[en] The stability and structure of systems comprising a negative ion and positrons are investigated by the stochastic variational method. It is shown that the H^- and the Li^- ions can bind not only one but two positrons. The binding energies of these double positronic atoms, E(H^- , e⁺, e⁺)=0.57 eV , and E(Li^- , e⁺, e⁺)=0.15 eV are somewhat smaller than those of their single positronic counterparts [E(HPs)=1.06 eV and E(LiPs)=0.32 eV ]. We have also found that two Ps^- 's can form a bound system with a proton. (c) 1999 The American Physical Society

[en] We derive a lower bound for the ground-state energy E^F(N,S) of N fermions with total spin S in terms of binding energies E^F(N-1,S±1/2) of (N-1) fermions. Numerical examples are provided for some simple short-range or confining potentials

[en] The adsorption of SO₄^2- (HSO₄^-) ions from diluted solutions (10^-6-10^-3 mol dm^-3) on powdered samples of a stainless steel (O8X18H10T used as constructional material in WWER nuclear reactors) was studied by a radiotracer method using ³⁵S labelled sulphuric acid. The observed pH and concentration dependence of the adsorption was explained by the high adsorbability of HSO₄^- ions that exceeds the adsorbability of SO₄^2- species. The sorption of HSO₄^- ions can be described by a logarithmic isotherm. (author) 19 refs.; 15 figs

[en] The electron transmission probability in nanodevices is calculated by solving an eigenvalue problem. The eigenvalues are the transmission probabilities and the number of nonzero eigenvalues is equal to the number of open quantum transmission eigenchannels. The number of open eigenchannels is typically a few dozen at most, thus the computational cost amounts to the calculation of a few outer eigenvalues of a complex Hermitian matrix (the transmission matrix). The method is implemented on a real space grid basis providing an alternative to localized atomic orbital based quantum transport calculations. Numerical examples are presented to illustrate the efficiency of the method.

[en] Variational calculations of the ground state of positronium hydride (HPs) are reported, including various expectation values, electron-positron annihilation rates, and leading relativistic corrections to the total and dissociation energies. The calculations have been performed using a basis set of 4000 thoroughly optimized explicitly correlated Gaussian basis functions. The relative accuracy of the variational energy upper bound is estimated to be of the order of 2x10^-10, which is a significant improvement over previous nonrelativistic results.

[en] In the framework of real time real space time-dependent density functional theory we have studied the electron-ion dynamics of a hydrogen-terminated silicon surface H-Si(111) subjected to intense laser irradiation. Two surface fragments of different sizes have been used in the simulations. When the intensity and duration of the laser exceed certain levels (which depend on the wavelength) we observe the desorption of the hydrogen atoms, while the underlying silicon layer remains essentially undamaged. Upon further increase of the laser intensity, the chemical bonds between silicon atoms break as well. The results of the simulations suggest that with an appropriate choice of laser parameters it should be possible to remove the hydrogen layer from the H-Si(111) surface in a matter of a few tens of femtoseconds. We have also observed that at high laser field intensities (2-4 V/A in this work) the desorption occurs even when the laser frequency is smaller than the optical gap of the silicon surface fragments. Therefore, nonlinear phenomena must play an essential role in such desorption processes.

[en] In the case of intact fuel claddings, the predominant source of radioactivity in the primary circuits of water-cooled nuclear reactors is the activation of corrosion products in the core. The most important corrosion product radionuclides in the primary coolant of pressurized water reactors (PWRs) are ⁶⁰Co, ⁵⁸Co, ⁵¹Cr, ⁵⁴Mn, ⁵⁹Fe (as well as ^110mAg in some Soviet-made VVER-type reactor). The second part of this series is focused on the complex studies of the formation and build-up of ⁶⁰Co-containing species on an austenitic stainless steel type 08X18H10T (GOST 5632-61) and magnetite-covered carbon steel often to be used in Soviet-planned VVERs. The kinetics and mechanism of the cobalt accumulation were studied by a combination (coupling) of an in situ radiotracer method and voltammetry in a model solution of the primary circuit coolant. In addition, independent techniques such as X-ray photoelectron spectroscopic (XPS) and ICP-OES are also used to analyze the chemical state of Co species in the passive layer formed on stainless steel as well as the chemical composition of model solution. The experimental results have revealed that: (i) The passive behavior of the austenitic stainless steel at open-circuit conditions, the slightly alkaline pH and the reducing water chemistry can be considered to be optimal to minimize the ⁶⁰Co contamination. (ii) The highly potential dependent deposition of various Co-oxides at E>1.10 V (vs. RHE) offers a unique possibility to elaborate a novel electrochemical method for the decrease or removal of cobalt traces from borate-containing coolants contaminated with ⁶⁰Co and/or ⁵⁸Co radionuclides

[en] The adsorption of H₂SO₄ on the surface of powdered samples of O8X18H10T austenitic stainless steel was studied by a radiotracer method using ³⁵S labelled sulfuric acid (10^-6-10^-3 mol dm^-3) in the presence of a great excess of supporting electrolyte (1 mol dm^-3 HClO₄ or 1 mol dm^-3 NaClO₄+HClO₄, pH=2). In accordance with previous findings, the phenomena observed can be interpreted as a specific sorption of HSO₄^-ions, a logarithmic adsorption isotherm with respect to HSO₄^-ions can be fitted to the experimental results. No remarkable potential dependence of the adsorption of HSO₄^- ions was found in the potential range studied (-100+1100 mV). (author) 10 refs.; 7 figs

[en] A new tracer technique and a measuring system consisting of electrochemical and nuclear measuring units were developed for the in-situ investigation of corrosion and contamination processes. The adsorption of sulfate ions labelled with sulfur 35 on the powdered surface of austenitic steel 08X18H10T was studied. The kinetics of the sorption, the effect of Na₂CrO₄ and the electrode potential were studied. It can be stated that the Cr³⁺ containing species of the oxide layer play an important role in the selective equilibrium sorption. (V.N.) 20 refs.; 12 figs

[en] A measuring method based both on the determination of the activity of the tracer isotope ³⁶Cl and on the control of electrochemical parameters was applied to study the sorption processes of chloride ions on powdered steel samples. The effect of CrO₄^2-, SO₄^2- and ClO₄^- anions on the sorption and desorption properties of chloride anions was investigated. The sorption ability of the anions on the steel sample of type DVO-16 proved to decrease in the following order: CrO₄^2- > Cl^- > SO₄^2- = ClO₄^-. (V.N.)