Experiments with a high-density positronium gas

We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3.3 x 10(10) cm-2, the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and PS2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2.9 x 10(-14) cm-2.     

Positronium quenching reactions and diffusion in solutions

The temperature effect on positronium quenching rates for p-benzoquinone and iodine in several solvents was determined. The quenching rate constantk ₁ follows ak ₁∝1/η relationship better than thek ₁∝T/η one, predicted by ordinary theory. A simple theoretical treatment based on diffusion theory shows that the temperatureT of the solute, positronium, may not be the same as the temperature of the solvent if the positronium is not fully thermalized. This temperature is found to be about twice the room temperature for the solvents investigated. The findings further support the theory that positronium reactions are “diffusion controlled” in liquid solutions. The activation energies of all the positronium reactions studied are either negligible or less than 0.1 eV.     

Static and dynamic quenching of luminescent species in polymer media

A method developed for quantitative determination of static and dynamic contributions to luminescence quenching is applied to Ru(II) complexes in polymer matrices (silica gel and polystyrene), quenched by oxygen. This method is based on both intensity and lifetime quenching experiments. The curvature of intensity Stern-Volmer plots is related to the results.     

Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.     

Formation and escaping of positronium in porous SiO2 films at low temperature

Positronium formation and escaping has been studied in porous silica thin films at temperature ranging from 13 to 300 K by 2-3 gamma ratio of positronium (3γ-PAS) measurements. Nanoporous silica thin films were deposited by spin coating on p-type (1 0 0) Si substrates and thermal treated in air at temperatures of 600 °C. Two different molar ratios of porogen (polyvinylpyrrolidone) were used in the TEOS-ethanol mixture to obtain samples with close porosity and connected porosity with the surfaces. In both types of sample a reduction of the 2-3 gamma ratio of positronium was observed by decreasing the temperature. This finding, in disagreement with the theoretical expectation, is discussed on the basis of the possible quenching mechanisms.     

Interactions between positronium atoms in porous silica

Interactions between pairs of positronium (Ps) atoms confined in porous silica films have been directly observed for the first time. Because of selection rules, the nature of such interactions should depend on the structure of the porous medium: if a Ps surface state exists, dipositronium (Ps2) molecules may be created, and if there is a continuum of cavity energy levels, spin exchanging collisions may occur. Using two structurally different silica films, we have been able to isolate and study these two processes. Our data indicate that Ps2 formation occurs primarily via a Langmuir-Hinshelwood-type mechanism on the internal pore surfaces, with an interaction length of the order of 7 x 10(-8) cm, and that the effective cross section for nonthermalized Ps-Ps spin exchange quenching in porous silica is around 9 x 10(-15) cm2.     

Pick-off quenching probability of ortho-positronium per collision with atoms and molecules

Positronium quenching through its collisions with gaseous atoms and molecules is discussed. It is found that the values of the normalized pick-off quenching parameter, ¹Z_eff, at room temperature are roughly proportional to the geometric collisional cross-sections estimated from the radius of the positronium and those of the atoms and molecules derived from the viscosity. This suggests that the probabilities of the pick-off quenching of the thermalized positronium per collision with various atoms and molecules are almost constant.     

TRIPLET TO SINGLET CONVERSION OF POSITRONIUM IN OXYGEN

Using a aerogel sample with introduced oxygen gas as a variable energy positronium source,the triplet to singlet conversion of positronium in the elastic and inelastic collisions between positronium atoms and oxygen molecules has been investigated with a time selecting energy spectrometer.We have found that the conversion cross section σ_0—p is proportional to 1/v for slow positronium in the elastic collisons,where v is the mean velocity of positronium,and for the inelastic processes,the cross sections are estimated to be σ_0—p=2.1×10^－17cm² and 6.6×10^－18cm² respectively at kinetic energies of positronium near two thresholds.     

Packing of phospholipid vesicles studied by oxygen quenching of Laurdan fluorescence

Steady-state fluorescence oxygen quenching experiments were performed on phospholipid vesicles where 2-dimethylamino-6-lauroylnaphthalene (Laurdan) was inserted. The quenching efficiency was found to be much higher in vesicles in the liquid-crystalline phase with respect to the gel phase, by a factor of about 50. Since the oxygen solubility in the two phospholipid phases can differ at most by a factor of 4 based on literature values, we concluded that oxygen diffusion must be responsible for the great difference in the quenching efficiency. A relatively high quenching efficiency was also found in vesicles composed of equimolar gel and liquid-crystalline phospholipids. Simulations were performed using the linear superposition of the properties of the pure phases to demonstrate that, in the case of vesicles composed of coexisting phases, the diffusional properties of oxygen in each phase are largely modified by the presence of the other. The addition of 10 mol% cholesterol to the gel phase rendered Laurdan fluorescence approximately as quenchable as in the equimolar mixture of the two phases. This result points out that molecules such as cholesterol, which introduce packing defects in the bilayer, favor oxygen diffusion. From the oxygen quenching experiments and using the properties of generalized polarization, the rate of Laurdan dipolar relaxation can be estimated.Abbreviations used Laudran 2-dimethylamino-6-lauroylnapthalene - DLPC dilauroylphosphatidylcholine - DMPC dimyristoylphosphatidylcholine - DPH 1,6-diphenyl-1,3,5-hexatriene - DPPC dipalmitoylphosphatidylcholine - TNS p-tofuidinyl-6-naphthalene sulfonic acid - PBS phosphate-buffered saline solution - GP generalized polarization - NMR nuclear magnetic resonance - EPR electron paramagnetic resonance     

