W.F. Magalhães

Positronium formation and inhibition in binary solid solutions on Al(III) and Co(III) tris(acetylacetonates)

charge stabilization at specific sites. These results also indicated that the positron annihilation parameters are sensitive to the liability of the solids to charge transfer processes. Furthermore, recent measurements on coordination and organometallic transition

A characterization study of xerogel silicapropylaniline powders

Abstract Silicapropylaniline nanometric materials with varying organic content were obtained using a sol–gel synthesis. By increasing the organic load, the scanning electron microscopy technique shows a slight increase in the average size of aggregated particles. N2 isotherms and positron annihilation lifetime spectroscopy measurements show that the average pore size decreases accompanied by a surface area reduction. FTIR thermal analysis was used to estimate the thermal stability of the organic phase and also to detect the presence of trapped organic groups in closed pores. From the organic coverage and surface area measurements the surface density of the immobilized organic molecules as well as the average intermolecular distance between them could be estimated.

Formation of positronium and kinetics of its quenching reactions in benzene solutions

Abstract The quenching reactions of positronium with the diamagnetic cobalt and with the paramagnetic chromium acetylacetonates have been studied in benzene solutions at various temperatures by both lifetime spectroscopy and the Doppler broadening of the annihilation radiation lineshape technique, DBARL. Both solutes inhibit Ps formation and quench its lifetime, respectively by redox and by spin conversion reactions for the Co(III) and Cr(III) compounds. The decrease upon heating of the overall apparent quenching rate constants and the Doppler data are both well explained on the basis of a new kinetic model involving the reversible formation of a Ps complex with solute S, [PsS], as an intermediate step toward Ps oxidation or spin conversion.

Solid Eu(III) complexes studied by positron annihilation, optical and Mössbauer spectroscopies: insights on the positronium formation mechanism.

In this work, positron annihilation lifetime (PALS), Doppler broadening annihilation radiation lineshape (DBARL), Mössbauer and optical spectroscopies measurements were performed in Eu(III) dipivaloylmetanate complex, Eu(dpm)(3), at 295 and 80 K. The Eu(dpm)(3) complex is not luminescent at 298 K and does not form positronium. On the other hand, it is highly luminescent at 80K, but still does not form positronium. The absence of positronium formation at 80K cannot be explained by a ligand/metal charge transfer process. We found strong evidences that the electronic delocalization does not occur at both temperatures. Despite the Mössbauer results being inconclusive regarding the Eu(III)/Eu(II) reduction hypothesis, previous results showing positronium formation in other Eu(III) complexes suggest that this process is not occurring. Thus, more studies are needed to explain the absence of positronium in Eu(III) complexes.

Inhibition of positronium formation and its quenching reactions in binary solid solutions and mixtures of metal tris-2,2,6,6-tetramethyl-3,5-heptanedionate (dpm) complexes

Abstract The inhibition of positronium formation and its quenching reactions have been studied in binary solid solutions formed by Al(dpm) 3 as a matrix, and Co(dpm) 3 , Fe(dpm) 3 and Mn(dpm) 3 as guest molecules, by lifetime and Doppler broadening spectroscopies. In all the solid solutions studied the quenching of positronium lifetime by redox reactions was observed. The total inhibition and quenching rate constants determined for each system are similar. The full-width at half-maximum of the annihilation line increases with increasing guest molecule mole fraction, indicating the presence of redox reactions combining with inhibition processes. The corresponding mixtures were studied as blanks.

Positron annihilation study in binary molecular solid solutions of metal acetylacetonate complexes using positron annihilation lifetime (PAL) and Doppler broadening (DBS) spectroscopies

