Positron annihilation in free volume elements of polymer structures

Free volume effects are important for positronium formation, pickoff annihilation and chemical reactions. In order to make the method of positron annihilation an effective technique for quantitative studies of molecular solids, for example polymers, the relation between the intensity of the longlived component of lifetime distribution of positron annihilation and the number of defects (elementary free volumes) was considered. The analysis assumes selective trapping of positrons and positronium in the defects of ordered and disordered (crystalline and amorphous) sites, respectively, the sizes of nonhomogeneities are assumed to be lower than positron diffusion length. The results on positronium annihilation in porous poly(phenylene oxide) allow one to estimate nontrapped positronium diffusion coefficient which is equal to . The relation between the positronium lifetime and effective size of free volume for large pores (effective radius 1 nm) is considered. Experimental results were obtained using the CONTIN program; some comments on its application for calculations of size distribution of elementary free volumes in polymers are discussed.     

An investigation of point defects in silicon carbide

Positron lifetime measurement on bulk samples of single crystal SiC wafers have shown that both 6H and 4H polytypes exhibit a bulk lifetime of 150±2ps. All samples contained a second, defect-related lifetime component, ranging in value from 250 to about 300 ps with rather low intensities. The defect structure exhibited by the nanocrystalline samples is, not unexpectedly, much more complex.Positron beam experiments on Electron Cyclotron Resonance Chemical Vapour Deposited (ECR-CVD) SiC thin films showed that the positrons are very sensitive to changes in important film parameters as a function of the deposition conditions. It was found that the film density is lower than expected, probably due to hydrogen incorporation; variations in compositions among different films were detected through variations in theS parameters, and differences were observed in the electric field at the film-substrate interface due to hydrogen passivation of dangling bonds and different substrate resistivities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE

We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Positronium atom in solids

The angular distribution of annihilation 's in quartz shows that the half-width of the narrow component of the correlation curves depends on the state of aggregation (whether the substance is a single crystal, a powder, or a glass). Before annihilation, the positronium atoms may be captured by defects (vacancies, pores, or regions of reduced electron density); if this is true, the half-width of the narrow component and the long positron lifetime depend on the characteristics of the defects or traps. The sizes of the defects in certain solids (quartz glass; powdered beryllium, magnesium, and aluminum oxides; and polytetrafluoroethylene) are evaluated from the experimental half-width of the narrow component and the long positron lifetime.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 59–64, May, 1971.     

Microstructural evolution of ZnS during sintering monitored by optical and positron annihilation techniques

Positron lifetime and optical absorption techniques were employed to track the microstructural evolution of polycrystalline ZnS grown by Chemical Vapor Deposition (CVD). As grown material and material treated with Hot Isostatic Pressure (HIP) was sintered at temperatures ranging from 400 to 1000°C for 2–18 h. A 290 ps defect lifetime could be resolved in all samples, while an additional longer lifetime (=430 ps) was found only in samples annealed at low temperatures. This component gradually disappeared during annealing at 800°C. Associated with the disappearance of the long-lived component, the apparent bulk lifetime of the material changed from 235 to 215 ps. A 215±2 ps bulk parameter was also found for HIP-treated material annealed at temperatures greater than 400°C and hence is taken to represent the delocalized state of the positrons in ZnS. Optical absorption measurements showed that annealing at 800°C also caused the absorption profiles of the CVD and HIP samples to converge. The rate of the bulk lifetime transition correlates with the absorption changes. The observed sharpening of the absorption profile is attributed to a decrease in scattering from grain boundaries and voids, and a decrease in absorption from point defects. The 430 ps lifetime is believed to be due to trapping at voids and grain boundaries, while the 290 ps lifetime likely is due to a monovacancy stabilized as a small complex.Paper presented at the 132nd WE-Heraeus-Seminr on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Solvent Exchange in Excited-State Relaxation in Mixed Solvents

The fluorescence of styrylthiazoloquinoxaline (STQ) in the solvent mixture methanol and dichloromethane (DCM) and 2-octanol have many common characteristics: biexponential fluorescence decay, wavelength-dependent amplitudes, a negative amplitude for the short-lifetime component at long emission wavelengths, and a time-dependent red shift of the emission spectrum. In octanol, the fluorescence lifetime decreases with increasing temperature, whereas the lifetime increases with temperature in the methanol/DCM mixture. The fluorescence characteristics in 2-octanol ( = 7.29 cP) are readily explained by the conventional model of excited-state relaxation kinetics by solvent reorientation. This model is not applicable for low-viscosity ( = 0.455 cP) solvent mixtures. A model of excited-state relaxation kinetics involving solvent exchange (versus solvent reorientation in pure solvents) in the excited state is proposed for the solvent mixture. The model assumes that the solvent compositions around the solute are different in the ground and excited states and the solvent composition is temperature dependent.     

