Electron-hole polarization

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Surprisingly it was found that PS luminescence excited by polarized light emerges from the sample preferentially with the same sense of polarization. This memory effect has been observed despite the fact that the electron-hole pair loses energy in the order of 1 eV in elastic processes with lifetimes in the order of... 

Fig. 10-32. Polarization curves of cell reaction for photoelectrolytic decomposition of water at a photoexdted n-type anode and at a photoezdted p-type cathode solid curve n-SC s anodic polarization curve of oxygen evolution at photoexdted n Qpe anode (Fermi level versus current curve) dashed curve n-SC = anodic polarization curve of oxygen evolution at dark p>type anode of the same semiconductor as photoexdted n-type anode (equivalent to the curve of current versus quasi-Fermi level of interfadal holes in photoezdted n-type anode) solid curve p-SC = cathodic polarization curve of hydrogen evolution at photoexdted p-type cathode (Fermi level versus current curve) dashed curve n-8Cr = cathodic polarization curve of hydrogen evolution at dark n-type electrode of the same semiconductor as photoezdted p-type cathode (equivalent to the curve of current versus quasi-Fermi level of interfadal electrons in photoexdted p-type cathode) > > = flat band potential of n-type (p-type) electrode nn.sc (v p sc) = inverse overvoltage for generation of photoexdted electrons (holes) in a p-type (n-type) electrode.

Charge transfer states (CT) are often found in molecular systems side by side with excitonic states. CT states describe polar nonconducting states bound by coulomb interaction of the electron-hole pairs. CT states may be ionized with localization of the charges on definite molecules. 

Figure 9-11. Surface pitting of AgBr due to supersaturation of electron holes near the polarized anode [T. Grofle (1991)] and schematic mechanism of surface pitting.

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

As a direct application of electric molecular terms we calculate the corresponding magnetic moments by studying the polarization of electronic (hole) states in a small external magnetic field. For a field H in z-direction we add to the potential (5) a... 

The interaction between light and matter can be viewed as the creation of a coherent quantum superposition of initial and final electron states that has an associated polarization [3], as shown in Figure 1. The coherence between states with different wave vector requires an intermediate virtual state and the presence of a coherent phonon. A transition between the initial and final states may occur when the coherence of the system is broken either due to the finite width of an optical wave packet or by scattering from the environment. The transition results in the absorption of a photon and the creation of a hot electron-hole pair. Otherwise, the photon is re-radiated with a different phase and, perhaps, polarisation. 

Lattice defects can function both as donors and as acceptors and create free electrons or electron holes. Crystalline surfaces containing unsaturated electron valences act as electron traps and capture free electrons. This leads to changes in binding conditions and in the charge state of e.g. metal ions their ability to polarize O- in a metal oxide decreases. Surface oxidation during the grinding process often causes deep alterations of the surface structure of solids (sulphides, graphite, coal). This usually leads to increases in affinity toward water and in reactivity with the surfactant. 

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [101] which can be observed at somewhat longer times via light/field modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFM EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the Ti02... 

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