Comparative study of the electron- and positron-atom bremsstrahlung

Fully relativistic treatment of the electron-atom and positron-atom bremsstrahlung is reported. The calculation is based on the partial-wave expansion of the Dirac scattering states in an external atomic field. A comparison of the electron and positron bremsstrahlung is presented for the single and double differential cross sections and the Stokes parameters of the emitted photon. It is demonstrated that the electron-positron symmetry of the bremsstrahlung spectra, which is nearly exact in the nonrelativistic regime, is to a large extent removed by the relativistic effects

Many-electron effects on the x-ray Rayleigh scattering by highly charged He-like ions

The Rayleigh scattering of x-rays by many-electron highly charged ions is studied theoretically. The many-electron perturbation theory, based on a rigorous quantum electrodynamics approach, is developed and implemented for the case of the elastic scattering of (high-energetic) photons by helium-like ion. Using this elaborate approach, we here investigate the many-electron effects beyond the independent-particle approximation (IPA) as conventionally employed for describing the Rayleigh scattering. The total and angle-differential cross sections are evaluated for the x-ray scattering by helium-like Ni$^{26+}$, Xe$^{52+}$, and Au$^{77+}$ ions in their ground state. The obtained results show that, for high-energetic photons, the effects beyond the IPA do not exceed 2% for the scattering by a closed $K$-shell.Comment: 15 pages, 11 figure

Non-perturbative calculation of the two-loop Lamb shift in Li-like ions

A calculation valid to all orders in the nuclear-strength parameter is presented for the two-loop Lamb shift, notably for the two-loop self-energy correction, to the 2p-2s transition energies in heavy Li-like ions. The calculation removes the largest theoretical uncertainty for these transitions and yields the first experimental identification of two-loop QED effects in the region of the strong binding field

Screened self-energy correction to the 2p3/2-2s transition energy in Li-like ions

We present an ab initio calculation of the screened self-energy correction for (1s)^2 2p3/2 and (1s)^2 2s states of Li-like ions with nuclear charge numbers in the range Z = 12-100. The evaluation is carried out to all orders in the nuclear-strength parameter Z \alpha. This investigation concludes our calculations of all two-electron QED corrections for the 2p3/2-2s transition energy in Li-like ions and thus considerably improves theoretical predictions for this transition for high-Z ions

Two-loop self-energy correction in high-Z hydrogen-like ions

A complete evaluation of the two-loop self-energy diagrams to all orders in Z\alpha is presented for the ground state of H-like ions with Z\ge 40.Comment: RevTeX, 5 figures, 1 tabl

Target effects in negative-continuum assisted dielectronic recombination

The process of recombination of a quasi-free electron into a bound state of an initially bare nucleus with the simultaneous creation of a bound-electron--free-positron pair is investigated. This process is called the negative-continuum assisted dielectronic recombination (NCDR). In a typical experimental setup, the initial electron is not free but bound in a light atomic target. In the present work, we study the effects of the atomic target on the single and double-differential cross sections of the positron production in the NCDR process. The calculations are performed within the relativistic framework based on QED theory, with accounting for the electron-electron interaction to first order in perturbation theory. We demonstrate how the momentum distribution of the target electrons removes the non-physical singularity of the differential cross section which occurs for the initially free and monochromatic electrons

Relativistic configuration-interaction calculation of KαK\alpha transition energies in beryllium-like iron

We perform relativistic configuration-interaction calculations of the energy levels of the low-lying and core-excited states of beryllium-like iron, Fe$^{22+}$. The results include the QED contributions calculated by two different methods, the model QED operator approach and the screening-potential approach. The uncertainties of theoretical energies are estimated systematically. The predicted wavelengths of the K\alpha transitions in beryllium-like iron improve previous theoretical results and compare favorably with the experimental data

Evidence for the absence of regularization corrections to the partial-wave renormalization procedure in one-loop self energy calculations in external fields

The equivalence of the covariant renormalization and the partial-wave renormaliz ation (PWR) approach is proven explicitly for the one-loop self-energy correction (SE) of a bound electron state in the presence of external perturbation potentials. No spurious correctio n terms to the noncovariant PWR scheme are generated for Coulomb-type screening potentia ls and for external magnetic fields. It is shown that in numerical calculations of the SE with Coulombic perturbation potential spurious terms result from an improper treatment of the unphysical high-energy contribution. A method for performing the PWR utilizing the relativistic B-spline approach for the construction of the Dirac spectrum in external magnetic fields is proposed. This method is applied for calculating QED corrections to the bound-electron $g$-factor in H-like ions. Within the level of accuracy of about 0.1% no spurious terms are generated in numerical calculations of the SE in magnetic fields.Comment: 22 pages, LaTeX, 1 figur