Electron-hole pairs, recombination

Fig. 2. Surface temperature dependence of the vibrational excitation of NO(v = 0 — 1) in collisions with a clean Ag(lll) surface. The observed thermal activation was attributed to hot electron-hole-pair recombination transferring energy to NO vibration. This work provided some of the first strong evidence that metal electrons can interact with an adsorbate molecule strongly enough to change its vibrational quantum numbers. (See Ref. 24.)...

For very long time (> 1 year), electron-hole pair recombination occurs and spectral features of neutral occluded DPB molecule are found to reappear (figure 5). Then, electron trapping in zeolite framework shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

In photoexcited n-type semiconductor electrodes, photoexcited electron-hole pairs recombine in the electrodes in addition to the transfer of holes or electrons across the electrode interface. The recombination of photoexcited holes with electrons in the space charge layer requires a cathodic electron flow from the electrode interior towards the electrode interface. The current associated with the recombination of cathodic holes, im, in n-type electrodes, at which the interfadal reaction is in equilibrium, has already been given by Eqn. 8-70. Assuming that Eqn. 8-70 applies not only to equilibrium but also to non-equilibrium transfer reactions involving interfadal holes, we obtain Eqn. 10-43 ... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

O Neil, M. Marohn, J. McLendon, G. Dynamics of electron-hole pair recombination in semiconductor clusters, J. Phys. Chem. 1990, 94, 4356. 

Step 3 This second virtual electron-hole pair recombines and a Raman-scattered photon is emitted. 

Another important contribution of radiation chemistry in photography was the enhancement of the sensitivity of photographic emulsions. The primary effect of photon absorption by silver halides is the formation of an electron-hole pair. However, because of the very fast and efficient electron-hole recombination and oxidation by hole of the newly formed silver atoms, the conversion yield of light is very low. The analogy with HO oxidation processes occurring in irradiated solutions led to the use of the same scavenging method to inhibit the electron-hole pair recombination and the oxidation by the... 

Many of the photogenerated electron-hole pairs recombine before they are separated by the built-in electric field. To maximize the built-in electric field, one electrode should have a work function which matches the tt- band of the polymer and the other electrode should have a work function which matches the ir -band. Even under these conditions, however, the quantum efficiency is typically only 0.05% [183]. In photodetectors the photosensitivity is usually increased by applying a reverse bias to the device [183,313]. 

Photochemistry of Titanium Dioxide Colloids. Another semiconductor colloid used in our studies is titanium dioxide which has a band gap of 3.2 eV. As in the case of cadmium sulfide, excitation of aqueous suspensions of this particle leads to electron-hole pair separation which can be intercepted with suitable redox reagents. In the absence of externally added solutes, the photogenerated electron-hole pair recombines to give the starting material and the light energy is dissipated to the medium as heat. Two types of TiOj samples are used in this study. TiOj prepared at high temperature (80°C) which behaves very similarly to commercial samples, and TlOj prepared at low temperature (35°C) which has a particle size of 300 100 A radius and shows different properties. 

The third part describes the electron/hole pair recombination efficiency r(E) which causes a further dependence of iph on the layer thickness for very thin films, high potentials, respectively (dscl = d ), and superimposes on the thickness dependence due to the optical part. 

Figure 3 Schematic representation of an electron-hole pair recombination in highly-excited Luttinger Uqtiid ring...

In the conventional ODMR techniques, one monitors the luminescence intensity to detect ESR signals in terms of changes in luminescence at resonance. These changes are a consequence of spin-dependent recombination processes. For trapped electron - hole pair recombination, illustrated in Fig. 1, the spin-dependent nature of the radiative recombination can be easily understood by considering that radiative recombination is allowed when spins of an electron and a hole are antiparallel and is forbidden, in principle, when they are parallel. 

Band-to-band kinetic models (presented in Fig. 5) allow electrons to have only valence or conduction-band energies. Absorption of the appropriate amount of thermal or electromagnetic energy creates an electron-hole pair recombination of an electron and a hole releases energy in the form of heat or light. The band-to-band model yields... 

Neutral species represent the majority of particles desorbed during electron stimulated desorption (ESD) experiments from solid surfaces or other desorption experiments with other types of ionizing radiation (Eeulner and Menzel 1995). However, the study of emission of these species has received little interest compared to charged particles. The comparative scarcity of information on this subject reflects both the difficulties in measuring the neutral species and the number and complexity of desorption mechanisms (Bazin et al. 2010). In fact, the origins of the neutral species are multiple DEA, electron-hole pair recombination, dipolar excitations, and multihole final states with or without recombination of different particles (Kimmel et al. 1994). Neutral species can also be produced by direct electronic excitation of a molecule to a repulsive state leading to electronic excitation dissociation (EED) (Eigure 16.3). 

The creation of an excess ehp requires an energy equal to the semiconductor band gap. When excess electron-hole pairs recombine they release this energy by one of several distinct physical mechanisms. When the energy is given to phonons or lattice vibrations, the recombination mechanism is known as multiphonon recombination or Shockley-Read-Hall (SRH) recombination. SRH recombination dominates in the indirect band gap semiconductors Si, Ge and GaP. 

The ability of porous photoelectrochemical systems to separate effectively electrons and holes is widely known since the presentation of the dye-sensitized particulate Ti02 solar cell [16, 105, 130-137]. In this system, the photocurrent quantum yield (the number of electrons counted in the external circuit as photocurrent divided by the number of absorbed photons) is close to unity. This means that electron-hole pair recombination is essentially absent. Efficient separation of photogenerated electrons and holes was demonstrated with several other photoelectrochemical systems [105, 130-137]. Photovoltaic devices based on permeated hole-conducting and... 

An electric field E is applied across the photoconductor to measure the photoconductivity. The field drives electrons toward x — l and holes toward X=0 see Fig. 4.8. The electrical contacts are assumed to be metallic and ohmic, so that photoexcited electron-hole pairs recombine there instantaneously thus... 

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