Effective empty trap density

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. ... 

This effect (not seen up to now) can be explained as follows The brush represents a much lower (5 times) electron density than the silica body nevertheless the potential step vacuum-brash is sufficiently high (more than kT) to be a trap for positronium, and o-Ps is confined in the empty part. The radius of this part is smaller, but Ps wavefunction penetrates a thick brash layer only, and depending on electron density (see Eq.(3)), is much smaller than in pure silica, so the lifetime increases. If the potential step at vacuum-brash boundary is small the penetration range 1/x increases, but the alkane layer is sufficiently thick to prevent Ps reaching silica. Removal of the hydrocarbon phase increases the radius, but increases too, giving as a result lifetime shortening. 

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