Measurement of the Gamma Ray Energy in Parapositronium 2γ Decay

Using a silica aerogel sample with oxygen gas introduced as the positronium source,the γ ray energy from the parapositronium 2γ annihilation has been measured as 510998.7(2.5)eV by means of the comparison with the known γ energy 511856.2(2.3)eV of the ¹⁰⁶Ru γ ray.The result,after a revision of the binding energy of positronium,is comparied with the adjusted value of the electron rest mass published in several years ago.     

Synthesis and spectroscopic characterization of Kiton Red-620 dye doped silica gel rods

The work was carried out to achieve two different but interdependent objectives; one to synthesis a transparent silica matrix with enough strength and thermal stability, by sol–gel technique, to host an organic dye molecule without quenching its fluorescence and two to find the probability if the said material is suitable for the construction of solid state dye lasers. Crack-free transparent silica gel rods with good mechanical strength, thermal stability and dimensions were successfully synthesized by sol–gel techniques. The rods were doped with Kiton Red-620 dye in different concentration separately. Effect of various synthesis parameters like time, temperature and aging condition was extensively studied to obtain crack-free silica rods doped with dye. Optical properties of prepared rods were studied by FTIR, UV/VIS–NIR and fluorescence spectroscopy. It is observed that Kiton Red-620 dye doped silica gel rods show good fluorescence with sharp peaks in the visible range. Their UV–VIS spectrum indicates the absorption in visible range. Thermal stability of rods were studied by DSC/TGA methods. Eventually it is found that these dye doped silica gel materials explore the possibility for new solid-state dye laser materials.     

Anisotropic magnetic quenching of positronium states in oriented crystals

We have carried out a theoretical analysis of the anisotropy of magnetic quenching of positronium states in noncubic crystals oriented relative to the direction of the external magnetic field. We show that an initial polarization of the positrons amplifies the anisotropy of magnetic quenching of positronium and lowers the magnitude of the magnetic field in which the anisotropy is maximum. We have obtained numerical estimates of the magnitude of the experimentally observed effect for quasipositronium in a single crystal of crystalline quartz and for a positronium complex in a single crystal of naphthalene. Fiz. Tverd. Tela (St. Petersburg) 41, 999–1002 (June 1999)     

The Effect of Cations on Uranyl Transport and Fluorescence in Mesoporous Silica Gel

The fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel was measured in the presence of six naturally occurring cations. It was shown that the presence of the cations can reduce the fluorescence intensity of the uranyl through collision quenching and through competition for the silica gel surface sites. Stern-Volmer quenching coefficients were obtained by measuring the uranyl fluorescence as a function of cation concentration. The cations compete with uranyl to occupy silica gel surface sites and cause a decrease in uranyl fluorescence intensity and a reduction in the uranyl saturation time constant. Energy-dispersive x-ray spectroscopy (EDS) was used to measure the weight percentage of uranium and the cations in the silica gel samples and these results correlated well with the results of the saturation time constant measurements. The results of this study show that, at high concentrations, the presence of cations in water can influence the fluorescence intensity and transport kinetics of uranyl into mesoporous silica gel.     

不同气氛下多孔硅中电子偶素湮没行为研究

使用正电子湮没谱学方法,在不同气氛下对电化学腐蚀法制备的多孔硅中电子偶素的湮没行为进行了系统的研究.正电子湮没寿命谱测试结果表明,样品中存在长达40 ns的电子偶素湮没成分,并且进入多孔硅膜层的正电子约有80%形成电子偶素,具有非常高的电子偶素产额;在氧气气氛下,由于气体导致o-Ps发生自旋转化猝灭是使多孔硅样品中电子偶素寿命缩短的主要原因.结合正电子寿命-动量关联谱测量结果,分析了不同气氛下多孔硅样品中电子偶素湮没寿命及动量变化关系,讨论了多孔硅中电子偶素的湮没机理以及气氛对孔径计算理论模型的影响. 关键词： 电子偶素 正电子湮没谱学方法 多孔硅     

Quenching of porous silicon photoluminescence by molecular oxygen and dependence of this phenomenon on storing media and method of preparation of pSi photosensitizer

