Positronium scattering by atoms and molecules at low energies

The recent theoretical and experimental activities in positronium (Ps) scattering by atoms and molecules are reviewed with special emphasis at low energies. We critically compare the results of different groups - theoretical and experimental. The theoretical approaches considered include the R-matrix and close-coupling methods applied to Ps-H, Ps-He and Ps-Li scattering, and a coupled-channel approach with a nonlocal model potential for Ps scattering by H, He, H-2, Ne, Ar, Li, Na, K, Rb, Cs and Ps and for pickoff quenching in Ps-He scattering. Results for scattering lengths, partial. total and differential cross-sections as well as resonance and binding energies in different systems are discussed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Collision of positronium with atoms, molecules and ions using a modified coupled-channel framework

Scattering of positronium (Ps) from atoms (H, He, Ne, Ar), molecule (H(2)) and ion (He(+)) have been investigated using a coupled-channel (CC) formalism with a regularised non-local exchange potential. The advantage of using such a regularized exchange potential in the close-coupling formalism and the normalizability aspect of the solution at low energies with a minimum effective coupling are discussed. Results for the elastic and total scattering cross-sections, resonance and binding energies in Ps-H, and pick-off annihilation results in Ps-He are found to be in excellent agreement with measurements and variational predictions. (C) 2000 Elsevier B.V. B.V. All rights reserved.

Stability and collapse of a hybrid Bose-Einstein condensate of atoms and molecules

The dynamics of stability and collapse of a trapped atomic Bose-Einstein condensate (BEC) coupled to a molecular one is studied using the time-dependent Gross-Pitaevskii (GP) equation including a nonlinear interaction term which can transform two atoms into a molecule and vice versa. We find an interesting oscillation of the number of atoms and molecules for a BEC of fixed mass. This oscillation is a consequence of continuous transformation in the condensate of two atoms into a molecule and vice versa. For the study of collapse an absorptive contact interaction and an imaginary quartic three-body recombination term are included in the GP equation. It is possible to have a collapse of one or both components when one or more of the nonlinear terms in the GP equation are attractive in nature, respectively.

Three helium atoms and the scaling limit

We show that a scaling limit approach, previously applied in three-body low-energy nuclear physics, is realized for the first excited state of He-4 trimer. The present result suggests that such approach has a wider application.

Study of a degenerate dipolar Fermi gas of Dy-161 atoms

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Radiative association of C and P, and Si and P atoms

The rate coefficients for the formation of carbon monophosphide (CP) and silicon monophosphide (SiP) by radiative association are estimated for temperatures ranging from 300 to 14 100 K. In this temperature range, the radiative association rate coefficients are found to vary from 1.14 x 10(-18) to 1.62 x 10(-18) cm(3) s(-1) and from 3.73 x 10(-20) to 7.03 x 10(-20) cm(3) s(-1) for CP and SiP, respectively. In both cases, rate coefficients increase slowly with the increase in temperature.

Radiative association of Ti and O atoms

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

The interaction between atoms of Au and Cu with clean Si(111) surface: A study combining synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

An ab initio study of oxygen vacancies and doping process of Nb and Cr atoms on TiO2 (110) surface models

We theoretically investigated how the formation of oxygen vacancies and the addition of niobium and chromium atoms as dopants modify the varistor properties of TiO2. The calculations were carried out at the HF level using a contracted basis set, developed by Huzinaga et al.. to represent the atomic centers on the (110) surface for the large (TiO2)(15) cluster model. The change of the values for the net atomic charges and band gap after oxygen vacancy formation and the presence of dopants in the lattice are analyzed and discussed. It is shown that the formation of oxygen vacancies decreases the band gap while an opposite effect is found when dopants are located in the reduced surface. The theoretical results are compared with available experimental data. A plausible explanation of the varistor behavior of this system is proposed. (C) 1997 John Wiley & Sons, Inc.

Local order around of germanium atoms in Ga10Ge25S65 glass by EXAFS

Irreversible photoexpansion effect has been observed in amorphous Ga10Ge2S65 glasses when its surface was exposed to light with energy greater than the band gap, 3.52 eV. A volume change of about 5% was reached in bulk samples by controlling illumination time and the laser power density. To understand the atomic scale processes of the photoexpansion effect, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used as a local probe of the germanium environment in the glass samples before and after illumination. Modifications are observed in the average coordination shell around Ge atoms in the illuminated sample compared to the non-illuminated one. For the non-illuminated sample, the Ge coordination shell is described by a distorted tetrahedron of sulfur atoms at around 2.20 Angstrom. After illumination, the EXAFS signal can be explained by introducing an additional contribution to this average environment. Based on an analysis of the EXAFS data we proposed a two-shell model of 0.5 oxygen atoms at 2.01 Angstrom and 3.6 sulfur atoms at a 2.20 Angstrom. The existence of Ge-O bonds in the glass after illumination was confirmed by infrared measurements. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Local order around tungsten atoms in tungstate fluorophosphate glasses by X-ray absorption spectroscopy

