Absorption of light at electron and positron states in quasi-zero-dimensional systems

A theory of interaction of the electromagnetic field with one-particle electron and positron Coulomb states emerging in nanopores of semiconductors is developed. It is established using the dipole approximation that the oscillator strengths of the transitions and the dipole moments of one-particle electron and positron states in nanopores assume giant values considerably exceeding the typical values of the corresponding quantities for semiconductors.     

Retarded dielectric theory of electron energy loss in multilayers and superlattices

This work uses computer algebra to develop the dielectric theory of energy loss for fast electrons travelling in multilayer systems and superlattices. The interaction of a relativistic electron beam is analysed in detail for beams travelling normal or parallel to the interfaces.     

Second-order theory of the dielectric response function of the electron gas

Using the Green function formalism an expression for the dielectric response function (or density-density function) is derived which is of second order with respect to the Coulomb interaction and applies for arbitrary wave vector and frequency. The employed procedure is somehow lengthy but quite clear from the mathematical point of view. Decoupling methods for Green functions are not used. The theory contains the well-known expressions of the HFA, RPA and the first-order local field theory. In addition to this all four-particle processes (two electrons and two holes) are taken into account and represented in a closed formula, which is the four-particle analog to the Lindhard function. As a first application the plasmon damping is investigated.     

Four pion final states with tagged photons at electron positron colliders

A Monte Carlo generator has been constructed to simulate the reaction , where the photon is assumed to be observed in the detector. Only initial state radiation is considered. Additional collinear photon radiation has been incorporated with the technique of structure functions. Predictions are presented for CMS energies of 1 GeV, 3 GeV and 10 GeV, corresponding to the energies of DANE, BEBC and of B-meson factories. The event rates are sufficiently high to allow for a precise measurement of in the region of Q between approximately 1 GeV and 2.5 GeV. For the construction of the program we employ isospin relations between the amplitudes governing decays into four pions and electron positron annihilation into four pions. Estimates of the kinematic breaking of these isospin relations as a consequence of the – mass difference are given. Received: 29 August 2000 / Revised version: 17 November 2000 / Published online: 21 December 2000     

Toward a theory of nuclear relaxation in dielectric glasses at ultralow temperatures

The temperature and frequency dependence of the nuclear relaxation rate in dielectric glasses is investigated. It is shown that at low and ultralow temperatures nuclear relaxation is due to an interaction between the nuclear quadrupole moment and fluctuations of the electric field created by dipole moments of two-level systems. Fluctuations of this field can be associated with the background relaxation or are due only to the dipole-dipole interaction between two-level systems. It is shown that at lower temperatures the second relaxation mechanism begins to dominate. Expressions are obtained for the temperature and frequency of crossover between different nuclear relaxation regimes. The possibility of experimental confirmation of our results is discussed. Zh. éksp. Teor. Fiz. 115, 2254–2262 (June 1999) Russian Scientific Center “Kurchatov Institute”     

Modification of the theory of diffracted channeling radiation for axial electron and positron channeling

A new type of combinational channeling radiation induced by subbarrier (interband) transitions for the transverse motion of relativistic electrons (positrons) is studied. It is known as diffracted channeling radiation (DCR). The formula describing the DCR angular distribution in the case of axial channeling is obtained by taking into account the band structure of energy levels for the transverse motion of electrons (positrons). It is shown that, in the two-wave approximation of the wave function A(r) of virtual photons, the DCR matrix elements in the dipole approximation for axial and plane channeling coincide formally (with the dimension of the problem taken into account). However, the formulas for DCR angular distributions in the cases of axial and plane channeling differ considerably.     

Bound excess electron states on subcritical size helium clusters in a magnetic field

Stable bound electron states are shown to arise on a helium cluster in an external magnetic field. They are qualitatively different from surface bound electron states and are stable for subcritical size helium clusters, for which there are no bound electron states in the absence of a magnetic field.     

Electron states in metal clusters

Radiative transitions in metal clusters are analyzed in terms of quantum transitions of valence electrons that interact with surrounding valence electrons and ion cores. The analysis is based on the solution of the Thomas-Fermi equation for valence electrons in a spherical cluster. The quantum states of valence electrons and the energy and the dipole moments of transitions are determined in the quasiclassical approximation. It is shown that the frequencies of dipole oscillations and the dipole moments of the transitions strongly depend on the size of a cluster.     

Advances in positron and electron scattering*

The topical issue on Advances in Positron and Electron Scattering” combines contributionsfrom POSMOL 2015 together with others devoted to celebrate the unprecedented scientificcareers of our loyal colleagues and trusted friends Steve Buckman (Australian NationalUniversity, Australia) and Michael Allan (University of Fribourg, Switzerland) on theoccasion of their retirements. POSMOL 2015, the XVIII International Workshop on Low-EnergyPositron and Positronium Physics and the XIX International Symposium on Electron-MoleculeCollisions and Swarms, was held at Universidade NOVA de Lisboa, Lisboa, Portugal, from17–20 July 2015. The international workshop and symposium allowed to achieve a veryprivileged forum of sharing and developing our scientific expertise on current aspects ofpositron, positronium and antiproton interactions with electrons, atoms, molecules andsolid surfaces, and related topics, as well as electron interactions with molecules inboth gaseous and condensed phases. Particular topics include studies of electroninteractions with biomolecules, electron induced surface chemistry and the study of plasmaprocesses. Recent developments in the study of swarms are also fully addressed.     

