Sergey V. Stepanov

Electric field effect on positronium formation in liquids

The comparison of different models (the Ore, spur, and blob models) of positronium (Ps) formation is presented. Because in molecular media Ps is formed in the terminal positron blob and not in an ordinary spur, the application of the blob model seems to be the most adequate. We extend this model for consideration of the Ps formation in the presence of an external electric field (<100 kV/cm). In the simplified limiting case, this approach provides a formula similar to the Onsager one for the geminate recombination probability. The influence of ion–electron recombination and other intrablob processes on Ps formation is taken into account. The role of quasifree positronium in the Ps formation process is discussed.

What do we learn from the electric field effect on Ps formation in liquids

Abstract The diffusion–recombination model has been extended for consideration of positronium formation in the presence of an external electric field. The important basic ideas and main issues are discussed. It is shown that because of electrical neutrality of the terminal positron blob, e + , easily escapes from the blob during thermalization while intratrack electrons are kept in the blob by an electric field of the primary ions. Ion–electron recombination does not appear to significantly compete with Ps formation.

Microscopic surface tension of liquids with curved free boundary studied by positron annihilation

Abstract A method to determine the microscopic surface tension of nanobubbles is presented, based on the combination of positron lifetime and ACAR spectroscopies.

The nanoheterogeneous structure of aqueous solutions of n-propanol

The results of positron spectroscopy and molecular light scattering studies, the data on radiationchemical yields and optical absorption spectra of solvated electrons formed under the action of ionizing radiation, and the data on the concentration dependences of viscosity, adiabatic compressibility, the velocity of sound, and partial molar volume were used to determine the structure of water-n-propanol mixtures. The conclusion was drawn that the insertion of alcohol molecules into water network voids over the range of alcohol mole fractions 0 < x2 < 0.05 strengthened the structure of water. A further increase in the concentration of the alcohol caused the destruction of the aqueous component and solution homogenization. At 0.01 < x2 < 0.3, mixtures resembled an emulsion of alcohol “nanodrops” suspended in water. At 0.3 < x2 < 0.9, the system again became homogeneous. Lastly, when water was added to pure n-propanol (1 > x2 > 0.9), its molecules combined into nanodrops.

Energy losses of subexcitation charged particles in polar media

A dielectric energy loss rate is calculated analytically for a charged light particle moving in either a straightforward or diffusive way in polar media within the framework of the excluded sphere and the uniformly charged sphere approximations. Some drawbacks of previous considerations are pointed out. As applications, we have calculated the energy loss rates in the slowing down of electrons, positrons and muons in liquid water at room temperature.

Formation of the Ps bubble in liquid media

Abstract The dynamics of the growth of the positronium bubble is considered in hydrodynamic approach. Driving force of the process is the exchange repulsion of the e − constituting Ps from the electrons of the surrounding molecules. Intermolecular forces (Laplace pressure) and friction forces (viscosity) hinder the bubble growth. Numerical calculations are done for a number of liquids having very different surface tensions and viscosities. In liquid He and glycerol the Ps bubble formation time is comparable or even exceeds the para-Ps lifetime.

Beyond the Point Ps Approximation

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.

Incorporation of the Magnetic Quenching Effect into the Blob Model of Ps Formation. Finite Sized Ps in a Potential Well

A self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given. By using the blob model of Ps for-mation we have obtained the contact density in the positronium atom in water, which is in agree-ment with the previous measurements.

A molecular basis of the bubble model of positronium annihilation in liquids

The bubble model of positronium annihilation in liquids universally used for the description of positron annihilation in liquids involves macroscopic notions, based on a continuum description of the liquid (such as surface tension extended to nano-particle dimensions), which is somewhat unsatisfactory. An elementary molecular level description is presented. The consequent clarification of underlying concepts (such as the location of the surface of tension) is injected into the bubble model to obtain a modified version. It is shown that there is a considerable difference between the effective surface tension in such microbubbles and the corresponding bulk values. The concept of a work function for the positronium in the liquid is introduced on the basis of this model from which some conclusions are drawn regarding the quasi-positronium, the precursor of the positronium in the bubble, as being a rather extended delocalized entity.

Effects of Local Heating and Premelting in the Terminal Part of the e+ Track

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.