Electron return

Any material which can form a color center contains two types of precursors as shown in Figure 2a. The hole center precursor is an atom, ion, molecule, impurity, or other defect which contains two paired electrons, one of which can be ejected by irradiation, leaving behind a hole center (Fig. 2b). The electron center precursor is an atom, ion, etc, which can produce an electron center by trapping the electron ejected from the hole center precursor. A hole and an electron center are thus formed simultaneously. Either or both can be the color center. Almost all materials have hole center precursors. If there is no electron center precursor, however, the displaced electron returns to its original place and the material remains unchanged. 

Fluorescence and phosphorescence are both forms of luminescence [3]. If the emission of radiation has decayed within 10 s after the exciting radiation is cut off it is known as fluorescence [4], if the decay phase lasts longer (because the electrons return to the ground state from a forbidden triplet state (Fig. 5), then the phenomenon is known as phosphorescence. A distinction is also made between... 

We can conclude now that one electron returns to the conductivity band during each act of formation of the vacant site to adsorb sensitizer. Because adsorption centers Zr(. ) are not accounted for by (2.81) the energetics of the process does not depend on the manner in which R is closing in A, i.e. on the fact which recombination mechanism (either Langmuire-Hinshelwood or Ili-Ridil) takes place. 

In contrast, when CH3N02 is exposed to radiation, neither CH3N02 nor CH3N02+ centres are formed [2]. We can be sure of this since the e.s.r. spectra of both these radical-ions have been thoroughly studied, and give well-defined spectra [3-4]. Instead, the major radicals detected by e.s.r. methods are CH3 and N02. These radicals, which are also formed by photolysis, are presumably formed by electron-return into an outer orbital followed by homolysis (3 and 4). In this case, electron or hole transfer, or... 

Changes in shape are not, of course, the only factors that can prevent electron-return. Other factors, such as a change in solvation or chemical reactions such as protonation, deprotonation, unimolecular break-down, rearrangement, etc., are summarised in Schemes 1 and 2. Some consequences of electron return are presented in Scheme 3. Here, AB stands for any species suffering the effects of radiation, including positive or negative ions as well as neutral molecules. 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

Firstly, if the electron returns directly to the ground state, the net effect would be evolution of heat. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

At room temperature, normally 99% of the molecules are in the ground state. On radiant excitation, the absorbed energy promotes electrons to a higher energy state of the chromophore. In chromophores with acceptable quantum yields, the fluorescence emission can be measured as the electrons return to the ground state. 

Eighth, when the electron returns to its former shell or lower energy level, it will emit the energy (photon), which represents energy the electron acquired to raise it to the higher energy level. 

It can be seen from the normal potentials E° (see p. 18) of the most important redox systems involved in the light reactions why two excitation processes are needed in order to transfer electrons from H2O to NADP"". After excitation in PS II, E° rises from around -IV back to positive values in plastocyanin (PC)—i. e., the energy of the electrons has to be increased again in PS I. If there is no NADP" available, photosynthetic electron transport can still be used for ATP synthesis. During cyclic photophosphorylation, electrons return from ferredoxin (Fd) via the plastoquinone pool to the b/f complex. This type of electron transport does not produce any NADPH, but does lead to the formation of an gradient and thus to ATP synthesis. 

A similar phenomenon is observed when molecules are vaporized and thermally excited. Electrons can be promoted from an occupied ground electronic state to a vacant excited state when an electron returns to the ground state, a photon of light may be emitted. 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

Hi) Even if electron transfer to H+(aq) does occur, the newly created H-(aq) species must find another H-(aq), and desolvate, to form H2 gas. If it does not, and is not scavenged by some other reactive species, then either the electron returns to the electrode or the H- atom diffuses into the metal causing hydriding (and usually embrittlement) of the metal, as explained in Section 5.7. 

The BaFBr Eu2 + phosphors are gaining increasing industrial importance because of their storage effect. On irradiation with X rays (or UV light) some of the Eu2 + ions are ionized to Eu3+ with loss of electrons to the conduction band. These electrons are captured by F+ centers (anion vacancies) and can be liberated from these traps by the action of light (stimulation). The electrons return to the Eu3 + ions via the conduction band, converting them to Eu2+ [5.413]-[5.417],... 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

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