Computed positron lifetimes in vacancies and vacancy-iron clusters in gold

Annihilation characteristics are calculated for positrons trapped in clean and impurity decorated vacancy clusters in Au. The positron lifetime depends strongly on the structure of the clusters. In a strongly relaxed vacancy cluster, the lifetime can become smaller than the lifetime in a single vacancy. The substitution of some neighbour atoms of a vacancy cluster by Fe atoms has only a minor effect on the positron lifetimes.     

Calculation of positron states in the BEDT-TFF based organic superconductors

Positron states in the BEDT-TTF based organic superconductors, namely -Cu(NCS)₂, -CuCN[N(CN)₂] and -Cu[N(CN)₂]Br salts, have been calculated using the superposedatom model and the numerical relaxation technique. For each salt positrons are distributed predominantly around the anion layers and have a little overlap with the TTF skeleton and the outer S atoms which are responsible for the conductivity.     

Positron states between positronium and “free positron” states in condensed matter

Positronium (Ps), the bound state of an electron and a positron, is formed in many insulating solids and liquids. Recently, several cases of a transition from a Ps state to a non-Ps state have been observed. This has been found for liquid CS₂ if the temperature is decreased from about –40 °C to the melting point, –110.8 °C. In some solid aromatic hydrocarbons, such as naphthalene, biphenyl and p-terphenyl, similar transitions occur upon decreasing the temperature. These transitions are unique in physics since Ps is the only bound state of two light particles in normal low-energy physics. Measurements of the positron lifetime, angular correlation of the annihilation radiation, Ps chemical reactions, and Zeeman effect of Ps give information on the transitions.Deceased.     

Surface positron and positronium states in real ionic crystals

Positron annihilation has been used to examine the positron and positronium states at the surfaces of (100) and (110) faces of KCl and KCl-5% AgCl single crystals. The measured and theoretical parameters differ from the analogous bulk values; exposure of KCl to Ps atoms produces mainly A₊, F₊, and D₊ centers. It is shown that this method can be used to examine surface states and surface electron-density distributions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 76–81, January, 1978.     

Test for the CPT invariance of bound states in QED and production of muonium or antimuonium in electron or positron scattering by nuclei

The possibility of testing the QED CPT invariance of bound states is analyzed for muonium or antimuonium produced in electron or positron scattering by nuclei. The number of muonium production events is estimated for modern accelerators. The method of muonium detection by measuring oscillations appearing in the decay curve owing to the interference of the muonium ground and excited states is discussed.     

Cohesive properties of mercury clusters in the ground and excited states

Optimized structures and cohesive energies of small mercury clusters (Hg_N; N = 3–7, 13, 19) are calculated with the spin-orbit diatomics-in-molecules method. The theory takes into account the effect of s-p mixing which tends to enhance the binding energies in the ground state. It is shown that excimer clusters have significantly short optimum bond lengths and their atomic geometries differ considerably from those in the ground state. Excitation energy gap depends sensitively on both cluster size and nearest-neighbor separation. Numerical results are compared with other theories and experiments.     

Measurement of the high-energy electron and positron spectrum in the PAMELA experiment

The PAMELA magnetic spectrometer onboard the Resurs DK1 satellite no. 1 was put into (Earth) orbit on June 15, 2006; measurements continue at the present time. The scientific objective of the spectrometer is the study of antiproton, proton, positron, electron and light nucleus fluxes in cosmic rays. In this paper, we present the technique for measuring electron and positron spectra in the energy range from 20 to 800 GeV.     

On the theory of dielectric relaxation

The Kirkwood expression for the static dielectric constant of a polar substance is extended to non-zero frequencies by a means which avoids the introduction of spherical specimens and the connection between the dipoledipole correlation functions of distinct spatial regions. The procedure is based on a previously derived relation between the dynamic dielectric constant and the current-current susceptibility and on the use of the Callen-Welton theorem to relate the susceptibility to fluctuations. Surface effects are eliminated at the outset by consideration of media which are infinite in extent. A general relation between the long-range dipole-dipole correlations (which fall off as the inverse cube of the distance) and short-range correlations in a specimen composed of non-polarizable molecules is found from a consistency relation. It is shown that the two microscopic relaxation times which result from the existence of but one relaxation time in the dielectric constant correspond to the transverse and longitudinal relaxation times with the longitudinal time being screened relative to the transverse time by the static dielectric constant.