Increase of the blue upconversion emission in YAG:Tm3+ nanopowders by codoping with Yb3+ ions

The YAG nanopowders were prepared by a co-precipitation method using nitrate and ammonium hydrogen carbonate as raw materials. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted. Therefore, single (Tm³⁺) and codoped (Tm³⁺–Yb³⁺) YAG nanopowders with a size between 40–90 nm have been obtained.Blue upconversion emission at around 480 nm has been found in YAG: Tm³⁺ nanopowders under excitation to the ³H₄ level of Tm³⁺ at around 800 nm. However, this upconversion emission in nanopowders codoped with Tm³⁺–Yb³⁺ ions is increased by a factor of about 10. The analysis of the temporal evolution of the involved levels and the dependence of the upconversion intensity on the pump power at 800 nm allowed to distinguish the upconversion mechanism. In YAG: Tm³⁺ nanopowders the upconversion mechanism is due to excited state absorption processes. However, in the codoped samples, Yb³⁺ ions acts as the sensitizers; in consequence, the blue upconversion is strongly increased.     

Particle resonance in the Dirac equation in the presence of a delta interaction and a perturbative hyperbolic potential

We show that the energy spectrum of the one-dimensional Dirac equation, in the presence of an attractive vectorial delta potential, exhibits a resonant behavior when one includes an asymptotically spatially vanishing weak electric field associated with a hyperbolic tangent potential. We solve the Dirac equation in terms of Gauss hyper-geometric functions and show explicitly how the resonant behavior depends on the strength of the electric field evaluated at the support of the point interaction. We derive an approximate expression for the value of the resonances and compare the results calculated for the hyperbolic potential with those obtained for a linear perturbative potential. Finally, we characterize the resonances with the help of the phase shift and the Wigner delay time.     

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. For some of the reactions, plots of k_obs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T^±) and the other anionic (T^?). In some cases, there is a kinetically significant proton transfer from T^± to an amine to yield T^?. Values of the rate micro constants k₁ (amine attack to form T^±), k_?1 (its back step), k₂ (nucleofuge expulsion from T^±), and k₃ (proton transfer from T^± to the amine) are determined for some reactions. The Brønsted plots for k₁ are linear with slopes β₁ = 0.2–0.4 in accordance with the slope values found when T^± formation is the rate‐determining step. The sensitivity of log k₁ and log k_?1 to the pK_a of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k₂ and k₃ steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal Behaviour of Oxovanadium(IV) Complexes of Halogenated Derivatives of 8-hydroxyquinoline

The thermal behaviour of a series of oxovanadium(IV) complexes of halogenated derivatives of 8-hydroxyquinoline was investigated by means of TG and DTA measurements in oxygen atmosphere. V₂O₅ was the final pyrolysis residue in all cases.This revised version was published online in November 2005 with corrections to the Cover Date.     

Validation of a gas chromatographic method for determining fatty alcohols that compose policosanol in five-milligram film-coated tablets.

A gas chromatographic method using a packed column and 1-eicosanol as an internal standard was developed and validated for determination of the aliphatic fatty alcohols that compose policosanol in 5 mg film-coated tablets. The alcohols were analyzed as trimethylsilyl (TMS) derivatives, prepared with N-methyl-N-trimethylsilylfluoroacetamide. The method can detect degradation products with high retention times without interfering with the peaks of the active principle. Good linearity (correlation coefficient = 0.9996) and accuracy (recovery = 100.44%) were proven over a range of 50-150% of the nominal concentration. Within-day and between-day precisions at the nominal 100% value met the acceptance criteria (< 2%). Ruggedness was examined through an intralaboratory experimental study in which 7 operational changes were made and the observed results were quantitation, repeatability, resolution, and relative retention time. Among these results, only the relative retention time (tC28,C20) was significantly affected when the column used was 2.1 m instead of 3.1 m. Repeatability and reproducibility (r = 0.1506 and R = 0.2450, respectively) were obtained from a uniform-level interlaboratory test. The method is suitable for quality control and stability studies of these tablets.     

Influence of local chain dynamics on diffusion of gases in polymers as determined by pulsed field gradient NMR

Diffusion of gases in polymers below the glass transition temperature, T_g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon‐13 PFG NMR measurements of [¹³C]O₂ diffusion in polymer films at 298 K are performed. Data obtained in polymers with T_g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T_g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T_g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010     

Quinoidal Oligothiophenes: Towards Biradical Ground‐State Species

A family of quinoidal oligothiophenes, from the dimer to the hexamer, with fused bis(butoxymethyl)cyclopentane groups has been extensively investigated by means of electronic and vibrational spectroscopy, electrochemical measurements, and density functional calculations. The latter predict that the electronic ground state always corresponds to a singlet state and that, for the longest oligomers, this state has biradical character that increases with increasing oligomer length. The shortest oligomers display closed‐shell quinoidal structures. Calculations also predict the existence of very low energy excited triplet states that can be populated at room temperature. Aromatization of the conjugated carbon backbone is the driving force that determines the increasing biradical character of the ground state and the appearance of low‐lying triplet states. UV/Vis, Raman, IR, and electrochemical experiments support the aromatic biradical structures predicted for the ground state of the longest oligomers and reveal that population of the low‐lying triplet state accounts for the magnetic activity displayed by these compounds.     

Application of the polar decomposition to light scattering particle systems

We have applied the polar decomposition theorem (PDT) to the scattering matrix of simple systems such as an isolated sphere, either metallic or dielectric. The discrete dipole approximation (DDA) method has been used as an intermediate tool to calculate these matrices. We show how the parameters introduced by the PDT for describing the scattering by a single sphere can be easily interpreted. It is also shown that the PDT provides us with an alternative frame to conventional Mueller matrix analysis, by characterizing the systems by means of a group of independent parameters representing magnitudes of simple (virtual) elements. This step is necessary in order to translate other complex system matrices into manageable information.     

Modeling the Electrochemical Response of Mesoporous Materials Toward Its Application to Biomolecular Detection

The performance of an electrochemical sensor based on the ability of a probe to cross a mesoporous membrane partially blocked by an analyte is predicted using a numerical model. The system comprehends a membrane placed close to the working electrode and the signal is generated by applying square wave voltammetry. The digital simulation allows comparing the responses for different situations regarding the way in which the membrane is blocked by the sample. The developed model is compared with experimental results. The effect of the sizes of the pore, analyte and probe on the system response is evaluated.