Characterization of diamond single crystals by means of double-crystal X-ray diffraction and positron annihilation

The crystallinity of synthesized and natural crystals of diamond was characterized by double-crystal X-ray diffraction and positron annihilation. The two-dimensional angular correlation of annihilation radiation and positron lifetime measurements revealed that in natural crystals positroniums are formed in a high fraction. The synthesized crystal Ib showed both an extremely small width for the diffraction and a positron lifetime spectrum with a single component of the lifetime of 115 ps. In contrast, the natural diamonds contain a long-lived component of lifetime longer than 2 ns. The diffusion length of positrons was also measured by a variable-energy positron beam. In the synthesized crystal IIa, a diffusion length of about 100.8 nm was observed.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Nucleation process of stacking fault tetrahedra in gold studied by positron lifetime spectroscopy

By component analyses of positron lifetime spectra in quenched gold, it was found that positrons are strongly trapped by stacking fault tetrahedra (SFT) at which the lifetime is about 160 ps remarkably smaller than that for monovacancies (200 ps) or divacancies (220 ps). Positron lifetimes at small vacancy clusters were also estimated in relation to the nucleation process of SFT, with the aid of computer simulation of kinetics of vacancy clustering during quenching and on subsequent isochronal annealing. The results show that large atomic relaxation occurs in small vacancy clusters among which pentavacancies have the largest open-space, thereby having the lifetime of about 230 ps larger than that of trior tetra-vacancies (160ps). It is also suggested that tetra- and penta-vacancies act as prenuclei for stable nuclei of SFT consisting of six or more vacancies.Present situation: Professor Emeritus     

Polarization dependence on the holographic recording in spiropyran-doped polymers

We report on the photoinduced anisotropy in the holographic recording in spiropyran doped polymers as a result of the photoizomerization of spiropyran into merocyanine. Photoizomerization is achieved with UV polarized light ( = 355 nm) from the third harmonic of Q-switched Nd:YAG laser. After UV illumination a strong and broad polarization-dependent absorption peak centered 600 nm appears. Absorption holograms were recorded with low power He–Ne lasers with 2.2% output diffraction efficiency for polarization parallel to the polarization of the UV laser. The extremely long lifetime of the merocyanine states (12 days in the dark) and their high resolution makes this material promising as an optical memory element.     

Time resolved photoluminescence of Cd-doped InSe

High resolution and time resolved photoluminescence in low resistivity Cd dopedp-type InSe in the 32K–155K temperature range are reported. The photoluminescence spectra consist of a broad band that turns out to have three components with different lifetimes (less than 0.1 s, 2.1±0.1 s and 17±1 s). For a given sample, the relative contribution of the components depends on the excitation conditions. The two components with longer lifetime have an asymmetric gaussian shape and a temperature dependence of both the intensity and the decay time which suggest that they are emitted by complex centers, in which the ground state originates from the Cd related acceptor. The fast component and the weak structures in the high energy tail of the broad band, with much shorter lifetime, are proposed to be due to distant donor-acceptor pair recombination.     

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (0.2 ns) due to positrons in the bulk; a component (0.45 ns) due to surface-trapped positrons; and a long-lived component (2 ns) ofo-Ps in the voids or the interfacial spaces of powders. Both bulk and surface positron lifetimes increase as a function of temperature. Correlations between the intensity of surface-trapped positrons and the surface area and between Ps formation and the surface area of graphite are found. The Ps formation probability increases as a function of temperature. A thermal desorption model interprets the emission process of Ps atoms from the surface of graphite to the vacuum and gives an activation energy of 0.23±0.02 eV.Preliminary results of this paper were presented at the March Meeting of American Physical Society, Los Angeles. Bull. Am. Phys. Soc.28, 347 (1983)     

Microstructure of polycarbonate seen by positrons as an in-situ probe

Positron Annihilation Lifetime Spectroscopy (PALS) is used to probe the micro-structural changes in the polymer polycarbonate in terms of the changes in free volume hole size and their content as a function of temperature. The measured spectra are best fitted to three lifetime component analysis. The average hole size in the amorphous regions is determined from the measured Ortho Positronium (O-Ps) lifetime ₃ by following the treatment of Nakanishi et al. On the other hand, information about defects in the crystalline regions is revealed by trapped positrons lifetime ₂. In this polymer the average hole radius varies from 2.71 Å to 2.77 Å. The present study indicates that this polymer has a glass transition temperature of 152°C. Further, we have calculated the trapping rates in the ordered and disordered regions of the polymer based on Goldanskii's kinetic equations and an attempt is made for the first time to estimate the activation energy in the amorphous and crystalline regions separately.     

