Electron — positron quantum droplets

A new physical object, electron-positron quantum droplet, is suggested. Structure, stability and dynamics of such objects are discussed. The analysis is based on the non-relativistic self-consistent local-density approximation. An essential role of many-body effects in the formation of the droplets is demonstrated. Their properties are compared with the known physical objects such as metal clusters and clusters of excitons in a solid.     

Electron and positron scattering from atomic potassium

The present coupled-channels-optical method (CCO) provides a comprehensive theoretical calculation of electron-potassium atom and positron-potassium atom scattering at intermediate energies. The CCO calculations for various physical observables are compared with the convergent close-coupling method (CCC) and other theoretical results, as well as experimental data where available. The CCO provides good to fair agreement with results from the convergent close coupling calculations, which are considered to be the state of the art for alkali atoms in the electron channel.     

Electron and positron planar channeling radiation from diamond

Channeling-radiation spectra have been obtained from a diamond with ≈ 54.5 MeV electrons and positrons. Theoretical calculations are found to fit the experimental results well for electrons, but significant discrepancies exist between theoretical predictions and the positron data.     

Electron density and positron trapping at a vacancy

The redistribution of valence electrons at a vacancy in aluminium is calculated in the Thomas-Fermi approximation. This result is then used in evaluating the annihilation characteristics of positrons trapped at vacancies. A good agreement with experiments is established.     

II. Photon induced nuclear processes

Photo-induced nuclear processes are revisited. The attention is focused on those topics where further investigations, carried out with the synchrotron radiation associated with LEP, are expected to promote an advance of our knowledge of the nuclear response function. A few of the possibilities offered by experimenting with laser photons backward scattered by the electrons of LEP are also considered.     

Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

Electron–positron annihilation into three pions and the radiative return

The Monte Carlo event generator PHOKHARA, which simulates hadron and muon production at electron–positron colliders through radiative return, has been extended to final states with three pions. A model for the form factor based on generalized vector dominance has been employed, which is consistent with presently available experimental observations.     

Electron capture processes in ion-atom collisions

The development of the theory of electron capture processes in ion-atom collisions is reviewed in the present work. The formal theory of scattering is used to obtain the Born and the distorted wave Born series for the rearrangement scattering matrix. On the basis of these series, the applications of the first and second Born approximation, the OBK approximation, the DWBA and the Padé approximants to electron capture processes are discussed. The impulse approximation, the Faddeev method and the second order potential method are also explained. The results of the investigations of the high energy behaviour of the charge transfer cross section based on the Faddeev-Watson multiple scattering expansion are analysed.The methods of molecular and atomic eigenfunction expansions are presented and their relative merits discussed. The semi-classical treatment for determining the differential electron capture probability is reviewed. Results obtained in works utilising expansions in Sturmian, Gaussian and pseudo-state wave functions are also presented. An integral equation approach to the close coupling approximation is described. The field theoretic approach to investigate the electron capture process is discussed in the last section.The results obtained by different theoretical methods are compared with available experimental results.     

Muon and positron induced luminescence in KBr and CuCl

The exciton luminescence induced by muon implantation in two selected halides is compared with that induced by positron irradiation. The data include wavelength and lifetime spectra for potassium bromide and lifetime spectra for cuprous chloride, recorded at the ISIS Pulsed Muon Facility. For each material, the spectra for positron-induced and muon-induced luminescence are indistinguishable; that is, no influence of muonium formation on the self-trapped exciton state can be discerned.     

Electron mass and physicochemical processes in photonic crystals

The effect of a change in the mass of an electronin a photonic crystal on the physicochemical processes in the latter is studied. The shift in the atomic energy levels of hydrogen due to this effect is calculated.     

Electron and positron characteristics of dioxides of transition metals of group IV

The linear method of MT-orbitals is used to perform a self-consistent calculation of the band structure of dioxides of titanium, zirconium, and hafnium. Calculations are performed to determine total and partial densities of state, partial charges, total energy, and optical conductivity. Quasi-free positron states, positron lifetime, and contributions to the process of the annihilation of different electronic states are computed, and a study is made of the effect of oxygen vacancies and substitutional metallic impurities (Y, Mg) on the positron characteristics of cubic zirconium dioxide.Institute of the Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 97–104, September, 1993.     

Electron phonon induced cyclotron resonance

The optical absorption coefficient of free polarons in a magnetic field is calculated for weak coupling at zero temperature in the E//H configuration. Characteristic fluctuations in the absorption are reported.     

Electron beam induced surface activation: a method for the lithographic fabrication of nanostructures via catalytic processes

In focused electron beam induced processing (FEBIP), the very narrow electron beam of a scanning electron microscope or transmission electron microscope is used to locally modify matter on the nanometer scale. Recently, the family of FEBIP could be considerably expanded by the technique of focused electron beam induced surface activation (EBISA). In EBISA, the surface itself gets chemically activated by the impact of the electron beam without the presence of precursor molecules. In the second EBISA processing step, the surface is exposed to a precursor molecule which is then catalytically decomposed at the pre-irradiated/activated areas and eventually continues to grow autocatalytically upon prolonged precursor dosage. In this way, electron irradiation and precursor dosage are effectively separated. One of the advantages is that, due to the autocatalytic growth, the size of the corresponding nanostructures can be controlled by the precursor dosage and corresponding electron proximity effects can be omitted. Another advantage is the parallel processing of the pre-irradiated regions during precursor dosage. This bears the potential to significantly reduce the fabrication times for larger deposits compared to the classical electron beam induced deposition approach, in which precursor molecules are sequentially dissociated by the impact of the electron. The fundamentals and apparent further developments as well as the potential and challenges of the comparably new EBISA technique, and more general of catalytic effects in FEBIP are presented and discussed.     

Electron induced modification in polypropylene

The effect of 23 kGy dose of 2 MeV electron irradiation on polypropylene has been studied by different characterisation techniques, viz. Fourier transform IR spectroscopy, electron spin resonance spectroscopy, thermogravimetric analysis, differential scanning calorimetry and X-ray diffraction analysis. The thermal stability of the polymer was found to be increasing due to electron irradiation. The thermal decomposition temperature as well as the melting temperature in case of irradiated polypropylene was found to increase due to electron bombardment. The isotactic nature of the polymer was found to be unaffected by electron irradiation. An increase in crystallinity of the polymer has also been observed after irradiation.     

Electron energy loss processes in X-ray photoelectron spectroscopy

A technique is established in X-ray photoelectron spectroscopy (XPS), using spectra emitted from successively evaporated metallic films, to distinguish between electron energy loss mechanisms identified as, respectively, extrinsic and intrinsic to the photoelectron excitation process. It is demonstrated that tailing on the high kinetic energy side of many XPS peaks is due to intrinsic processes, while the background emission at energies generally some 30 eV below the peaks arises from extrinsic processes. Plasmon energy-loss peaks are believed to contain contributions from both intrinsic and extrinsic processes.