Pores electron trapping

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. 

Another technique is gaining interest because of the ease of regeneration and improved flow characteristics (small and constant pressure drops). Instead of physically trapping a pollutant in its pores, the technique involves direct attachment of the contaminant molecules to the sorption material, usually a polymer. All molecules are composed of a number of atoms with a confluence of electrons spinning around them in what is called an electrostatic field or electron cloud. The cloud, however, is not necessarily uniformly distributed. 

Particle irradiation effects in halides and especially in alkali halides have been intensively studied. One reason is that salt mines can be used to store radioactive waste. Alkali halides in thermal equilibrium are Schottky-type disordered materials. Defects in NaCl which form under electron bombardment at low temperature are neutral anion vacancies (Vx) and a corresponding number of anion interstitials (Xf). Even at liquid nitrogen temperature, these primary radiation defects are still somewhat mobile. Thus, they can either recombine (Xf+Vx = Xx) or form clusters. First, clusters will form according to /i-Xf = X j. Also, Xf and Xf j may be trapped at impurities. Later, vacancies will cluster as well. If X is trapped by a vacancy pair [VA Vx] (which is, in other words, an empty site of a lattice molecule, i.e., the smallest possible pore ) we have the smallest possible halogen molecule bubble . Further clustering of these defects may lead to dislocation loops. In contrast, aggregates of only anion vacancies are equivalent to small metal colloid particles. 

It is also possible to prepare crystalline electrides in which a trapped electron acts in effect as the anion. The bnUc of the excess electron density in electrides resides in the X-ray empty cavities and in the intercoimecting chaimels. Stmctures of electri-dides [Li(2,l,l-crypt)]+ e [K(2,2,2-crypt)]+ e , [Rb(2,2,2-crypt)]+ e, [Cs(18-crown-6)2]+ e, [Cs(15-crown-5)2]" e and mixed-sandwich electride [Cs(18-crown-6)(15-crown-5)+e ]6 18-crown-6 are known. Silica-zeolites with pore diameters of vA have been used to prepare silica-based electrides. The potassium species contains weakly bound electron pairs which appear to be delocalized, whereas the cesium species have optical and magnetic properties indicative of electron locahzation in cavities with little interaction between the electrons or between them and the cation. The structural model of the stable cesium electride synthesized by intercalating cesium in zeohte ITQ-4 has been coirfirmed by the atomic pair distribution function (PDF) analysis. The synthetic methods, structures, spectroscopic properties, and magnetic behavior of some electrides have been reviewed. Theoretical study on structural and electronic properties of inorganic electrides has also been addressed recently. ... 

Pore dimensions can be determined also by positron annihilation lifetime spectroscopy (PALS). Positron in a solid can create a bound structure with an electron, called positronium (Ps). Its triplet state (ortho-Ps) has an intrinsic lifetime in vacuum 142 ns, but when trapped in a free volume, like a pore, it lives shorter. The o-Ps lifetime is... 

Almost a linear dependence between pore size and positrons lifetime can be observed which was not clearly obtained in previous studies. This relationship is expected because when the pores are wider the probability of interaction between the positrons and the surface electron density in the pore walls decreases. This results in a lower rate of positrons annihilation with the surrounding electrons and then a higher lifetime. A simple model for the annihilation process can be constructed assuming that the positron is trapped in a spherical pore of radius R of constant potential. The resolution of the Schroedinger equation shows that the lifetime of positrons is a function of R [5]. 

In summary, we first of all have used TEM results to determine that rp 20 nm and Np 5 x 1015 cm-3, and thus have been able to calculate the sample porosity P = (4/3)jrrp3Np s= 0.2. That is, about 20 % of the free-carrier loss in our sample is due to the removal of material. Then, an analysis of DLTS results has shown that acceptorlike surface states on the pores produce a depleted volume of radius w 34 nm around each pore. Therefore, the total fractional volume depleted of free electrons is (4/3)nw3Np 0.8. This means that about 60 % of the carrier depletion is due to traps on the pore surfaces, not the pores themselves. The calculated carrier depletion (80%) is quite consistent with C-V measurements (Figure 9.2), which indicate that n (averaged over the depleted regions [3]) has fallen from mid-1018 to about 1017 cm-3 in the region sampled by the DLTS experiment. 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

The open and closed conformations on binding ligands to the nicotinic acetylcholine receptor channel were already trapped and observed by electron microscopy. Here again, the cooperative binding of ligands induces large rotational movements of the pore-defining subunit-. 

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