102,586 research outputs found
G. Imbriano, Der Begriff der Politik. Die Moderne als Krisenzeit im Werk von Reinhart Koselleck
No full textRecensione a G. Imbriano, Der Begriff der Politik. Die Moderne als Krisenzeit im Werk von Reinhart Kosellec
Background measurement with constant detector geometry in positronium three quanta annihilation experiments
No full textShapes of the free volume holes in amorphous polymers as estimated by positron annihilation lifetime spectroscopy
No full textIntermolecular spaces in polymer chains form the free volume, a useful concept to understand mechanical and transport properties of polymers. Quantification of the free volume can be obtained theoretically, using appropriate lattice models, as well as experimentally, through suitable probes. Among these, positronium (Ps) has become widespread due to the nondestructive character of the technique and the correlation between the Ps lifetime and the size of the holes. In most of the investigations, the cavity is approximated to a sphere. However, this may bias the evaluation of the free volume fraction. We show that by coupling results from Ps lifetime and specific volume measurement for amorphous polymers at equilibrium and the predictions of the Simha–Somcynsky equation of state, it is possible to shed light on dimensions of the holes as well as on their morphology
Positronium trapping in small voids: influence of their shape on positron annihilation results
No full textPositron annihilation lifetime spectroscopy is extensively used to get information on the sizes of the nanoholes existing in porous and nonmetallic solids. Cavities are most often modeled as spherical, although their shape is irregular. In the present paper the relationship between lifetime distribution as extracted from a time annihilation spectrum and size (and volume) distributions of the holes for some nonspherical geometries is discussed. It is shown that the same lifetime distribution can produce cavity size distributions having different momenta (centroid and standard deviation), if the shape of the hole is modified. Nevertheless, the ratio between the two momenta is largely independent of the geometry. Furthermore, parameters derived from the positron measurements (such as the free volume fraction) are not strongly influenced by the choice of the hole geometry
Formal calculation of exchange correlation effects on annihilation lifetimes of positronium confined in small cavities
No full textPositronium atoms (Ps) are widely used as a probe to characterize voids or vacancies in non-metallic materials. The annihilation lifetime of trapped Ps is strongly modified by pickoff, depending on the cavity size and on the appropriate external electron density. The connection between these material characteristics and Ps annihilation lifetimes is usually based on models that do not consider the requirements of full electron indistinguishability, that must be taken into account for a correct description of pickoff annihilation processes. In this report, we provide a formal theoretical framework in which exchange correlation effects between confined Ps and surrounding electrons are introduced in a natural way, giving a clear and versatile picture of the various contributions to pickoff annihilation. Moreover, our results provide a simple explanation of the lowering of the contact density (the Ps-electron density at the positron position) as a direct consequence of the electrons indistinguishability, at variance with previous interpretation based on spatial deformations of Ps wavefunction. Calculations are performed within the 'symmetry adapted perturbation theory' approach, and the results are compared with available experimental data on Ps lifetimes for polymers and molecular solids. Finally, our analysis gives a formal justification to the approximations involved in early models based on the well known Tao-Eldrup approach, and gives a simple interpretation of Ps properties in subnanometric voids
Positronium confined in nanocavities: The role of electron exchange correlations
No full textPositronium atoms (Ps) are commonly employed as a probe to characterize nanometric or subnanometric voids or vacancies in nonmetallic materials, where Ps can end up confined. The annihilation lifetime of a trapped Ps is strongly modified by pickoff and depends on the cavity size and on the electron density in the confining cavity surface. Here, we develop a theory of the Ps annihilation in nanocavities based on the fundamental role of the exchange correlations between the Ps-electron and the outer electrons, which are not usually considered but must be considered to correctly theorize the pickoff annihilation processes. We obtain an important relation connecting the two relevant annihilation rates (for the p-Ps and the o-Ps ) with the electron density, which has the property of being totally independent of the geometrical characteristics of the nanoporous medium. This general relation can be used to gather information on the electron density and on the average cavity radius of the confining medium, starting from the experimental data on PALS annihilation spectra. Moreover, by analyzing our results, we also highlight that a reliable interpretation of the PALS spectra can only be obtained if the rule of 1/3 between the intensities of p-Ps and o-Ps lifetimes can be fulfilled
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