Abstract PAL and DBS techniques have been used to study inhibition of positronium (Ps) formation in binary molecular solid solutions of Al(III) (tris)-acetylacetonate (acac), as matrix, and Co(III), Fe(III), Ru(III), Rh(III), and Ir(III) (tris)-acetylacetonates, as guest molecules. For all the studied systems, no Ps quenching reactions have been observed. For Al (1− x ) Co ( x ) (acac) 3 , Al (1− x ) Fe ( x ) (acac) 3 , and Al (1− x ) Ru ( x ) (acac) 3 systems, a strong inhibition of Ps formation was observed with increasing of guest molecule concentration. The obtained Ps yields measured by PAL were fitted by the Stern-Volmer equation. The DBS results confirm the inhibition constants ( k ) previously determined by PAL technique, except for the Al (1− x ) Ru ( x ) (acac) 3 system, where the experimental and calculated Doppler broadening curves are quite different. Only in Al (1− x ) Ir ( x ) (acac) 3 system, the inhiition effect was not detected. The highest total inhibition constants belong to Al (1− x ) Co ( x ) (acac) 3 and Al (1− x ) Ru ( x ) (acac) 3 systems and the smallest one to Al (1− x ) Rh ( x ) (acac) 3 system. The magnitude of the obtained k values was associated with the ability of the guest complexes to scavenge electrons and are closely related to the redox properties of the complexes. X-ray diffraction measurements were made to confirm the hypothesis that all studied samples are single phase systems and that the introduction of the guest molecules in the matrix lattice does not result in significant crystalline changes. These results are well explained in the framework of the spur model and show that the chemical properties determine the Ps yields in molecular solids, regardless of their free volume characteristics.

ANALYTICAL APPLICATION OF POSITRON LIFETIME SPECTROSCOPY IN THE CHARACTERIZATION OF BINARY SOLID SOLUTIONS

Positron Lifetime Spectroscopy (LS) measurements were performed in a series of binary molecular solid solutions of the general formula M1−xGxL3, where L(ligand)=acetylacetone, dipivaloylmethane, or N-benzoyl-N-phenylhydroxylamine, M=Al(III), Ga(III) or In(III), as matrix, and G=Cr(III), Mn(III), Fe(III), Co(III), Ru(III), Rh(III) and Ir(III), as guest molecules, and the corresponding mechanical mixtures. For the solid solutions, the o-Ps yield values (I3) decrease very rapidly with the increase of the guest mole fraction, showing a high efficient Ps inhibition process, while for the corresponding mechanical mixtures, these values decrease linearly with the increase of the “guest” molecule concentration, indicating the presence of two distinct phases. These results confirm our previous proposal that the LS technique is able to characterize solid solutions formed by one matrix, in whichI3 values are high, and one guest, in which theI3 values are very low.

Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Substitutional Tb1-xEux(dpm)3 Solid Solutions Studied by Optical and Positron Annihilation Spectroscopies

Positron annihilation lifetime (PALS) and photoluminescence spectroscopies measurements were performed in Tb(III) and Eu(III) dipivaloylmethanates, Tb(dpm)3 and Eu(dpm)3, and also on their binary solid solutions of general formula Tb1-xEux(dpm)3. A correlation between the 5D4 Tb(III) energy level lifetime and the positronium formation probability was observed, indicating that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition. From these results, a new model is proposed, showing that excited electronic states have a relevant role in the positronium formation mechanism.

Positron Annihilation Studies on Reactor Irradiated and Thermal Annealed Ferrocene

Retention and thermal annealing following (η,γ) reaction in solid ferrocene, Fe(C5H5)2, were studied by positron annihilation lifetime spectroscopy (PAL). Positronium (Ps) formation was observed in the non-irradiated compound with a probability or intensity (/,) of 30%. Upon irradiation of the compound with thermal neutrons in a nuclear reactor, I3 decreases with increasing irradiation time. Thermal treatment again increases /, values from 16% to 25%, revealing an important proportion of molecular reformation without variation of the ortho-positronium lifetime (τ,). These results point out the major influence of the electronic structure as determining the Ps yields in the pure complex. In the irradiated and non irradiated complexes the results are satisfactorily explained on the basis of the spur model.

Estimação da Incerteza de Medição de um Procedimento Analítico Gravimétrico Aplicado À Hidrossedimentologia Fluvial - Um Exemplo Didático para o Ensino de Metrologia Química nos Cursos Técnico e de Graduação em Química

The measurement uncertainty is fundamental to assess the ‘quality’ of the results of any experiment. The measurement result with the corresponding uncertainty statement ensures its fitness for purpose by giving a measure of the confidence that can be placed on the results. This paper presents a detailed and didactic evaluation of the uncertainties for suspended sediment concentration determined by a gravimetric analytical method by filtration and for the river solid discharge using both the variance propagation law and its particular cases for sums (subtractions) and multiplications (divisions). The sources of uncertainties considered to estimate the uncertainty of the suspended sediment concentration were the weighing (resolution and calibration of the balances), the resolution of the conversion factor, the intermediate precision of the whole analytical procedure and the repeatability precision due to the direct measurements. The major uncertainty contributions for the suspended sediment concentration uncertainty were the intermediate precision and weighing of the samples. DOI: 10.5935/1984-6835.20160058