Studies on the microstructure of phase transitions in two kinds of liquid crystals by PAT and DSC

The positron lifetime and DSC measerments for EBBA and DOBAMBC have been made with heating and cooling clycles. The experimental results show that a shorter lifetime (₁) is essentially independent of temperature while the longer lifetime (₂) and the intensity (I ₂) change abruptly double or triple with temperature. Consequently, the EBBA has only nematic phases while the DOBAMBC has two liquid-crystalline phases (smectic C* and smectic A) with transition temperatures as follows: for EBBA, solid nematic (304.5 K), nematic isotropic (356.5 K), isotropic nematic (356.5 K), nematic solid (301 K); and for DOBAMBC, solid smectic C* (346 K), smectic C* smectic A (357.5 K), smectic A isotropic (389 K). These critical temperatures are in accordance with the transition temperatures measured by DSC. In addition, the difference in the solid-nematic transition temperature in the heating and cooling cycles was also observed. A discussion about the correlation of the observed changes in lifetime (₂) with the changes in molecular orientational order (S) and dielectric anisotropy () is presented.     

Phenylalanine fluorescence and phosphorescence used as a probe of conformation for cod parvalbumin

The fluorescence emission and triplet absorption properties of phenylalanine in cod fish parvalbumin type II, a protein that contains no Trp or Tyr, was examined in the time scale ranging from nanoseconds to microseconds at 25°C in aqueous buffer (pH 7.0). In the presence of Ca(II), the decay of fluorescence gave two lifetimes (5.9 and 53 ns) and the triplet lifetime was 425 s. Upon removal of Ca, the fluorescence intensity decreased to values approaching that for free Phe, while the longest fluorescence decay component was 17 ns. At the same time, the decay of triplet showed complex nonexponential kinetics with decay rates faster than in the presence of Ca. Quenching and denaturation analyses suggest that the Phe's are present in a hydrophobic environment in the Ca-bound protein but that the Ca-free protein is relatively unstructured. It is concluded that Phe luminescence in proteins is sensitive to conformation and that the long lifetime of Phe excited states needs to be considered when studying its photochemistry in proteins.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

Structural relaxation in polyethylene in the presence of silver oxide investigated by positron-lifetime spectroscopy

Structural relaxation in high-density virgin polyethylene and silver oxide doped polyethylene has been investigated using positron annihilation lifetime technique at 30 °C and 100 °C ageing temperatures. The ortho-positronium (o-Ps) pick-off lifetime and its intensity show no changes with isothermal ageing time for the virgin sample at 30 °C. In the case of the doped sample at 30 °C, the o-Ps intensity shows exponential relaxation in the short-time range. At 100 °C, the o-Ps lifetime remains almost constant, whereas the o-Ps intensity exhibits an exponential character in the virgin and doped samples, which can be fitted with exponential decay curves. The relaxation times have been evaluated from the structural relaxation function constructed using o-Ps intensity values, and the stretching exponent has been estimated using the Kohlrausch–Williams–Watts (KWW) function. Positron results indicate that the stretching exponent seems to be temperature dependent. PACS 78.70.Bj     

Positron annihilation in synthetic zeolites (II) magnetic quenching effect

Positron lifetime spectra were re-measured for a series of synthetic zeolites using a large time window of observation. Magnetic quenching experiments were also performed for the zeolites, and it has been confirmed that both the ₄ and the ₃ components are due to o-Ps. The annihilation rate of the third component, ₃, showed a good correlation with the size of the largest voids, which is similar to the correlation reported for other compounds. However the annihilation rate of the longest-lived component, ₄, showed a poor correlation with the void size. The ₃ component has thus been assigned to o-Ps in the regular voids of the zeolites, and the ₄ component to that escaped to inter-particle open spaces.     

Positrons and electron-irradiation induced defects in the layered semiconductor InSe

The positron annihilation characteristics of the layered semiconductor InSe have been investigated. No evidence for low temperature positron trapping is found in as-grown and heavily deformed InSe. The temperature dependence of the S-parameter in these sample exhibits an increase rate in good agreement with the linear expansion coefficient along the c-axis. The positron lifetime spectra of electron-irradiated 0.01% Sn-doped InSe show a long-lifetime component of 336 ps which is tentatively attributed to positrons trapped at isolated In vacancies. Isochronal annealing experiments performed on these samples show that the recovery of the positron lifetime measured at 77K is accomplished in two stages. The first, starting after annealing at 150K, could be induced by the formation of complexes (V_In-Sn_In). The second stage, observed at temperatures T375K, is attributed to the dissociation of these complexes and subsequent annealing of the In vacancies.     

卟啉和金属卟啉的荧光寿命

在单光子计数仪器上测量了一系列卟啉自由碱和金属卟啉的荧光寿命.在光动力治癌疗法中用作光敏剂的HPD缓冲溶液呈现双指数荧光褒减.τ=3ns的短寿命组分是聚集态HPD的表征,而τ=14ns的长寿命组分是单体HPD的表征,HP缓冲溶液却呈现单指数衰减.有七个卟啉或其金属络合物的寿命是初次报导.某些卟啉化合物的实测寿命与计算值符合良好.     

The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.