Genetic algorithms for positron lifetime data

Recently, genetic algorithms have been applied for ultrafast optical spectrometry in systems with several convoluted lifetimes. We apply these algorithms and compare the results with POSFIT (by Kirkegaard and Eldrup) and LT programme (by Kansy). The analysis was applied to three types of samples: molybdenum monocrystals, Czochralski-grown silicon with oxygen precipitates, Si with under-surface cavities obtained by He + H ion co- implantation. In all three tests, the genetic algorithm performs very well, in particular for short lifetimes. Further developments to model the resolution function in genetic algorithms are needed

Decoration of Buried Surfaces in Si detected by Positron Annihilation Spectroscopy

The terminations of buried surfaces of two different cavity types (nano- and microcavities) produced in the same He+-H+ co-implanted p-type Si (100) sample annealed at 900 °C, are studied and characterized by positron annihilation spectroscopy. The characterization was carried out by means of three complementary positron techniques: Doppler broadening and coincidence-Doppler broadening spectroscopy with a continuous slow positron beam, and lifetime spectroscopy with a pulsed slow positron beam. It was found that the nanocavities have a pristine surface of Si, while the surfaces of the microcavities, formed below protruding blisters, are oxygen decorated. This case study opens the interesting use of the positron spectroscopy tool in the topical subject of empty space for microelectronics applications.Fil: Brusa, R.S.. Universita degli Studi di Trento; ItaliaFil: Macchi, Carlos Eugenio. Universita degli Studi di Trento; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; ArgentinaFil: Mariazzi, S.. Universita degli Studi di Trento; ItaliaFil: Karwasz, G.P.. Universita degli Studi di Trento; ItaliaFil: Egger, W.. Universität der Bundeswehr München; AlemaniaFil: Sperr, P.. Universität der Bundeswehr München; AlemaniaFil: Kögel, G.. Universität der Bundeswehr München; Alemani

Protonated water clusters

A complete set of protonated water clusters conformers up to n = 8 have been evaluated by ab-initio Hartree-Fock method in 6-311G** molecular orbitals basis set and with quadratic complete basis set method. Configurations of ground-state conformers are in a good accord with those available in literature but present dissociation energies for the light clusters agree better with experimental values. A tentative analysis of experimental conditions for observation of certain clusters is presented in comparison to selected formation paths of clusters. Dissociation energies show a broad range of values

Positron and electron scattering on atoms and molecules–modified effective range theory revisited

New experiments on the very low-energy electron and positron scattering allow to verify the old question on applicability of modified effective range theory (MERT). We perform it using an analytical solution of the Schrödinger equation with the long-range polarization potential. In this work two atomic (He, Ar) and molecular (H2, CH4) target are studied using this approach. Total cross sections were used for obtaining parameters characterizing the scattering phase shifts related to the short-range interaction potential; differential cross sections were used for comparison. Differently from previous works, we conclude that MERT with few (2-3) partial waves applies very well up to energies of few eV in all four targets studied. For positrons, reliable experimental data indicate occurrence of zeros in the s-wave phase shifts for all four targets. This should be recognized as Ramsauer-Townsend minima

Total cross sections for positron scattering on Argon and Krypton at intermediate and high energies

Preliminary total cross section data for positron scattering on argon and krypton between 250 and 5000 eV are presented. A Ramsauer-type apparatus, previously used for electron-scattering measurements has been modified. To check for the convergence between electron and positron total cross sections, a semiempirical analysis has been applied. The analysis for argon indicates an asymptotic convergence for electron and positron projectiles – the high energy parameters of the both fits are equal within the uncertainty limits. No such a clear conclusion is possible with the present data up to 5000 eV for krypton

A very low-energy apparatus for positron scattering on atoms and molecules

An apparatus for very low-energy positron scattering measurements in gases is presented. The apparatus uses electrostatic optics, two stages of positron moderation, magnetic focusing in the scattering chamber and a pseudo-random time-of-flight correlation technique

Reactions in nanofilms of trifluoroacetic acid (CF3COOH) driven by low energy electrons

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