Low-energy correlations in the positronium-hydrogen-atom system

Employing a nonlocal model potential for electron exchange we study positronium-hydrogen-atom (Ps-H) scattering using a five-state coupled-channel model allowing for Ps(2s,2p)H(1s) and Ps(1s)H(2s,2p) excitations. We find remarkable correlations among S-wave Ps-H binding energy, scattering length, effective range, and resonance energy in the electronic singlet state. Using these correlations we predict fairly accurate values of singlet Ps-H scattering length (3.50a0) and effective range (1.65a0).

Specific and nonspecific collapse in protein folding funnels

Experiments with fast folding proteins are beginning to address the relationship between collapse and folding. We investigate how different scenarios for folding can arise depending on whether the folding and collapse transitions are concurrent or whether a nonspecific collapse precedes folding. Many earlier studies have focused on the limit in which collapse is fast compared to the folding time; in this work we focus on the opposite limit where, at the folding temperature, collapse and folding occur simultaneously. Real proteins exist in both of these limits. The folding mechanism varies substantially in these two regimes. In the regime of concurrent folding and collapse, nonspecific collapse now occurs at a temperature below the folding temperature (but slightly above the glass transition temperature).

Frequency upconversion in rare-earth doped fluoroindate glasses

We present recent results on frequency upconversion (UPC) obtained in fluoroindate glasses (FIG) doped with Ho3+, Tm3+ and Nd3+ ions and codoped with Pr3+/Nd3+ and Yb3+/Tb3+ ions. The results for the Ho3+-doped samples show strong evidence of energy transfer (ET) between Ho3+ ions resonantly excited at 640 nm. The origin of the blue-green upconverted fluorescence observed was identified and the dynamics of the signals revealed the pathways involved in the UPC process. In the case of Tm3+-doped FIG, the samples were resonantly excited at 650 nm and the main mechanism that contributes for the red-to-blue upconversion is excited-state absorption (ESA). The FIG samples codoped with Pr3+/Nd3+ were excited at 588 nm in resonance with transitions starting from the ground state of the Nd 3+ and the Pr3+ ions. It was observed that the presence of Nd3+ ions enhanced the Pr3+ emission at 480 nm by two orders of magnitude. Multiphonon (MP)-assisted upconversion is also discussed for Nd3+-doped FIG pumped at 866 nm. Emission at 750 nm with a peculiar linear dependence with the laser intensity was observed and explained. A rate-equation model that includes MP absorption via thermally coupled electronic excited states of Nd3+ was developed and describes well the experimental results. The role played by effective phonon modes is clearly demonstrated. MP-assisted UPC process was also studied in Yb3+/ Tb3+-codoped FIG samples excited at 1064 nm, which is off-resonance with electronic transitions starting from the ground state. It was determined that the mechanism leading to Tb3+ emission in the blue is due to ET from a pair of excited Yb3+ ions followed by ESA in the Tb 3+ ions. © 2002 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS.

Structural control of photoluminescence of four poly(urethane-urea-co-1,3,5-triazine)s: Synthesis and characterization

A series of segmented poly(urethane-urea)s containing 1,3,5 triazine in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers in which the chromophore concentration varied from 4.2% to 18.1%. Although triazine emission is located in the UV region, the films with higher content of the chromophore emitted a visible blue light (425 nm) when excited at the very red-edge of the absorption band. The photophysical properties of the materials were strongly dependent on the relative amount of triazine moieties along the main chain. Isolated moieties emit in copolymers with small amount of triazine groups, indicating that even though in solid state, these moieties tend to be apart. Two photophysical consequences were observed when the amount of triazine increases: there is some energy transfer process involving isolated moieties with consequent decrease of the lifetime and an additional red-edge emission attributed to aggregated lumophores. The mono-exponential decay observed for the isolated form is substituted by a bi-exponential decay of the aggregated species. The materials were not strong emitters, but since the N-containing triazine moieties are good electron transport groups, the polymers have potential application as electron transport enhancers in various applications. © 2006 Elsevier B.V. All rights reserved.