X-ray absorption spectroscopy was used to study the local environment of tungsten atoms in NaPO3-BaF2,-WO3 glasses and the results were compared with crystalline references Na2WO4 and WO3. XANES measurements at the W-L-1 edge allowed to determine a distorted octahedral environment of tungsten atoms in these glasses similar to the local order of tungsten in monoclinic WO3. Extended X-ray absorption fine structure (EXAFS) has been used as a local probe to monitor the effect of WO3 concentration on the tungsten environment. Based on an analysis of the EXAFS data, we proposed a three-shell model of oxygen atoms around tungsten as in monoclinic WO3. With increasing WO3 concentration, it was found that R-2 decreases from 1.96 to 1.92 angstrom whereas R-3 increases from 2.07 to 2.12 angstrom. (c) 2005 Elsevier B.V. All rights reserved.

Positronium scattering from ions, atoms and molecules

Elastic and inelastic scattering of positronium (Ps) by H, He, He(+) and H(2) have been studied using coupled-channel calculations with a regularised nonlocal model exchange potential. Suitability and reliability of the theoretical scheme have been demonstrated by studying scattering cross-sections in various systems and also by addressing critical dynamic features like binding and resonances of Ps to open-shell-atoms. Results are found to be in good agreement with Variational predictions and experimental observations. (C) 2000 Elsevier B.V. Ltd. All rights reserved.

Self-bound droplet of Bose and Fermi atoms in one dimension: Collective properties in mean-field and Tonks-Girardeau regimes

We investigate a dilute mixture of bosons and spin-polarized fermions in one dimension. With an attractive Bose-Fermi scattering length the ground state is a self-bound droplet, i.e., a Bose-Fermi bright soliton where the Bose and Fermi clouds are superimposed. We find that the quantum fluctuations stabilize the Bose-Fermi soliton such that the one-dimensional bright soliton exists for any finite attractive Bose-Fermi scattering length. We study density profile and collective excitations of the atomic bright soliton showing that they depend on the bosonic regime involved: mean-field or Tonks-Girardeau.

Enhanced emission from Eu(III) beta-diketone complex combined with ether-type oxygen atoms of di-ureasil organic-inorganic hybrids

Organic-inorganic hybrids, named di-ureasils and described by polyether-based chains grafted to both ends to a siliceous backbone through urea cross linkages, were used as hosts for incorporation of the well-known coordination complex of trivalent europium (Eu3+) ions described by the formula [Eu(TTA)(3)(H2O)(2)] (where TTA stands for thenoyltrifluoroacetone). By comparing with Eu3+-doped di-ureasil without complex form the new materials prepared here enhanced the quantum efficiency for photoemission of Eu3+ ions. The enhancement can be explained by the coordination ability of the organic counterpart of the host structure which is strong enough to displace water molecules in [Eu(TTA)(3)(H2O)(2)] from the rare earth neighbourhood after the incorporation process. High intensity of Eu3+ emission was observed with a low non-radiative decay rate under ultraviolet excitation. The quantum efficiency calculated from the decay of D-5(0) emission was 74%, which in the same range of values previously obtained for the most efficient Eu3+ coordination compounds reported in literature. Luminescence, X-ray absorption and infrared absorption results considered together leads to a picture where the first coordination shell of Eu3+ is composed of the 6 oxygen atoms of the 3 beta-diketonate ligands and 2 ether-like oxygen atoms of the host. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Stress-induced ordering due heavy interstitial atoms in Nb-0.3 wt.% Ti alloys

The mechanical properties of metals with bee structure, such as niobium and their alloys, are changed of a significant way by the introduction of heavy interstitial elements. These interstitial elements (oxygen, for example) present in the metallic matrix occupy octahedral sites and constitute an elastic dipole of tetragonal symmetry and might produce anelastic relaxation. Polycrystalline samples of Nb-0.3 wt.% Ti (Nb-Ti) alloy with oxygen in solid solution were analysed. The anelastic spectroscopy measurements had been made in a torsion pendulum, with frequencies in the Hz range, in a temperature range between 300 and 700 K. The results showed thermally activated relaxation structures were identified four relaxation process attributed to stress-induced ordering of single oxygen, nitrogen and carbon atoms around niobium and stress-induced ordering of single oxygen atoms around titanium atoms. (c) 2005 Elsevier B.V. All rights reserved.