a neighboring bond, the result is a hole moving in a direction opposite to that of the electron. Thus the hole may be considered as a (fictitious) particle with... 

From the preceding discussion it may be concluded that the main resonance line at g — 2.0006 in irradiated frozen alkali hydroxide solutions is attributable to the radiation-produced electron trapped around a hydrated O- radical ion. Insofar as the latter is an electron vacancy created by the reaction of the radiation produced holes with the OH ions, the trapped electron may be considered to be analogous to an F center formed in alkali halide crystals, where, however, the electron vacancies exist even prior to irradiation. The term trapped electron (symbolized T ) has been used throughout the present paper. This model will be... 

The boron atom contains an electron hole or an electron vacancy. In order to fill this vacancy, the doped atom takes up an electron from the mother material (arrow). In this way a new vacancy is formed in the valence band, et cetera. The additional electrons in the energy level of the doped atoms enable conduction to take place. This conduction is the transport of vacancies. A vacancy has a shortage of one electron and is thus positively charged. That is why this semiconductor is of the / -type. 

The equation can also be illustrated in Figure 9.1. When a semiconductor such as Ti02 absorbs photons, the valence band electrons are excited to the conduction band. For this to occur, the energy of a photon must match or exceed the band-gap energy of the semiconductor. This excitation results in the formation of an electronic vacancy or positive hole at the valence band edge. A positive hole is a highly localized electron vacancy in the lattice of the irradiated Ti02 particle. This hole can initiate further interfacial electron transfer with the surface bound anions. 

Platinum-based bi-metallics (Pt M, M = Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the OER. Several rationales have been proposed including (1) enhanced chemisorption of intermediates (2) a lattice change of Pt that results in the shortening of Pt-Pt interatomic distances by alloying (3) the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer caused by the underlying alloy and the anchor effect of alloy metals on a carbon carrier.93,94... 

The electron-transfer process must take place without violating the Franck-Condon restriction, namely, that none of the atoms involved change position during the instant (< 10-14 sec) of the actual electron transfer. As it is rather unlikely that the acceptor molecule should have an electron vacancy at its vibrational and electronic ground state, most eJTq reactions must result in the formation of excited molecules as their primary products. It is probable that after accommodating an additional electron the interatomic bond distances, and in many cases the whole... 

The BLW method can be considered as an extension of the orbital deletion procedure (ODP) (51,52), a simpler method that can only be applied to carbocations (52) and boranes (51). The ODP consists of representing a resonance structure displaying an electronic vacancy (Lewis acid character) by deleting the primitive basis functions corresponding to the empty site before launching the SCF calculation. As a typical example, the ODP has been applied to calculate the resonance energy of the allyl cation (52). 

The absorption of exciting photons, most often in the ultraviolet spectral range, by a semiconductor promotes electrons from the filled valence band (where electron vacancies, electron deficiencies, or holes are thus formally created) to the vacant conduction band (Fig. 1). The electron-hole pairs can recombine either directly (band-to-band recombination) or, most... 

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