On nonlinear dynamics in a free fluid surface of a tank excited by a non-ideal power source

We investigate the nonlinear oscillations in a free surface of a fluid in a cylinder tank excited by non-ideal power source, an electric motor with limited power supply. We study the possibility of parametric resonance in this system, showing that the excitation mechanism can generate chaotic response. Additionally, the dynamics of parametrically excited surface waves in the tank can reveal new characteristics of the system. The fluid-dynamic system is modeled in such way as to obtain a nonlinear differential equation system. Numerical experiments are carried out to find the regions of chaotic solutions. Simulation results are presented as phase-portrait diagrams characterizing the resonant vibrations of free fluid surface and the existence of several types of regular and chaotic attractors. We also describe the energy transfer in the interaction process between the hydrodynamic system and the electric motor. Copyright © 2011 by ASME.

Strongly luminescent, highly ionic europium in a lanthanum diphenylphosphinate matrix

A series of insoluble, chemically inert and thermally stable compounds La1-xEux(DPP)3 (x = 0.50, 0.20, 0.10, and 0.050; DPP = diphenylphosphinate) was synthesized and characterized by elemental and thermogravimetric analysis, FT Infrared spectroscopy and X-ray powder diffraction. Luminescence spectroscopy at both 77 and 298 K showed changes in the intensity of the hypersensitive transition 5D 0 → 7F2 of Eu3+ which are dependent of the excitation wavelength, suggesting that the europium occupies two different sites in the compounds. The large quantum efficiency and quantum yield, as well as the long radiative lifetime of the 5D0 Eu3+ level of the series of compounds, which are desirable qualities for light-conversion molecular devices, are discussed in terms of the interactions and the energy transfer process between the ligands and the metal ion. © 2012 Elsevier B.V. All rights reserved.

Luminescent properties of hybrid materials prepared by the polymeric precursor method

Rare earth complexes (RE) can be incorporated in silica matrixes, originating organic/inorganic hybrid materials with good thermal stability and high rare earth emission lines. In this work, the hybrid material was obtained by the polymeric precursor method and ultrasonic dispersed with spherical silica particles prepared by the Stöber Method. The Raman spectra indicated that the Eu3+ ions are involved in a polymeric structure formed as consequence of the chelation and polyesterification reactions of this ion with citric acid and ethylene glycol. After the ultrasonic stirring, 2-hydroxynicotinic ligand will also compose this polymeric rigid structure. The TGA/DTA analysis showed that this polymeric material was thermal decomposed at 300 °C. Moreover, this process allows the chelating process of the 2-hydroxynicotinic acid ligand to the Eu3+ ions. The 29Si NMR showed that the ultrasonic dispersion of the reactants was not able to promote the functionalization of the silica particles with the 2-hydroxynicotinic acid ligand. Moreover, heat treatment promotes the [Eu(HnicO2)3] complex particles incorporation into silica pores. At this temperature, the TGA curve showed that only the thermal degradation of ethylene glycol and citric acid used during the experimental procedure occurs. The silica and hybrid materials are composed by spherical and aggregated particles with particle size of approximately 450 nm, which can be influenced by the heat treatment. These materials also present an absorption band located at 337 nm. The photoluminescent study showed that when the hybrid samples were excited at 337 nm wavelength, the ligand absorbs the excitation light. Part of this energy is transferred to the Eu3+ ion, which main emission, 5D0→ 7F2, is observed in the emission spectrum at 612 nm. As the heating temperature increases to 300 C, the energy transfer is more favorable. The lifetime values showed that the Eu3+ emission is enhanced due to the energy transfer process in the powders. © 2013 Elsevier B.V. All rights reserved.

Interação do complexo luminescente [Eu(tta)3] com sílica mesoporosa

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Modulação das propriedade ópticas de semicondutores pela incorporação de íons terras raras

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Propriedades e mecanismos de cintilação dos materiais 'GD''AL''O IND.3' e ('GD''LU')'AL''O IND.3' dopados com 'EU POT. 3+', 'TB POT. 3+' e 'PR POT. 3+'

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Híbridos orgânicos-inorgânicos siloxanos àcidos-graxos emissores de luz branca

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Complexos de európio e /ou de zinco com ácido 3-piridinocarboxílico

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Fotoluminescência e transporte elétrico em 'SNO IND. 2' dopado com os íons terras-raras 'ER POT. 3'/' e 'EU POT. 3'/'

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estudo dos difenilfosfinatos de lantânio ativados por cério, európio e térbio emissores luminescentes multicores

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Vidros a base de óxido de telúrio para dispositivos fotônicos

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