The quenching of porous silicon photoluminescence (pSi PL) by molecular oxygen has been studied in different storing media in an attempt to clarify the mechanism of the energy transfer from the silicon photosensitizer to the oxygen acceptor. Luminescent materials have been prepared by two methods: electrochemical anodizing and chemical etching. Different structural forms were used: porous layers on silicon wafer and two kinds of differently prepared powder. Dry air and liquid water were employed as storing media; quenching behaviour was under observation until total degradation of quenching properties. Singlet oxygen molecules generation through energy transfer from photoluminescent pSi was the only photosensitizing mechanism observed under dry gas conditions. This PL quenching process was preferentially developed at 760 nm (1.63 eV) that corresponds to the formation of the ¹Σ singlet oxygen state. Oxidation of the pSi photosensitizer was the main factor that led to its total deactivation in a time scale of few weeks. Regarding water medium, different photosensitizing behaviour was observed. In watery conditions, two preferred energy levels were found: the one detected in dry gas and another centred at approximately 2.2 eV (550 nm). Formation of reactive oxygen species (ROS) different from singlet oxygen, such as superoxide anion or superoxide radical, can be responsible for the second one. This second quenching process developed gradually after the initial contact of pSi photosensitizer with water and then degraded. The process lasted only several hours. Therefore, functionalization of the pSi photosensitizer is probably required to stabilize its PL and quenching properties in the watery physiological conditions required for biomedical applications.     

Theory of positronium formation on solid surfaces: II. Velocity distribution of re-emitted positronium

The velocity distribution of re-emitted positronium form solid surfaces is calculated using the resonant transition model presented in the preceding paper. Thermal smearing of the Fermi surface is included in the calculation. The profiles of velocity distribution are rather more sensitive to temperature than the positronium fractions calculated in the preceding paper. The results agree with experiments for Al. Moreover, the difference between the results and the experiment can be explained by a new ionization process of re-emitted positronium. We can use the new process to measure level shift and level broadening of positronium near solid surfaces.     

Oxygen quenching of toluene fluorescence at elevated temperatures

Gas-phase oxygen quenching of toluene laser-induced fluorescence (LIF) is studied between 300 and 650 K in a nitrogen/oxygen bath gas of 1-bar total pressure with oxygen partial pressures up to 400 mbar. With increasing vibrational excitation of the laser-excited toluene, intramolecular decay becomes faster, resulting in a decreasing relative strength of collisional quenching by oxygen. Additionally, Stern–Volmer plots are found to be non-linear for temperatures above 500 K in the case of 266-nm excitation and at all temperatures for 248-nm excitation. This is attributed to the onset of internal conversion from specific vibrational levels. A photophysical model is developed that describes the experimental data and predicts toluene LIF signal strengths for higher oxygen partial pressures. One important result for practical application is that oxygen quenching is not the dominant de-excitation process for engine-related temperature and pressure conditions, and thus application of the popular fuel–air ratio LIF (FARLIF) concept leads to erroneous signal interpretation.This revised version was published online in August 2005 with a corrected cover date.     

Scattering of positronic atoms on optical phonons in ionic crystals. Positron thermalization

The time for positron thermalization in ionic crystals due to the interaction of positronic atoms with polarizational optical oscillations of the lattice is calculated. The positronic atom is considered as a dipole with moment p induced by the internal crystal field. The dipole moment is evaluated from the value of the positronic-atom deformation parameter determined from data on the magnetic quenching of positronium. For an NaCl crystal, p 1.91D. The positronium thermalization time in ionic, crystals (for the example of NaCl) is estimated at 10^–1 sec. After this time, not only ortho positronium but also para positronium reached total thermalization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizida, No. 5, pp. 66–70, May, 1979.     

Theory of positronium formation on solid surfaces: I. General theory and its application to positronium formation probability

We consider the micro-processes of positronium formation on surfaces with a new Hamiltonian, in which we treat the positron as a quantum particle. First we calculate the temperature dependence and the positronium work function dependence of the positronium formation probability in the case of no surface binding states. We succeeded in deriving a slightly increasing profile of the positronium fraction at low temperatures, as was found in the experiment of Lynn et al. We also calculated the probability of the positronium formation at excited levels and of the formation of the negative positronium ion. Furthermore, the positronium fraction is shown to be proportional to the length at which the surface electron density drops to half the maximum value.     

Synthesis and characterization of Ce doped silica (SiO2) nanoparticles

A series of silica doped with different mol percentages of Ce³⁺ concentration was synthesized using a sol-gel method to determine the dependence of photoluminescence wavelengths and intensity on the concentrations of the dopants. Sol-gel glasses are porous networks that have been densified through chemical processing and heat treatment. Rare-earths (REs) are insoluble in silica; due to this insolubility RE ions in silicate glasses enter as network modifiers and compete for non-bridging oxygen in order to complete their coordination. The morphological, structural, thermal and optical properties of the phosphors were characterized by X-ray diffraction, scanning electron microscopy, UV-vis absorption, photoluminescence, thermogravimetric analyses and differential scanning calorimeter. Silica (SiO₂) gel containing Ce³⁺ ions was sputter coated with Au (gold) in order to monitor surface morphology of the samples. The highest emission intensity was found for the sample with a composition of 0.5 mol% Ce³⁺. Cerium doped silica glasses had broad blue emission corresponding to the ²D_3/2-²F_J transition at 448 nm but exhibited apparent concentration quenching above concentrations of 0.5 mol% Ce³⁺.