K-shell single-electron capture in collision of fast protons with multi-electron atoms

Single-electron capture from the K shell of atomic targets by impact of protons at moderate and high energies has been studied using a first-order three-body Coulomb-Born continuum distorted wave approximation. The applied formalism satisfies the correct Coulomb boundary conditions. Single-zeta Roothaan-Hartree-Fock wave functions are used to describe the initial electronic bound state of the exchanged electron. Both differential and integral capture cross sections are calculated for impact of protons on carbon, nitrogen, oxygen, neon and argon atoms. The results are compared with the available measurements and other theories. The agreement between the calculations and experimental data is remarkable.     

Antihydrogen formation in low-energy antiproton collisions with excited-state positronium atoms

The convergent close-coupling method is used to obtain cross sections for antihydrogen formation in low-energy antiproton collisions with positronium (Ps) atoms in specified initial excited states with principal quantum numbers n_i ≤?5. The threshold behaviour as a function of the Ps kinetic energy, E, is consistent with the 1/E law expected from threshold theory for all initial states. We find that the increase in the cross sections is muted above n_i =?3 and that here their scaling is roughly consistent with \({n_{i}^{2}}\), rather than the classically expected increase as \({n_{i}^{4}}\).     

Theoretical studies of antihydrogen formation in collisions between positronium atoms and antiprotons

The current state of the theoretical methods that are used in calculations of cross sections for the production of antihydrogen in collisions between antiprotons and positronium atoms is reviewed. A broad outline of available methods together with the results of recent computations are presented. The main emphasis is made on the general close-coupling approach that allows any reaction channel to be taken as the initial state of the collision system. In this way, and on account of charge-conjugation invariance, the formation of antihydrogen in collisions between antiprotons and positronium atoms becomes linked to positron-hydrogen scattering and the same computational methods can be applied to either reaction. The review gives references to recent papers on the subject.     

Unified treatment of positron annihilation and positronium formation

The effects of the positron (e(+)) annihilation in e(+) scattering by the H atom are included directly in the Hamiltonian as an absorption potential, and hence the finite lifetimes Gamma(ns) of positronium (Ps) in states ns are automatically taken account of. The Schr?dinger equation is solved using the hyperspherical close-coupling method. The annihilation and Ps formation are shown to be inseparable near the Ps(1s) threshold E(th); Ps formation constitutes just an indirect pathway to annihilation. The annihilation cross section, which would diverge near E(th) if Gamma(1s) were infinite, connects smoothly across E(th) to the cross section for Ps formation, which is meaningful only at energies well above E(th).     

Magnetic dichroism in K-shell photoemission from laser excited Li atoms

Magnetic dichroism in the angular distribution has been demonstrated for single-electron photoemission from inner ns(2) subshells of gaseous atomic targets using the example of K-shell photoionization of polarized Li atoms laser prepared in the 1s(2)2p (2)P(3/2) excited state. The effect is pronounced for the conjugate shakeup and conjugate shakedown photoelectron lines, and less important, though observable, for the main and direct shakeup lines. The phenomenon is caused by configuration interaction in the final continuum state and is quantitatively described by the close-coupling R-matrix calculations.     

Collisions of fast electrons with positronium atoms

Summary Elastic and inelastic collisions of fast non-relativistic electrons with positronium atoms have been studies in the Born-Ochkur approximation. It has been shown that exchange plays an important role for transitions between states with the same parity as in this case the direct scattering amplitude vanishes. Numerical results for the total and ortho-para conversion cross-sections for the 1s→1s, 1s→2s and 1s→2p transitions have been presented for projectile energies varying from 0.1 to 10 keV. According to charge symmetry, presented results apply also to positron projectiles.     

Annihilation of positronium atoms in an oscillatory field

Annihilation parameters (the angular correlation curve and positronium lifetime) are calculated for annihilation of positronium atoms in a three-dimensional oscillatory well. This well can serve as a model of a positron trap in rigid bodies. The model lends itself to exact mathematical analysis, and it admits separation of variables of the center-of-mass motion and the relative motion of the particle. A calculation is given for the wave functions of a positronium, which oscillatory field. The wave function of the relative motion of the particles at small distances is similar to the wave function of free positronium, which gives us a basis to speak of the annihilation of an electron-positron pair as the annihilation of a positronium atom in an oscillatory field. With a decrease in the size of the trap, broadening of the correlation curve occurs, which has Gaussian form for the annihilation of positronium from the minimum condition for center-of-mass motion. The bound state of the electron and positron exists even in traps of the size of the Bohr radius. The model permits evaluation of the size of traps over an experimentally chosen narrow component of the correlation curve.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 82–87, March, 1989.     

Fragmentation of positronium in collision with He atoms

The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.     

K-shell ionization of atoms and ions by relativistic projectiles

We evaluate the total cross section for the single K-shell ionization of atoms and ions by the impact of relativistic electrons. The study is performed to leading orders of the QED perturbation theory with respect to the parameters αZ and 1/Z. The results obtained are in good agreement with experimental data for different atomic targets. In the case of moderate values of the nuclear charge Z, the total cross section is described by a simple analytic formula. The K-shell ionization by relativistic heavy particles is also considered.     

Double K-shell photoionization of low-Z atoms and He-like ions

We report on the investigation of the photon energy dependence of double 1s photoionization of light atoms and compare the cross sections for hollow atom and He-like ion production. Measurements of the Kα hypersatellite x-ray spectra of Mg, Al, and Si were carried out using the Fribourg high-resolution x-ray spectrometer installed at the ID21 and ID26 beam lines at the ESRF. The double-to-single photoionization cross section ratios were derived as a function of the incident photon beam energy and compared to convergent close-coupling (CCC) calculations for He-like ions. The dynamical electron-electron scattering contribution to the DPI cross-sections was found to be more important for neutral atoms than for the He isoelectronic series.     

K-shell ionisation of atoms by positron and electron impacts

Recent developments in the calculation of the K-shell ionisation cross sections of atoms by positron and electron impacts are reviewed. The application of the first Born and other approximations derived by the modification of the first Born are discussed. It is pointed out that the first Born approximation modified to include exchange and the effect of the atomic field on the projectile yields satisfactory results for the light atoms.     

Thermalization of positrons and positronium atoms in ionic crystals

It is shown that total positron scattering on acoustic and optical lattice vibrations in atomic semiconductors and ionic crystals causes total positron thermalization before annihilation at both high and low (T≤10 K) temperatures.     

Positron-lithium scattering with the inclusion of positronium formation

Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100 ₀ ² at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10 ₀ ² at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60 ₀ ² just below the first excitation threshold at 1.84 eV.     

Sequential over-barrier ionization of multi-electron atoms in the tens-to-hundreds keV/u energy range

Our previous work on the classical over-barrier ionization model for helium double ionization is extended to the complex multi-electron system of Ne.The total and q-fold ionization cross sections are calculated at energies ranging from a few tens to several hundred keV/u.The calculation results are in good agreement with the experimental data,and the energy dependence of the cross sections suggests that the multi-ionization of a strong perturbated complex atom is probably the sequential over-barrier ionization process.     

The role of coherent radiation and an active medium in the annihilation of positrons and the formation of excited positronium atoms

The irradiation by laser beams and the use of an active medium can increase the probability of formation of positronium atoms in excited states. Corresponding estimates are given.