Abstract: In application to positron annihilation spectroscopy, Ps atom is considered not as a point particle, but as a finite size e+e- pair localized in a bubble-like state in a medium. It is shown that during transition from quasifree to localized state, variation of the internal Coulombic e+-e- attraction energy (several eV) plays an important role, which may govern Ps localization process.
Abstract: The terminal part of the e+ track (the positron blob) is formed during ionization slowing down and subsequent ion-electron recombinations produced by a positron. It releases up to 1 keV of energy, which is converted into heat within few picoseconds. If a bulk temperature of a medium is below, but close enough to its melting point, some region of a substance may melt, yielding a peculiar temperature dependence of the lifetime (LT) spectra. We have estimated properties of the molten region with a help of macroscopic heat con- duction equation and suggested a model describing temperature dependence of the ortho- positronium lifetime in frozen methanol, ethanol, butanol and water close to their melting points.
Abstract: Ps and radiolytic hydrogen yields anticorrelate in saturated hydrocarbons when molecular structure changes from a normal to a cyclic form. This fact is explained by much higher mobility of primary radical-cations in cyclic hydrocarbons than in normal ones.
Abstract: Five samples of porous polymers were investigated using positron annihilation lifetime spectroscopy and liquid nitrogen sorption techniques. Temperature dependencies of o-Ps lifetime were used to estimate the ETE model parameters (R and Δ) for porous polymers. The value of the Δ parameter was found to be 0.13-0.15 nm. A significant difference between pore radii obtained by both techniques was found.
Abstract: Ortho-positronium lifetime in alkanes and alcohols was measured in a broad range of temperatures. The results were compared with predictions of the bubble model. The bubble radius can be determined from the Tao-Eldrup equation as well as calculated using the model. The depth of potential well and surface tension are the main factors ruling the Ps bubble size. The best fit of the bubble model to the experimental data is obtained assuming the rectangular potential well to be 1 eV deep (equivalent to an infinite one broadened by 0.166 nm) and the microscopic surface tension increased about 3 times comparing to the macroscopic one.
Abstract: In the present contribution the dynamics of the intermolecular free volume is investigated. For this purpose a new computational algorithm has been proposed and programmed. The free volume is analyzed along the atomistic trajectory obtained by molecular dynamics simulations and a picture on time averaged free volume structure is obtained. The picture showed two distinct regimes of the free volume dynamics at pico-second and nano-second time-scale in liquid. The analysis has shown that at the time scale of the ortho-positronium lifetime and in the liquid phase the free volume microstructure ceases to exist in rigid cavities. This picture is consistent with the common considerations on the o-Ps existence, which suppose the o-Ps atom digging its own holes/cavities in liquids around the temperature of the o-Ps lifetime saturates.
Abstract: The many-body wavefunction of the unbound He-Ps system has been studied by the exact diagonalization of a explicitly correlated gaussians basis optimized by a stochastic variational method. The nucleus-positron distance has been varied by constraining the parameters of the nucleus-positron correlated gaussian term. The constraining technique allows to describe He and Ps interacting at different distances. The calculated wavefunction can be approximated as composed by weakly perturbed He and Ps atoms. The electron forming the Ps tends to be farther from the nucleus than the positron due to the strong electron-electron Pauli repulsion with the electrons of He. The described technique gives accurate energy and wave functions for Ps interacting with atoms that can be used to calculate the interaction potential of Ps with molecular matter.
Abstract: Commercial composite membranes for water purification were studied by means of the variable-energy positron annihilation γ-ray technique. The positron incident energy E dependence of R parameter (3γ/2γ ratio) analyzed from positron annihilation γ-ray spectra revealed the layer structure consisting of a dense top layer and a porous substrate for the membranes. Thickness of the dense layer for each membrane was evaluated from the R(E) based on a positron implantation stopping profile.